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Geology of the Fareham and Portsmouth district. Sheet description for the British Geological Survey 1:50 000 Series Sheet 316 and the mainland portion of Sheet 331 (England and Wales)

By P M Hopson

Bibliographical reference: HOPSON, P M. 1999. Geology of the Fareham and Portsmouth district. Sheet Description of the British Geological Survey, Sheet 316 and part of 331 (England and Wales).

Geology of the Fareham and Portsmouth district. Sheet description for the British Geological Survey 1:50 000 Series Sheet 316 and the mainland portion of Sheet 331 (England and Wales)

Author: P M Hopson. Contributor: M A Woods, BSc

Keyworth, Nottingham: British Geological Survey 1999. © NERC 1999. All rights reserved. ISBN 0 85272 353 9

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: GD 272 191/1999. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) HMS Victory, the flagship of the Royal Navy, looking east from the harbour. The natural harbour at Portsmouth created by the drowning of the lower reaches of southward flowing streams provided a safe haven for ships. This, together with a hinterland of oak, yew and beech forests growing on the Palaeogene strata of the Bere Forest and the chalk of the Downs encouraged the growth of Portsmouth as a ship building centre and the home of the navy.

(Back cover)

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The British Geological Survey is a component body of the Natural Environment Research Council.

Acknowledgements

This Sheet Description was compiled and largely written by P M Hopson from data held in the open file Technical Reports for this district. S Holloway is thanked for his comments on the structure, concealed geology and hydrocarbon sections and M A Woods is thanked for his review of the biostratigraphy. The manuscript was edited by A A Jackson and R D Lake.

We acknowledge the Department of the Environment (DoE: now DETR) for their support of the field mapping of the southern portion of the Fareham sheet and the mainland portion of the Portsmouth sheet between 1978 and 1984. Landowners, tenants and quarry companies are thanked for permitting access to their lands.

Notes

The area covered by Sheet 316 Fareham and the mainland part of Sheet 331 Portsmouth is referred to as the district.

National grid references are given in the form [SU 1234 1234] or [SU 123 123]. Unless otherwise stated all such references fall within the grid square SU.

Symbols in brackets, for example (LeCk), refer to symbols used on the 1:50 000 Series map.

Numbers at the end of photograph descriptions refer to the official collection of the British Geological Survey.

Geology of the Fareham and Portsmouth district—summary

This Sheet Description covers the area shown on Sheet 316 Fareham and Sheet 331 Portsmouth, but excludes the north-eastern part of the Isle of Wight, which is described elsewhere. Thus the district includes part of Hampshire and West Sussex.

The chalk of the South Downs and an inlier of chalk forming the Portsdown anticline dominate the landscape. It provides a characteristic east - west-trending, escarpment topography to this district that forms the south-western part of the Wealden anticline. Palaeogene strata form the low ground of the Bere Forest and also the coastal plain with its extensive natural harbours of Chichester, Langstone and Portsmouth. The broad coastal plain is urbanised and industrialised around the naval and ferry port of Portsmouth and its satellites Gosport, Fareham, Horndean and Havant. Numerous north-south orientated valleys cut across the east-west chalk escarpments; many are ‘dry’ for much of their length. The larger valleys that support permanent surface flows are the River Meon, in the west, and the River Ems in the east.

This report contains a brief history of geological research in the district, which started with the Geological Survey’s one inch mapping in the 1850s, and includes the search for oil and gas in the 1980s. A summary of the geological history spans the end Carboniferous Variscan orogeny, Permian and Triassic basin development, Mesozoic, and Cainozoic deposition, basin inversion and erosion.

The main part of the report describes both the rocks that have been proved in boreholes (Devonian, Permian/Triassic, Jurassic and lowest Cretaceous) and those that crop out in the district. The Lower Greensand Group and the Gault crop out in the north-east, but the Chalk Group underlies most of the district. The lithostratigraphy of all the formations and members shown on the 1:50 000 Series map is described. Particular emphasis is given to the deposition and diagenetic processes involved in the formation of the chalk.

The largely unconsolidated Palaeogene beds have been worked from many quarries in the past. They include the Lambeth, Thames, Bracklesham and Barton groups, and the sequence consists predominantly of clay, silt and sand.

The Quaternary deposits are described in relation to their mode of origin and they include the residual deposits, fluvial and organic deposits, aeolian deposits, marine and estuarine deposits, landslip and made ground.

A section is devoted to applied geological issues, such as geotechnical factors that should be taken into consideration in any land development. The chalk is a major aquifer in the region and an account of its hydrogeology is given. Other resources described are sand and gravel, building stone and hydrocarbons.

The Information Sources lists all the BGS publications relevant to the district and gives information on how to gain access to BGS collections and databases, including borehole records, geophysical, geochemical and geotechnical data. References cited in the text are available from BGS Library, Keyworth.

(Table 1) Geological succession of the Fareham and Portsmouth district. *timescale from Gradstein and Ogg 1996.

Chapter 1 Introduction

This Sheet Description of the Fareham and Portsmouth district covers the south-east Hampshire coastal plain and its extension into West Sussex, together with the chalk downland and primary scarp of the South Downs in the north (Figure 1a) and (Figure 1b). It does not describe the north-east portion of the Isle of Wight shown on the Portsmouth geological sheet.

The coastal plain and large, natural embayments of Chichester, Langstone and Portsmouth harbours are founded on Quaternary and Palaeogene strata. The broad plain is urbanised and industrialised with the conurbation of Portsmouth and its satellites Gosport, Fareham, Horndean and Havant centred around the naval and ferry port of Portsmouth Harbour.

Northwards, Ports Down with its extensive chalk quarries presents a skyline dominated by massive nineteenth century forts and modern radar installations. Farther north are the characteristic long dip slopes and scarps of the Chalk downland with the Wealden anticline lying to the north-east of the district.

The east-west geological grain of the country is cut across by numerous north-south orientated valleys; many are ‘dry’ for much of their length. Most notable of those with permanent surface flows are the River Meon, in the west, and the smaller River Ems in the east (Figure 1a).

The area is traversed by major routes, from west to east by the M27/A27 and the south coast railway, and from south to north, by the A3 and mainline railway London-bound routes. The A32 connecting Fareham with Alton (to the north of this district) utilises the natural routeway of the Meon valley.

History of research

The district was first surveyed at the ‘one-inch’ scale by H W Bristow and published on ‘Old Series’ One-inch Geological Sheets 9, 10 and 11 in 1856 to 1864. A descriptive memoir The Geology of the Weald covering a part of this and adjacent sheets was compiled by W Topley and published in 1875. The first ‘six-inch’ survey was completed by C E Hawkins, C Reid and W Whitaker and published as ‘New Series’ One-inch Geological Sheets in 1893 (sheet 331) and 1900 (sheet 316). F H Edmunds revised and made additions to sheet 316 in 1928. The descriptive memoir for Sheet 316 was published in 1913, and for Sheets 330/331 in 1915, both by H J Osborne White.

The initial 1:10 000 scale survey of the district was carried out in 1978 to 1984, and this re-survey was done in 1993 to 1996. The geological succession is shown in (Table 1).

Several of the Geological Survey’s ‘water supply’ memoirs have dealt in part with the district (Whitaker and Reid, 1899; Whitaker, 1911; Edmunds, 1928; and Hargreaves, 1982).

The stratigraphy of the Weald was first discussed by Gilbert White (1789) who recognised the principal formations of the Lower Greensand upwards. Subsequently Mantell (1822), Murchison (1826) and Fitton (1836) elaborated on this basic stratigraphy.

In an important memoir devoted to western Sussex, P J Martin, (1828) was one of the earliest workers to clearly recognise the major subdivisions of the Lower Greensand and he introduced a nomenclature to cover the sequence from the Hastings Beds to the Lower Chalk. Members of the Weald Research Committee also published accounts (Wooldridge, 1928; Kirkaldy, 1933). The base of the Gault was studied by Kirkaldy (1935). Humphries (1964) studied the Lower Greensand in the western Weald and was able to trace the Marehill Clay and Pulborough Sandrock throughout. Ruffell (1992) correlated the Hythe Formation of southern England.

Brydone (1912) produced biozonal maps for the chalk of Hampshire. A great deal of his collection is registered in the BGS Survey and Type and Stratigraphical collections with the prefix Ya or Yt. Gaster in a series of papers in the 1930s and 1940s produced biozonal maps for the Chalk of the South Downs in Sussex. His 1944 paper describes the area from the Arun Gap westward to the Sussex/Hampshire border.

Aspects of the sedimentology of the sequences found in the district have been described by several authors. Wood (1957) described the heavy mineralogy of the Lower Greensand; Narayan (1963, 1971) and Allen and Narayan (1964) gave current flow directions for this unit.

Casey (1961) reviewed the palaeontology of the Lower Greensand and gave a reliable zonal framework to which various formation and member names could be assigned. Owen’s studies (1963, 1971 and 1975) of the Gault and part of the Upper Greensand provide a detailed zonal scheme for these formations. Correlation of the Lower Chalk of south-east England was described by Kennedy (1969) and Robinson (1986), The petrology, conditions of deposition and diagenesis of the Chalk Group were considered by Hancock (1975). The detailed stratigraphy for the Middle and Upper Chalk of East Sussex was demonstrated by Mortimore (1986, 1987). His named members can be successfully traced into this district and form the basis of the description of the Chalk Group.

The search for oil and gas in the Wessex Basin, including this district, provided the data for a number of structural studies including those of Stoneley (1982), Chadwick (1985, 1986), Sellwood and Scott (1986), Lake and Karner (1987) and Ruffell (1992).

The lithostratigraphy and biostratigraphy of the Palaeogene strata are discussed in King (1981), King and Kemp (1982), Insole and Daley (1985), and Edwards and Freshney (1987). These studies build upon previous research by, most notably, Prestwich (1850, 1852 and 1854), Hester (1965), Cooper (1976), and Curry, King and Stinton (1977).

The Quaternary sequences (and the contained palaeolithic artifacts) of the Sussex coastal plain (the ‘raised beaches’) have been described by Fowler (1932), Calkin (1934), Martin (1937, 1938) and reviewed in Kellaway et al. (1975) and Mottershead (1976). The research has gained added impetus since the early 1980s with more detailed mapping and stratigraphical investigations as part of the British Geological Survey’s mapping programme, described in Technical Reports (p.26) and the archaeological excavations at Boxgrove (Roberts, 1986).

The Quaternary sequences of the chalk dip slopes, principally the clay-with-flints, were described by Hodgson et al. (1967).

Chapter 2 Structure

Structurally, the district falls within the Wessex Basin (Figure 2)a; (Whittaker, 1985) that covered most of southern England, south of the London Platform and Mendip Hills during Permian to Mesozoic times. It is underlain by Palaeozoic strata which were strongly deformed during the Variscan Orogeny, a period of tectonic compression and mountain building which culminated at the end of the Carboniferous. The rocks of the ‘Variscan Basement’ are only lightly metamorphosed; they consist of Old Red Sandstone of probable Devonian age. Several major southwards-dipping thrust zones and north-west-oriented wrench faults have been tentatively identified in the basement, principally from seismic reflection data. These are thought to have originated during the Variscan Orogeny.

The Variscan Orogeny was followed by a long period of erosion and a major unconformity marks the base of the Permo-Triassic sequence.

In Permian times, subsidence associated with periods of tectonic extension began to affect southern England initiating the development of a number of smaller basins, bounded by large faults, within the Wessex Basin. Sedimentation began in the western part of the Wessex Basin to the west of this district. Deposition gradually spread eastwards so that the earliest rocks present, at depth, within the district are red beds thought to be of Triassic age. Crustal extension was accommodated along pre-existing planes of weakness in the Variscan basement, principally the former Variscan thrusts, which were reactivated in an extensional sense. The majority of the faults within the Wessex Basin show evidence of syndepositional downthrow to the south during Permian and Mesozoic times (Figure 2)b. The largest of these faults divide the region into a series of structural provinces (Chadwick, 1986) such as the Weald and Channel basins, separated by the Hampshire–Dieppe High (also known as the Cranborne–Fordingbridge High) (Figure 2).

The Fareham and Portsmouth district straddles the northern margin of the Hampshire–Dieppe High and part of the Weald Basin (Figure 2)a; the boundary between these two structural provinces lies along the Portsdown- Middleton faults (Figure 2)c, which underlie the northern margin of the Portsdown and Littlehampton anticlines.

During periods of active crustal extension, syndepositional movement on the major faults resulted in thick sequences being laid down on the downthrown (hanging wall) sides and thin sequences on the upthrown (footwall) sides; the beds commonly thin up the dip slope of the tilted fault blocks. In the district, changes in the thickness of the strata across the Portsdown–Middleton faults (Figure 2)c, indicate major periods of active faulting during Early Jurassic and Late Jurassic times and during deposition of the Wealden ‘Group’ of the Lower Cretaceous. Episodes of fault movement were interspersed with periods of tectonic quiescence, when more regional subsidence took place, and strata thickened more evenly towards the depocentre of the Weald Basin.

The sea began to flood into the Wessex Basin in Rhaetian (Late Triassic) times, depositing the sediments which make up the Penarth Group. The area of deposition increased gradually throughout the Jurassic, interrupted by relatively minor periods of erosion, occurring mainly at the basin margins, until Upper Oxfordian to Kimmeridgian times, when the London Platform was probably entirely submerged. Towards the end of Kimmeridgian times, this process was reversed and the London Platform began to re-emerge, probably as a result of global sea-level fall and a reduction in the rate of tectonic subsidence. This resulted in erosion on the margins of the Wessex Basin and the beginning of the development of the Late-Cimmerian unconformity. This marine regression continued to Cretaceous times. Thus the environment of deposition changed from offshore marine (Kimmeridge Clay Formation) to shallow marine (Portland Group), to brackish water and evaporitic conditions (Purbeck Group), and eventually to lagoonal deposition (Wealden ‘Group’). The final period of extensional fault movement, marked by normal faulting, in the Wessex Basin took place in Early Cretaceous times. This resulted in the accumulation of thick sequences of Wealden Group sediments in the main fault-bounded troughs in the eastern Wessex Basin, whilst the intervening exposed highs suffered severe erosion.

A period of regional subsidence followed, during which there was a gradual expansion of the depositional area. This, combined with eustatic sea-level rise, led to renewed marine transgression of the Wessex Basin, and the deposition of the Lower Greensand, Gault and Upper Greensand, and eventually the Chalk Group, which probably covered all the surrounding high areas including the London Platform.

The global sea-level fall at the end of the Cretaceous, resulted in erosion of parts of the Upper Chalk and the development of a pre-Cainozoic unconformity. Later deposition in Palaeocene to Oligocene times was followed by the onset of the compressive tectonic regime during mid-Tertiary ‘Alpine’ earth movements. This compressive event effectively reversed the sense of movement on the major bounding faults of the Wessex and Channel Basins causing inversion of the basins and highs. Uplift is estimated at about 1500 m (Simpson et al., 1989) for both the Weald and Channel depocentres. Subsequently, erosion has unroofed these inverted basins giving rise to the present-day landscape.

Cross-sections showing the main structures are presented on Sheet 316 Fareham and Sheet 331 Portsmouth. The major listric growth faults bounding the Hampshire–Dieppe High can be seen beneath Portsdown (Section 1 and 2, Sheet 316) to the north, and to the south beneath the Arreton Downs (Section 1, Sheet 331), on the Isle of Wight, where the high is bounded by the Portland–Wight Fault. They are both associated with monoclinal inversions. Asymmetric fold structures, such as the Portsdown Anticline, the Bere Forest Syncline, and the Warnford Dome and East Meon anticlines (Plate 1), also affect both the Cretaceous and Tertiary strata.

Erosion associated with the Late-Cimmerian unconformity is demonstrated on Section 1 on Sheet 331 Portsmouth, where strata are cut out beneath the Gault and the Carstone (upper part of the Lower Greensand) down to the Oxford Clay. Elsewhere in the Wessex Basin, this erosion cuts down to the Inferior Oolite in mid-Dorset, the Lias at Golden Cap in west Dorset and to the Trias in Devon.

The northward thickening of the Jurassic and the Wealden strata into the Weald Basin, away from the Hampshire–Dieppe High, can be clearly seen on the cross-section on Sheet 316 Fareham.

Chapter 3 Pre-Cretaceous geology

The stratigraphy of the rocks buried beneath the district is known from boreholes sunk primarily for the hydrocarbon industry. Those at Lomer, Hinton Manor, Horndean, Portsdown and Potwell form the basis of this account. A summary of thicknesses is shown in (Table 2).

Pre-Jurassic

A thin Permo-Triassic sequence of limestone, siltstone, sandstone and breccia overlies beds of siltstone and orthoquartzite tentatively assigned to the Devonian ‘basement’. The structural contours and subcrops of the sub-Permian surface for the southern half of the United Kingdom (Smith, 1985) show a broad band of Devonian rocks stretching from south of the Mendips south-eastwards into the eastern part of the English Channel. A similar map (Sellwood and Scott, 1986) of the sub-Mesozoic floor beneath southern England incorporates the thin and patchy Triassic cover with the older ‘basement’.

Devonian (D-C)

Devonian rocks proved beneath the district are predominantly cleaved, reddish brown and purplish grey, calcareous siltstone and claystone with subordinate siliceously cemented sandstone. The presence of mica, chlorite and epidote indicates that they have been subjected to lowgrade metamorphism. The thickest proved sequence within the district is in the Horndean Borehole where 85.95 m (282 feet) of strata, dipping 26° towards the north-northwest, were encountered beneath a marked angular unconformity. Similar rocks known from boreholes outside the district provide valuable information about the origin of these sedimentary rocks.

The Marchwood Borehole (SU31SE/227) [SU 3991 1118] to the north-west of the district, penetrated a total of 890m of Devonian strata without reaching the base. In the borehole sandstone predominates over siltstone and claystone. This sequence contains fining-upwards cycles of fluviatile origin similar to the Old Red Sandstone (Whittaker, 1980). Similar sequences are known from the Arreton Borehole (SZ58NW/1) [SZ 5320 8580] on the Isle of Wight, where ⁴⁰Ar/³⁹Ar dating indicates deposition between 380 Ma and 390 Ma years ago, and having suffered low-grade metamorphism at approximately 340 Ma years ago. These Early Devonian continental sequences were deposited on the Brabant Massif to the north of the Cornwall Basin (Ziegler, 1982), a part of the Variscan Foredeep Basin; both basins derived much of their sediment from the Old Red Sandstone continent of Laurasia to the north.

Permo-Triassic (P-T)

A thick sequence of Permian and Triassic strata is preserved to the west of this district in the Portland–Wight Basin (Hamblin et al., 1992) but in this district only a thin sequence of doubtful age is known. Permian and Triassic red-beds and Rhaetian (Penarth Group) have been tentatively identified in boreholes. The Lomer and Horndean boreholes proved 35.4 and 13.4 m respectively of limestone, dolomitic limestone and breccia. These lithologies may be the equivalent of the much thicker sequence seen to the west, or more likely they represent a basal conglomerate of the Mercia Mudstone Group, analogous to the Dolomitic Conglomerate of the Mendips. Whatever their age they were probably preserved against bounding faults of the Hampshire–Dieppe High. There is some evidence to suggest an angular discordance between these basal beds and the overlying strata.

Above the basal beds, the Mercia Mudstone Group and Penarth Group have a maximum proved thickness of 31.7 m in this district. The Mercia Mudstone Group consists of mottled reddish brown and greyish green calcareous siltstone and claystone (marl) with some thin sandstone. The overlying Penarth Group comprises fissile dark grey claystone and white to pale brown limestone. The Penarth Group represents a widespread Rhaetian marine transgression.

Jurassic

The whole of the Jurassic System is represented in rocks (Table 3) at depth below the district. They are mainly marine in origin and were deposited within the subsiding Wessex Basin. They rest conformably on the Penarth Group, and reflect predominantly shallow-marine deposition.

The relatively uniform, cyclical sequences of the Jurassic provide evidence for the eastward shift of the area of maximum subsidence in the Wessex Basin as the faults bounding the Hampshire–Dieppe High became active. The Weald and Channel basin depocentres developed at this time.

Lower Jurassic

This epoch represents a period of renewed extension in the Wessex Basin with predominantly shallow-water deposition; variable sediment thicknesses resulted from differential movement of normal faults.

Lias Group (Li)

Apart from the lowest few metres, which are of Triassic age, the Lias Group is Early Jurassic in age (Table 3). It consists predominantly of cyclic alternations of calcareous or bituminous mudstone and pyritic shale with thin, variably argillaceous, micritic limestone. All the divisions of the Lias Group that are seen at outcrop on the Dorset coast can be correlated, using geophysical logs, with deep boreholes in Dorset, Hampshire and West Sussex. However, the Bridport Sand Formation are not present in the Upper Lias of this district; east of Bournemouth they pass laterally into ferruginous, slightly arenaceous limestones, siltstones and shales.

Middle Jurassic

The early Middle Jurassic represents a period of shallow-water carbonate-shelf deposition throughout the onshore part of the Wessex Basin. The sequence is characterised by fossiliferous limestones and limy muds with hardgrounds marking non-sequences and short periods of erosion between lithostratigraphical units. The late Middle Jurassic shows a deepening of the depositional environment, mainly represented by mudstone with thin sandstone and limestone beds. In early Callovian to mid-Oxfordian times, when the Kellaways and Oxford Clay formations were deposited, the Wessex–Channel Basin was characterised by uniform subsidence and stratal thicknesses.

Inferior Oolite Group (InO)

Over the Hampshire–Dieppe High the beds are characterised by sandy, ferruginous, sparsely oolitic limestone and calcareous siltstone in the Lower and Middle Inferior Oolite with more typical ‘Cotswold facies’ grainstone, oolitic limestone and rubbly argillaceous limestone in the Upper part. Northwards into the Weald Basin, the sequence thickens, and all of the Inferior Oolite is in typical carbonate-platform ‘Cotswold facies’.

Great Oolite Group (GtO)

This group comprises in ascending order the Fuller’s Earth, Great Oolite, Forest Marble and Cornbrash formations. Each formation is developed to different degrees, depending on the timing of penecontemporaneous fault movement, in the major structural provinces of the Wessex Basin.

The Fuller’s Earth comprises dark grey, calcareous, fissile, pyritic siltstone and claystone with thin argillaceous limestone.

It passes up into the Great Oolite Formation, which dominates the sequence at depth in this district, and consists of pale grey to cream coloured, ooidal, shelly, hard, pelletal packstones and grainstones with localised argillaceous partings. It is the principal oil reservoir in the Weald Basin; for example it is the producing horizon in the Horndean Oilfield.

The succeeding Forest Marble and Cornbrash are thin in this district (17.7 m in the Horndean 1 Borehole). They consist of pale grey calcareous mudstone and argillaceous limestone passing up into hard limestone.

Kellaways Formation (Kys)

This formation consists of pale to dark grey, silty, micaceous, calcareous mudstone which passes up into fine-grained shelly sandstone. Geophysical logs suggest a single coarsening upwards sequence, less than 20 m thick, throughout the district.

Oxford Clay Formation (OxC)

This is divided into the Peterborough, Stewartby and Weymouth members (formerly Lower, Middle and Upper Oxford Clay). The Weymouth Member is the lowest part of the Late Jurassic. The beds are between 125 and 140m thick in this district, and consist mainly of subfissile dark mudstone with varying proportions of silt, carbonate and carbonaceous material, and thin cementstone and limestone beds some of which are sufficiently widespread to be named.

Upper Jurassic

The maximum Jurassic transgression occurred in Kimmeridgian times and the base of the Kimmeridge Clay Formation may represent an erosion surface in this district. However, at the end of Late Jurassic times, a regression due to a fall in sea-level resulted in the gradual emergence of the London Platform to the north and the eventual separation of the Wessex and North Sea basins. A thin sequence of shallow-water sediments was laid down in this district at that time.

Corallian Group (Cr)

The succession consists of limestone, sandstone and siltstone with mudstone in the middle of the sequence. It is divided into upper and lower parts. Over the Hampshire–Dieppe High the sequence thins southward away from the Portsdown Fault and only the Lower Corallian is present. The late Oxfordian Ampthill Clay has been identified at the top of the sequence in both the Potwell and Lomer boreholes where light to dark grey pyritic, calcareous and silty claystone occurs.

Kimmeridge Clay Formation (KC)

Within the district the Kimmeridge Clay may rest on an erosion surface. The formation is composed of a series of sedimentary cycles consisting of mudstone, shale, oil-shale and thin limestone. The lower part consists of interbedded thin siltstone and silty mudstone with sparse thin pale grey argillaceous limestones that pass up into medium to dark grey shelly, fissile mudstones. The upper part of the formation comprises brownish black, shelly and phosphatic mudstone overlain by grey calcareous mudstone and argillaceous limestone. From the middle of the sequence the formation becomes progressively more calcareous upwards.

The Kimmeridge Clay thickens from the south over the Hampshire–Dieppe High to reach a maximum recorded thickness of 335.9 m in the Portsdown Borehole on the downthrown side of the Portsdown Fault. To the north of the Portsdown Fault, on the upthrown side, the Kimmeridge Clay is much thinner but again thickens northwards into the Weald Basin.

Portland Group (Pl)

Following the maximum flooding of the Wessex Basin during deposition of the Kimmeridge Clay, the Portland Group and succeeding Purbeck Group represent a period of regression with a return initially to shallow marine deposition. The Portland Group has been removed by pre-Cretaceous erosion on the southern part of the Hampshire–Dieppe High, but thin sequences have been proved in boreholes onshore south of Portsdown. To the north, the succession thickens into the Weald basin, but the absence of the upper beds in some boreholes suggest a non-sequence at this level. The group ranges in thickness between 25 and 75 m.

The group comprises the Portland Sand overlain by the Portland Stone. The Portland Sand consists of thin fine-grained, glauconitic sandstone and argillaceous sandstone interbedded with siltstone and mudstone. The Portland Stone comprises glauconitic and fossiliferous limestone.

Purbeck Group (Pb)

The traditional division into Lower, Middle and Upper Purbeck has been used in borehole descriptions within the district. Recent revision of the nomenclature (Clements, 1993; Westhead and Mather, 1996) divides the group into two formations, the Lulworth Formation, of Jurassic age, and the succeeding Durlston Formation of Cretaceous age. However the exact position of the Jurassic-Cretaceous boundary is still not determined. Casey (1963) placed it at the level of the Cinder Bed, a stratigraphically significant oyster lumachelle; this lies at the base of the Durlston Formation as defined by Westhead and Masher (1996). The lower part of the Lulworth Formation is also known as the Purbeck Anhydrite because of the preponderance of beds of evaporitic minerals in the sequence.

The group thickens into the central Weald Basin where it is known to be over 140 m thick.

The Purbeck Group rests with a sharp contact, which is possibly erosional, on the Portland Group. Thick beds of anhydrite interbedded with thin argillaceous limestones are common in the lower part. Many of the limestones are brecciated as the result of evaporite diagenesis and consequent volume changes. These pass up into cherty marls and impure limestones, formerly the Middle Purbeck.

The Durlston Formation is represented in this district by shelly limestone and shale, with bituminous shale, which pass up into dark grey calcareous sandstone and shaly clay with thin shelly limestone beds.

Chapter 4 Lower Cretaceous

The Cretaceous period opened with a short-lived marine transgression which produced the characteristic Cinder Bed and associated deposits of the Durlston Formation (Purbeck Group). Despite renewed subsidence at this time the clastic deposition in the Weald area was maintained in non-marine facies by the abundant sediment supply derived from the uprising London–Brabant Ridge to the north, Armorica to the south, and other landmasses to the west and south-west. These early Cretaceous sediments are informally called the Wealden ‘Group’ here and include the Hastings Group and Weald Clay Formation. This section will not describe the Lower Cretaceous sequence exposed on the Isle of Wight which was deposited south of the Portland–Wight fault and with the exception of the Carstone is not known at depth in this district.

The Wealden ‘Group’ sediments were deposited in predominantly freshwater conditions, in a large shallow lake or lagoon that occupied much of the present Hampshire and Weald areas. Some indications of periodic erosion and shallow-water brackish conditions occur, suggesting minor flood events from the ‘East Anglian Sea’ to the east (Allen, 1975). Alluvial and lagoonal mud plains were periodically covered by braided rivers carrying coarser material. Some of the major siltstone-sandstone bodies are thought to have formed by lateral accretion from migrating channels, but the thickest sand units are attributed to accretion of sediment transported into the basin as a result of erosion on a rejuvenated block-faulted source area.

At this time, the Channel and Weald basins are thought to have been separated by the ‘Portsdown Swell’ (the successor to the Hampshire–Dieppe High), and the Wealden ‘Group’ is known to thicken northwards away from this structure.

Rising sea level in Aptian times flooded the Wessex Basin and eventually led to the re-establishment of a marine connection with the North Sea Basin around the western end of the London–Brabant Ridge. The boundary between the lower, essentially non-marine sequence, the Wealden ‘Group’, and the upper marine sequence, the Lower Greensand Group, is marked by the Late-Cimmerian Unconformity. The unconformity represents a gap in the sequence that is greatest at the margins of the Weald Basin, where much of the Lower Cretaceous is missing, and reduces progressively towards the centre of the basin such that in the central Weald the unconformity is represented by a number of closely spaced minor erosion surfaces, close to the boundary between the Wealden Group and the overlying Lower Greensand Group (Chadwick, 1986; Ruffell, 1992).

Brackish deposition in the upper part of the Wealden ‘Group’ gave way to tidally influenced shallow-marine and shoreline sands and clays of the Lower Greensand. Thick marine sequences were deposited in the Wessex Basin which was now subsiding relatively faster than the London–Brabant Ridge.

Deepening of the basin continued into Albian times and the Gault, a sequence of deeper-water marine clays, was deposited. By late Albian times, the London–Brabant Ridge had been overstepped by the Gault. The Upper Greensand is, in part, the lateral equivalent of the Gault (mainly Upper Gault in this district) and represents a shallow-water nearshore environment. It gradually replaces the Gault towards the western part of the Wessex Basin.

The Wealden ‘Group’ and the lower part of the Lower Greensand Group do not crop out in the area but are described here for completeness. Nomenclature for the Lower Cretaceous is shown in (Table 4).

The concealed Lower Cretaceous of this district includes the strata from the Durlston Formation (see Purbeck Group) to the Upper Pulborough Sandrock within the Sandgate Formation. The remainder of the Lower Cretaceous crops out in the north-east of the district.

Wealden ‘Group’

Formations and members of the lower part of the Lower Cretaceous are shown in (Table 4). The ‘group’ shows a general thickening towards the centre of the Weald Basin.

Hastings Group (HB)

Four lithological units make up this group in the Weald Basin. The essentially arenaceous Ashdown and (Lower and Upper) Tunbridge Wells formations are split by the transgressive argillaceous deposits of the Wadhurst Clay Formation and Grinstead Clay; the whole represents three major coarsening-upwards cycles.

Weald Clay Formation (WC)

This formation is less varied than the Hastings Group, but two major sequences occur, the lower characterised by thin limestones containing small forms of Paludina and the upper by limestones containing large forms of the same gastropod.

The Weald Clay Formation is characteristically light grey and yellow-brown in colour but may be variegated greyish green and brick red in places. It consists of noncalcareous, carbonaceous claystone with subordinate thin sandstones and rare limestones, the latter particularly near its base.

Lower Greensand Group (LGS)

Within the Weald Basin the Lower Greensand Group is divided into four formations (Table 4). The lower two formations, the Hythe and Atherfield Clay are concealed within the district, and the Sandgate and Folkestone formations crop out in the north-east.

At depth in the south-east of Sheet 331 Portsmouth and at outcrop on the Isle of Wight, the Lower Greensand Group is represented, in ascending order, by the Atherfield Clay, Ferruginous Sands, Sandrock and Carstone. The lower three of these units are not known over the Hampshire–Dieppe High beneath the mainland. The Carstone is younger, being entirely Albian in age, than the Lower Greensand of the Weald Basin.

Atherfield Clay Formation

The Atherfield Clay Formation consists of dark grey or brown, stiff, shaly, fossiliferous, silty clay or clayey silt of early Aptian age. In places it is sandy and generally contains variable proportions of glauconite. Lomer Borehole proved 9 m of the Atherfield Clay, but elsewhere at depth in the district the beds have not been identified. The formation is estimated to be about 20 m thick to the north of the district around Petersfield, reflecting a general thickening into the Weald.

Hythe Formation

This formation comprises a succession dominantly of medium-grained, glauconitic sandstone; thin siliceously cemented sandstone and chert beds are a common feature. The beds span the Lower Aptian/Upper Aptian boundary and range from deshayesi Zone at the base to at least the bowerbanki Zone at the top. The higher martinoides Zone is thought to be absent in the Chichester district to the east. The thickness of the Hythe Formation is not known within the district, but at Greatham just to the north 64 m have been proved. The beds thicken rapidly to the north and east into the Weald.

Sandgate Formation

The upper part only of this formation is exposed in the extreme north-east of the district. Bristow (1991) gives a detailed description and subdivision of these rocks around Petersfield to the north. However due to the paucity of information it is not clear if these subdivisions can be traced southwards, at depth, where overstep of lithologies may have occurred on to the Portsdown Structure (the fading remnant of the Hampshire–Dieppe High). Neither the Rogate Beds, the lowest division of the Sandgate Formation, nor the succeeding Lower Pulborough Sandrock, is seen at outcrop within the district. Outcrops of the Lower and Upper Marehill Clay are separated by the Upper Pulborough Sandrock.

Pulborough Sandrock Member (PSk)

This grey sandstone weathers to yellowish brown; it is friable, uniformly fine grained, glauconitic and rarely cross-bedded. Locally, the beds are richly fossiliferous; the fauna includes Parahoplites cunningtoni, the subzonal index fossil at the top of the P. nutfieldensis Zone.

Marehill Clay Member (MhC)

This consists of dark grey to purplish grey, silty, locally glauconitic clay. It is sparsely fossiliferous, containing only undiagnostic foraminifera. The Upper and Lower Marehill Clay are respectively 4 and 8 m thick in a borehole at Ryefield. About 4.6 m of the Upper Marehill Clay is poorly exposed in the railway cutting just north of the district at West Heath [SU 7865 2305], where it rests on the iron-cemented top of the Upper Pulborough Sandrock and is overlain by the basal ironstone bed of the succeeding Folkestone Formation. It consists of dark clay which becomes sandy in the lower 1.6 m (see also Topley, 1875).

Folkestone Formation (F)

This formation comprises about 10 to 54 m of fine to coarse-grained, cross-bedded, sands and sandstones. The upper part of the succession contains common, white, grey or lilac clay seams. Much of the succession is exposed in the West Heath sand pit [SU 785 228] where it consists of 25 m of cross bedded, yellow to yellow-brown medium to coarse-grained sand, showing an overall southerly dip of about 4°. The sands are characteristically arranged in large-scale cross beds, in units up to 3 m thick, but the upper part of the succession consists of 6 m of tabular friable sandstones each about 0.1 m thick and separated by thin (10 mm) grey clays. Allen and Narayan (1964) describe various aspects of the cross-stratification encountered at the West Heath sand pit. Mean azimuths indicate that the palaeocurrents were from the north-west (Narayan, 1971) and deposition took place by lateral migration of sand-waves in a shallow sea, possibly also under tidal conditions.

The thickness at Ryefield is 30 m but is known to increase to the north and east. Only fossil wood has been noted in these beds, but they are thought to span the Upper Aptian/Lower Albian boundary.

Excellent roadside exposures and old quarry sections can be seen near Sandhill House [SU 805 220] where up to 10 m of medium to coarse-grained cross-bedded sandstones were noted. Foresets dip steeply south, and fine upwards away from a coarse basal sand a few centimetres thick, marking a difference between these exposures and those at West Heath.

At the top of the Folkestone Beds, a thin, brightly coloured, sandy ironstone, the Iron Grit (up to 0.1 m), is well developed in the Chichester district to the east. The most westerly exposure in the Weald was noted near Petersfield around [SU 725 236], (White, 1910) and suggests that it may be present in the north-east part of this district.

Gault Formation (G)

The Gault gives rise to an undulating topography on a heavy wet clayey soil, given over largely to woodland and permanent pasture. It is poorly exposed, but can be seen in small meander bluffs along the incised streams draining from springs at the base of the Upper Greensand and West Melbury Marly Chalk. It is 80 to 95 m thick in this district.

The boundary with the underlying Folkestone Beds is the Iron Grit (see above). The upper boundary is marked by a negative break of slope at the base of the Upper Greensand scarp, but in this district the contact is masked by thin veneers of alluvium, head, and hillwash or landslip. The Gault consists of mainly pale to dark grey silty fissured soft clay with scattered phosphatic nodules up to 15mm across. The weathered profile of many natural exposures shows a gradation up into very soft pale yellow brown plastic clay beneath the active soil layer and it is this material which is recognised in auger traverses across the interfluves.

The Gault of the Alresford district to the north (Owen, 1971) was deposited on the northern flank of a ‘high’ which affected sedimentation during the Early and earliest Middle Albian. Part at least of the basal Douvilleiceras mammillatum Zone of the Gault is thought to be absent in the north-eastern part of this district. There is no direct evidence for the age of the top of the Gault, but it is believed to be of Late Albian Mortoniceras inflatum Zone age, since the basal beds of the Upper Greensand are of Callihoplites auritus Subzone age (Owen, 1975).

Exposures around Nyewood [SU 800 218] (Owen, 1963) yielded an ammonite fauna indicating the Hoplites dentatus Zone of the Lower Gault.

Upper Greensand Formation (UGS)

The Upper Greensand forms a distinct scarp along its outcrop from East Harting to Ramsdean. The dip-slopes to the south indicate a general dip of 4° towards the southsouth-east and the south-south-west. A small inlier was also mapped in the sunken lane east of East Meon [SU 691 220]. It is between 20 to 35 m thick in this district, but thickens rapidly northward to about 60 m around Selborne [SU 741 339] (White, 1910).

There are a large number of good roadside exposures along the characteristic sunken lanes over the whole of the Upper Greensand outcrop. The majority show white-weathering siltstones and fine silty sandstones.

The formation consists of bedded pale yellow-brown, pale grey and greenish grey bioturbated siltstone and very fine-grained silty sandstone. The beds show a characteristically wispy-bedded structure due to small lenses of clay and sand. The beds contain small and variable amounts of mica and glauconite. The outcrop is characteristically marked by small blocky angular brash that weathers to a greenish hue from which the formation derives its name.

In this district there are apparently lenticular masses of uniform siltstone that are very hard, grey to bluish grey, calcareous with a porcellaneous texture and weather to a buff or white colour. These can be traced for short distances in places, but it cannot be demonstrated whether they represent a single horizon of lenses or a number of such bodies at different levels in the sequence. They appear to be concentrated in the higher part of the formation and this siltstone is commonly seen in brash on the dip slope below the West Melbury Marly Chalk. These siltstones have been worked for building stone, particularly around South Harting (Hopson, 1996), where they are colloquially known as the ‘bluestone’ or ‘malmstone’ (Plate 6a) and (Plate 6b). The latter term has also been used for the local facies of the Upper Greensand as a whole.

The Upper Greensand corresponds to the uppermost M. inflatum Zone and the whole of the succeeding S. dispar Zone of the Upper Albian (Owen, 1975).

Chapter 5 Upper Cretaceous

Upper Cretaceous rocks occupy much of the district. They form the scarp of the South Downs in the north-east, the extensive dip slopes to the south and west and the anticlinal feature of Ports Down in the south (Figure 1b). Principally composed of chalk, the beds are approximately 500m thick. The nomenclature for the Upper Cretaceous used in this district, together with earlier schemes, and the biostratigraphical zonation are shown in (Table 5). The scheme used here is based on those of Mortimore (1983, 1986a) and Bristow et al. (1994, 1997). The Chalk Group is divided informally into Lower, Middle and Upper Chalk formations, whose stratotypes have not yet been designated, and ten members that form the basis of the lithological mapping (Bristow et al., 1997).

In the district, the Chalk Group spans the chronostratigraphical stages of the Cenomanian, Turonian, Coniacian Santonian and Campanian.

In Cenomanian times, emergent landmasses were present in south-west England, Wales, Scotland and Northern Ireland, and farther afield in Brittany, the Vosges, the Ardennes and the Baltic Shield. Southern Britain lay approximately 10 degrees of latitude farther south than at present. Chalk accumulated on the outer shelf of an epicontinental subtropical sea of normal salinity and with little terrigenous input.

The chalk that makes up most of the group consists of predominantly soft white to off-white very fine-grained and very pure, microporous limestone with subordinate hardgrounds and beds of marl, calcarenite and flint. The lowest part of the succession comprises alternations of marl and marly limestone.

Chalk is composed largely of the microscopic calcareous skeletal remains of planktonic algae (coccoliths). Other coarser carbonate material is present, some in rock-building proportions; this includes foraminifera, ostracodes and calcispheres, together with entire and finely comminuted echinoderm, bryozoan, coral and bivalve remains, and disaggregated prisms of inoceramids.

Other, generally minor, constituents of a depositional and early diagenetic origin are present.

Mud-grade material, consisting chiefly of the clay minerals illite, smectite and kaolinite, in varying relative amounts, forms an appreciable proportion (30 to 40 per cent, Destombes and Shephard-Thorn, 1971) of the marly parts of the Lower Chalk and of thin marl seams elsewhere in the Middle and Upper Chalk. Throughout the main body of the chalk, above the West Melbury Marly Chalk, the proportion of argillaceous material falls progressively, until in the Middle and Upper Chalk it does not exceed 5 per cent. The geochemical signature of individual marl seams in the Middle and Upper Chalk is proving to be sufficiently characteristic to correlate the basinal sequences of Sussex and Kent (Wray and Gale, 1993).

A coarser silt and fine to medium-grade sand fraction, predominantly of detrital quartz, forms less than 1 per cent of the chalk overall. Certain beds, most notably the Glauconitic Marl in this district, contain other stable mineral species such as mica, zircon, rutile and tourmaline (Jukes-Browne and Hill, 1903). Another minor siliceous element is derived from skeletal material such as sponge spicules and radiolaria. These are commonly replaced by secondary calcite or pyrite.

The authigenic minerals glauconite and calcium phosphate occur throughout the chalk succession, but are most conspicuous in winnowed and condensed horizons such as the Glauconitic Marl and Jukes-Browne Bed 7 (a possible non-sequence in the Zig Zag Chalk). The phosphate occurs most commonly as reworked nodules within these winnowed levels. Hardgrounds within the chalk commonly have concentrations of glauconite and phosphate as impregnations and coatings. Much of this material is the product of early diagenesis. Finely disseminated pyrite is a common authigenic mineral in the more argillaceous parts of the succession and pyrite nodules and burrow fills are a conspicuous feature of much of the chalk.

For parts of the Middle and much of the Upper Chalk, the most important non-carbonate constituent is flint which occurs in nodular and tabular form in seams which parallel the bedding, and also along cross-cutting joints and fissures (see diagenesis below).

The diagenesis of the chalk occurred in two distinct phases. An early phase, associated with interruptions in sedimentation, affected unconsolidated sediment at or just below the sea floor. A late phase was associated with deeper burial, compaction, silicification and carbonate dissolution.

Early diagenesis of the chalk in response to changing water depth, deposition rates and erosion gave rise to a variety of bedding surfaces and associated lithologies. These range from simple omission surfaces, demonstrating non-deposition, to complicated scoured, burrowed and mineralised surfaces (hardgrounds) overlying lithified chalk (chalkstone). They provide a framework of lithological markers within the Chalk Group. The fact that many of these surfaces and lithologies are the result of basinwide changes in depositional conditions has permitted detailed regional correlations (Mortimore, 1986, 1987; Bromley and Gale, 1982; Robinson, 1986). The progression in the development of hardgrounds is given in Kennedy and Garrison (1975).

Late-stage diagenesis modified the framework of chalk lithostratigraphy created during early sedimentation. Carbonate dissolution as the result of deep burial and compaction has produced a variety of effects. In hard chalks, microstylolites are common, but in the softer chalks stylolites are absent and anastomosing residual clay seams are widespread. Where dissolution has been extensive, the softer chalk takes on a ‘flaser’ appearance with ‘augen’ of white chalk enveloped by greyish marl. The ‘flaser’-like limestones described by Kennedy and Garrison (1975) seem to be the same as the ‘griotte’ chalks described by Mortimore (1979).

The most conspicuous diagenetic process in the chalk is silicification. The major product of this silicification is flint which is considered to have resulted from the segregation of silica (SiO₂), presumably derived from dissolution of original biogenic sponge material and skeletons of other siliceous organisms, notably radiolarians and diatoms, in layers, parallel to the sea floor. Flint is a chert, with a particularly well-developed, conchoidal fracture, which is composed of an aggregate of ultramicroscopic quartz crystals only a few microns across. It is generally present in pure white chalks that contain an insignificant clay content, and is thus most prevalent in the Middle and Upper Chalk formations. Clayton (1986) suggests that this precipitation was a multistage process, initiated by dissolution of host carbonate and occurred 5 to 10 m below the sediment surface, at the oxic/anoxic boundary. It was aided by an excess of dissolved sulphide (initiating acid dissolution) in the pore waters. Local porosity in excess of 75 to 80 per cent and permeability variations, particularly in response to burrowing (trace fossils are mainly Thalassinoides and Zoophycos), produced the characteristic burrow-fill form of most flint nodules. Less marked permeability resulted in tabular flint-bands. Similar sheet flints formed along near-horizontal fractures in the chalk. Even later stage silicification and remobilisation of silica can be demonstrated by cross-cutting, sheet-like bodies which line open joints and faults. The most strongly developed flint beds can be traced over large distances.

Eustatic changes in sea level, subsidence and variable sediment accumulation rates influenced the deposition of the chalk, but these factors did not have a uniform effect over the whole area. Thick successions accumulated in basinal areas and incomplete or condensed successions on adjacent highs, but through time the different areas of deposition migrated in response to these changes in environment.

Evidence of small-scale cyclical deposition occurred in the West Melbury and lower Zig Zag members of the Lower Chalk. Elsewhere in the succession, a lack of suitable markers or colour variations precludes the identification of these cycles. The cyclicity may reflect a linkage with the Earth’s orbital cycles (Milankovitch Cycles) (Gale, 1989).

Marl seams in some parts of the Chalk Group, particularly the Middle Chalk, appear unrelated to sedimentary rhythms and must therefore be regarded as episodic. Their origin is not clearly understood, but Wray and Gale (1993) regard them as representing an increased supply of detrital material, at times of falling sea level. However, some at least may represent volcanic ash accumulations analogous to the contemporaneous tuffs identified in the north German chalk succession.

There has been much argument over the depth of the ‘Chalk Sea’, with early estimates ranging from 250 m (Cayeux, 1897) up to 1280 m (Jukes Browne and Hill, 1904). More recent estimates based on hexactinellid sponge assemblages (Reid, 1973) suggest a range of 200 to 600 m: modern sponges are most abundant and faunally diverse between these depths. Shallow-water horizons such as the cyclicly bedded West Melbury Marly Chalk and the hardgrounds of the Lewes Chalk indicate that deposition took place in the photic mobile zone, perhaps in water depths as shallow as 50 m. Kennedy and Garrison (1975) suggest a range of depth for the ‘Chalk Sea’ of between 50 and 300 m, but deeper estimates of 100 to 600 m are more generally accepted. It follows, therefore, that typical white chalk is neither a deep oceanic ooze nor a deposit of shallow-water origin.

In general, relative sea level rose throughout the Upper Cretaceous, until a marked fall in the Maastrichtian (Hancock and Kauffman, 1979). However, short-term, possibly isochronous reversals to this progression produced regionally correlatable changes in deposition. The maximum transgression in Campanian times probably only left the highest parts of the Welsh Massif above sea level.

The sea floor, by analogy to modern equivalents, is considered to have been soft and adaptions of the fauna, particularly amongst bivalves, lend support to this argument. At winnowed horizons, where the hard substrate became exposed, encrusting organisms such as oysters became common.

Lower Chalk

The Lower Chalk comprises three members, namely the Glauconitic Marl, the West Melbury Marly Chalk and the Zig Zag Chalk. Together they are approximately 90 m thick.

Glauconitic Marl (GM)

This member occurs as inliers in the sunken green lanes east of East Meon and can be traced almost continuously eastwards from Barrow Hill [SU 700 224] to South Harting. It has an erosive contact with the underlying Upper Greensand. To the east of South Harting, the member cannot be followed and is known to be absent from the succession in a road cutting [SU 795 191] near Turkey Island, and to the east of the district (Hopson, 1994), suggesting that a second period of erosion (or non-sequence) occurred before the deposition of the West Melbury Chalk Member.

The member comprises between 1 and 3 m of partly indurated fine to medium-grained calcareous sand with fine gravel and coarse sand-sized black phosphatic nodules. It is a bright olive-green colour and is highly glauconitic and bioturbated (Plate 2). In exposures it is a friable rock, but in the subsoil it breaks down to give a dark green, loose, clayey sand.

The Glauconitic Marl is exposed in the roadside at the crossroads in the centre of Buriton and extensively in the banks of Greenway Lane east of East Meon where it overlies the Upper Greensand (Figure 3); (Plate 2). The Greenway exposure contains an extensive fauna (Woods, 1996[21R]) including Aucellina gryphaeoides, Terebratulina protostriatula, Plicatula minuta and Grasirhynchia grasiana indicating a lowest Cenomanian, lower Neostlingoceras carcitanense Subzone age for the beds above the strongly interburrowed contact with the Upper Greensand.

Micropalaeontological samples (Wilkinson, 1994a) collected from a site near Treyford to the east of the district indicate that the beds below the same strongly interburrowed contact contain Arenobulimina advena and Flourensina intermedia (which indicate an age no older than the highest Albian), while the occurrence of Arenobulimina chapmani (which becomes extinct before the close of the Albian) implies Foraminiferal zone 6a. Analysis of the beds above the contact (i.e. within the glauconitic sand) suggest intraformational reworking has occurred, and that the glauconitic sand could be either latest Albian or earliest Cenomanian in age.

West Melbury Marly Chalk (WMCk)

The soft marly chalks and limestones of this member form a gently sloping shelf at the foot of the South Downs scarp (primary scarp) in the north-east of the district, and the broad floor of both the East Meon and Warnford Dome structures. They are generally separated by a marked negative break of slope from the firm grey to white chalks and marls of the Zig Zag Chalk that underlies the steeply rising ground at the base of the primary scarp and other structures.

The West Melbury Chalk is dominated by cycles of soft, pale to medium, grey marly chalks with thin grey to brown limestones. The base of the succession is marked by a grey marl with variable proportions of glauconite, which rests on an eroded surface of the Glauconitic Marl and Upper Greensand. The top of the succession is the top of the Tenuis Limestone which forms a small positive feature in some places but is generally concealed within the broader negative feature at the base of the primary scarp. The thickness estimated from the outcrop pattern ranges from approximately 10 to 35 m.

The basal bed of the West Melbury Chalk comprises a marl with conspicuous glauconite; it is only a metre or so in thickness but sparse glauconite ranges up to 2 to 3 m into the overlying strata. A hard spongiferous limestone, or perhaps series of limestones occurs 1 to 2 m above the basal bed and these may also contain sporadic glauconite grains.

A characteristic pale greyish brown, rough textured (10 to 30 cm) limestone packed with Schloenbachia marks the middle of the West Melbury succession. Woods (1994) tentatively equates this bed with the M3 limestone at Folkestone (Gale, 1989). Above and below this limestone, a number of thin, grey, poorly fossiliferous limestones occur. In general, those below the M3 limestone contain sponges. The hardness of all of these limestones varies. Some appear locally as ‘cemented lenses’, but all are laterally impersistent, so individual beds cannot be identified reliably by ‘counting up’ (or down) the sequence.

The Tenuis Limestone at the top of the sequence is similar in appearance to the M3 limestone. It is a pale greyish brown rough textured limestone, with Schloenbachia and is distinguished by the inoceramid bivalve I. tenuis and by its uneven hackly fracture (particularly in frosted blocks).

Large exposures of the West Melbury Marly Chalk are rare and only sections showing a few metres of beds are seen in this district; these are principally along sunken tracks and in small degraded pits.

Zig Zag Chalk (ZCk)

This member crops out along the lower face of the primary scarp, along the inward facing lower slopes of Warnford Dome centred around [SU 625 215], around East Meon [SU 680 220], and as outliers capping Torberry Hill [SU 780 204] and Barrow Hill [SU 700 224]. The deep, cleft-like coombes and the buttress-like spurs beneath the Melbourn Rock (see Holywell Chalk) are developed in this horizon.

In this district the Zig Zag Chalk is estimated to be between 40 and 60 m thick.

The Zig Zag Chalk is composed typically of medium-hard greyish to white blocky chalk. The lower part is more marly and contains some thin limestones; it equates with the upper part of the Chalk Marl of the traditional scheme (Table 5).

The base of this member is taken at the Cast Bed (Bristow et al., 1997), a very fossiliferous silty chalk immediately above the Tenuis Limestone. Some 3 to 4 m higher, a pale grey hard splintery limestone with conspicuous Sciponoceras is the only other marker identified during this survey. Higher in the succession the Zig Zag Chalk becomes less marly and is pale cream or white in colour. This colour change could not be located reliably on the steep wooded slopes of the scarp. However, in exposed sections it occurs at the Jukes-Browne Bed 7 level (a calcarenite bed with phosphatic nodules) and marks the base of the ‘Grey Chalk’. The top of the member is taken at the top of the Plenus Marls; these are rarely exposed but can be identified in field brash at a number of localities.

The base of the member is in the Acanthoceras rhotomagense Zone, Turrilites costatus Subzone and the top within the Metoicoceras geslinianum Zone.

The lowest beds of the Zig Zag Chalk have been quarried in a number of places in the past, presumably to provide the raw material for the ‘liming’ of arable fields underlain by the Upper Greensand. Most of these quarries are now degraded and show little stratigraphical detail. However, a large number of chalk faces remain at the Butser Hill quarry [SU 726 203], east of the A3 trunk road, and provide the largest and most continuous exposure of these beds (Figure 4). The highest part of the Zig Zag Chalk including the Plenus Marls is exposed in a pit [SU 6495 2125] south of Whitewool Farm (Whitewood on the map).

The basal bed of the Zig Zag Chalk, the Cast Bed, was identified in brash [SU 7770 2022] on the west slope of Torberry Hill where an extensive fauna of brachiopods, bivalves and ammonoids (PMH 1860 to 1917) was collected from a recently ploughed field. The fauna included Capillithyris squamosa and Kingena concinna which are indicative components of the Cast Bed fauna.

Middle Chalk

The Middle Chalk, as traditionally defined, included the beds from the base of the Melbourn Rock to the base of the Chalk Rock and spanned the Inoceramus labiatus (renamed the Mytiloides spp. Zone) and Terebratulina lata biozones. The Chalk Rock does not occur in the basinal successions of the South Downs and, in the scheme used here, the top of the Middle Chalk is taken at the appearance of hard nodular chalks that define the base of the Lewes Chalk. This horizon is somewhat lower, within the

T. lata Zone, than in the traditional scheme where the top of the Middle Chalk approximates to the base of the S. plana Zone (Table 5).

The Middle Chalk of this district is divided into two members. The lower and thinner member, the Holywell Nodular Chalk, includes the Melbourn Rock at its base. The upper, thicker member, is the New Pit Chalk. The combined thickness of the members is about 90 m. The only large-scale exposure of the Middle Chalk in the district is along the deep steep-sided cuttings for the A3 trunk road where it crosses the scarp beneath Butser Hill. Access to these terraced and partially vegetated slopes is limited by the large volume of traffic and the possibility of dislodging material on to the carriageway.

Holywell Nodular Chalk (HCk)

This member has a narrow outcrop over much of the district, broadening somewhat in the ‘wind-gaps’ and where the scarp takes on a stepped geomorphological expression. Three small outliers identified from brash of the Melbourn Rock are mapped on Hemner Hill [SU 773 195], above the railway tunnel south of Buriton [SU 740 196] and west of Old Winchester Hill [SU 632 208]. The member generally occupies the middle part of the steep primary scarp above a positive feature formed by the Melbourn Rock. However, between South Harting and Buriton the Melbourn Rock is mapped immediately above a strong negative feature, presumably the result of erosion of the Plenus Marls.

The Holywell Nodular Chalk is composed of medium hard to very hard and nodular chalks with flaser marls throughout. It is generally shelly to very shelly and has a gritty texture. The Melbourn Rock at its base is a very hard nodular chalk but generally lacks significant shell debris. Including the Melbourn Rock (which is about 5 m thick) this member is between 15 and 35 m thick based on field estimates.

Biostratigraphically the member spans the Cenomanian/ Turonian boundary with the boundary close to the top of the Melbourn Rock. The member covers the highest part of the M. geslinianum Zone, the Neocardioceras juddii Zone (entirely in Melbourn Rock facies) and much of the Mytiloides spp. Zone (Table 5). In the standard succession in Sussex, the member covers the interval between the Foyle Marl and the base of the Gun Gardens Main Marl (Mortimore, 1986a).

The very hard nodular chalks of the Melbourn Rock give rise to the strong positive feature at the base. This feature is most pronounced on the less steep slopes of the scarp buttresses, but can also be identified together with its characteristic brash on all but the very steepest parts of the scarp (but see above). It is one of the most easily mapped boundaries within the whole chalk succession.

The distinctive hard nodular brash of the Melbourn Rock can be traced throughout the district and can be seen at least in part at both Butser Hill quarry and Whitewool Farm pit. A number of small pits were noted within the member but clear sections are rare. However, exposures in fallen tree roots abound in this district reflecting the damage caused by the 1987 hurricane. For the most part these exposures show hard to very hard white rubbly chalk usually packed with pink shell debris and scattered three-dimensional Mytiloides mytiloides.

Above the Melbourn Rock, the remainder of the Holywell Nodular Chalk is characteristically nodular with common shells of the inoceramid bivalve Mytiloides, locally in rock-building proportions. The higher part of the Holywell Chalk is conspicuously more shelly than the lower part. In hand specimen the rock has a grainy rough texture, a feature that, in the absence of significant shell debris, helps distinguish these beds from the chalks above and below. The sequence also contains thin interbedded plexus marls, but these are only readily apparent in exposures. The upper limit of this member is at the top of the highest shell-detrital nodular chalk.

New Pit Chalk (NPCk)

The member occupies the steeply sloping ground near the top of the primary scarp and around the rim of the Warnford Dome and the East Meon valley. As a consequence of its geomorphological setting its outcrop is usually narrow. An inlier in the deeply eroded valley east of Harting Down [SU 797 183] also showed outcropping New Pit Chalk. The member is between 35 and 42 m thick in this district.

The New Pit Chalk comprises medium-hard, massive-bedded, pure white chalk with regularly spaced pairs or groups of marls, each up to 15 cm thick. It is sparsely fossiliferous with brachiopods dominant. In this district flints are confined to the upper half of the succession although elsewhere in Sussex they are known to occur sparsely down to within a few metres of the base of the member.

Biostratigraphically the New Pit Chalk covers the uppermost part of the Mytiloides spp Zone and all but the highest part of the Terebratulina lata Zone (Table 5). It is equivalent to the strata between the Gun Gardens Main Marl and the base of Glynde Marl 1 in Sussex (Mortimore, 1986a).

Apart from exposures along the A3 cuttings [SU 720 193] to [SU 724 202], exposure of the member is limited to small pits. The most notable are in the Butser Hill quarry complex, and in the abandoned railway cutting [SU 633 228] near Warnford Park where the whole of the New Pit Chalk was formerly exposed. The contact with the overlying Lewes Chalk is exposed at Teglease Down (Figure 5).

Upper Chalk

The base of the Upper Chalk is taken at the base of the Lewes Chalk, which approximately coincides with the incoming of hard nodular chalks at the top of the New Pit Chalk. This level lies within the upper part of the Terebratulina lata Zone. It is therefore somewhat lower than the base as defined in the traditional scheme within which it was taken at the base of the Chalk Rock or, where this is absent, at the first appearance of abundant flint. In general the Upper Chalk is characterised by white chalk with numerous flint seams.

In Sussex and Hampshire the Upper Chalk is divided into six members and all are mapped in this district. The formation is approximately 320 m thick, nearly twice as thick as the Lower and Middle Chalk combined.

The Upper Chalk crops out from the top of the primary scarp southward to the margin of the Palaeogene strata preserved in the Bere Forest Syncline, a maximum distance of 11 km, and forms Portsdown Hill. The outcrop forms a series of minor scarps and long dip slopes.

Lewes Nodular Chalk (LeCk)

The outcrop gives rise to a positive feature on the primary scarp of the South Downs; it forms the rim around the Warnford Dome and East Meon valley and part of the dip slope to the south. Three inliers in incised valleys west of the Warnford Dome indicate that this upwarping of the chalk continues to the west-north-west, probably as far as Cheesefoot Head [SU 53 28] to the north of the district. The base of the member is mapped on the appearance of hard nodular chalks; this is generally 15 to 20 m below the maximum positive break of slope on the primary scarp but in some places it coincides with the break of slope. In the deeply incised valleys, the change from the relatively softer New Pit Chalk to the Lewes Chalk may give rise to a small negative feature, and the hardest beds within the Lewes Chalk stand out as small positive features.

The member is generally between 50 and 55 m thick over much of its outcrop, but it may be as little as 35 m on the southern side of the Warnford Dome and up to 75 m in the east around Beacon Hill [SU 807 184].

The Lewes Chalk comprises interbedded hard to very hard nodular chalks, with soft to medium-hard chalks and marls. The first persistent seams of flint appear near the base. The flints are typically black or bluish black with a thick white cortex.

Biostratigraphically the Lewes Chalk includes the topmost part of the Terebratulina lata Zone and all of the succeeding Sternotaxis plana and Micraster cortestudinarium zones (Table 5). It correlates with the strata from the Glynde Marl 1 to the base of the Shoreham Marl 2 in Mortimore (1986a). In this district the lithological boundary with the New Pit Chalk is invariably mapped between the Glynde Marls and Southerham Marl 1 of Mortimore’s scheme.

The Lewes Chalk is divided into two units by the paired Lewes Marl and the Lewes Flints, which comprise an extensive system of black cylindrical burrow-form flints. The lower unit consists of medium to high-density chalk and conspicuously iron-stained hard nodular chalks. The upper unit is mainly low to medium-density chalks with regular thin nodular beds; it yields the bivalve Cremnoceramus (Mortimore, 1986a)

The most extensive exposure of the Lewes Chalk in the district is along the A3 cuttings near the Queen Elizabeth Country Park [SU 717 188]. Sections in this area were published by Mortimore and Pomerol (1987, p.102).

Two other exposures of note occur at Tegleaze Down [SU 657 199] where a basal section of the Lewes Chalk is exposed, and at Downley Hanger [SU 752 178], where the Turonian/Coniacian boundary is exposed (Figure 5). There are numerous exposures of the Lewes Chalk in cuttings along the abandoned Meon valley railway now part of a long distance walk. The most extensive of these are east of Warnford Park [SU 633 228] and north of Droxford Station [SU 614 187].

Seaford Chalk (SCk)

This member has a broad outcrop on the long, smooth, even dip slopes south of the South Downs scarp, Warnford Dome and East Meon valley, with only limited outliers at Old Winchester Hill [SU 647 205], Butser Hill [SU 715 204], on West Harting Down [SU 768 170] and near to Telegraph House [SU 806 175]. The top of the member is generally taken at the negative break of slope at the base of secondary scarp features (Figure 1). In places where this negative break is absent (e.g. west of Cowdown Farm [SU 764 157]) the junction with the overlying member is inferred (see Newhaven Chalk). In this district the Seaford Chalk is 55 to 80 m thick. The Seaford Chalk is composed primarily of soft white chalk with seams of large nodular and semitabular flint. Near the base, thin harder nodular chalk seams also occur associated with seams of carious flints giving this member a similar appearance to the upper part of the Lewes Chalk; the boundary is not clear-cut in mapping terms. Higher in the sequence the flints are black and bluish black, mottled grey, with a thin white cortex, and they commonly contain shell fragments. Typically, brash from the lower part of the Seaford Chalk contains an abundance of fragments of the bivalves Volviceramus and Platyceramus; brash from the upper part contains Cladoceramus and Platyceramus (Mortimore, 1986a). In the absence of these bivalves, the flaggy-bedded nature and pure whiteness of the soft chalk serves to distinguish it from the Lewes Chalk below.

Biostratigraphically, the Seaford Chalk is co-extensive with the Micraster coranguinum Zone and crosses the Coniacian/Santonian boundary which is placed at the Michel Dean Flint (Mortimore, 1986a). This boundary is also marked by the appearance of Cladoceramus. It correlates with the strata from Shoreham Marl 2 to the base of Buckle Marl 1 of Mortimore (1986a).

The lower part of the Seaford Chalk succession is exposed along the A3 cuttings [SU 714 165] to [SU 717 174] (see Mortimore and Pomerol, 1987). It can also be seen in the cutting for the Meon valley railway south of Droxford [SU 612 183].

Newhaven Chalk (NCk)

This member crops out on the steep face of the secondary scarp, see (Figure 1) below the maximum break of slope, and in the floor of deeply incised valleys cutting through that feature. The Newhaven Chalk is also present at the surface on the steep southern face of Ports Down. In this district, the base of the Newhaven Chalk is generally marked by the lower of two negative breaks of slope at the base of the secondary scarp. In places, the secondary scarp is subdued or absent and broad dip slopes of the Newhaven Chalk are mapped, for example west of Cowdown Farm. Where the negative break of slope marking the base of the Newhaven Chalk is absent, individual marls towards the base of the succession give rise to a number of slight positive features, and this geomorphological pattern, together with the change in character and reduced abundance of flints and the distinctive crinoid fauna of this member, can be useful in distinguishing the Newhaven Chalk from the Seaford Chalk. In this district the Newhaven Chalk is estimated to be 50 to 75 m thick.

The Newhaven Chalk is composed of soft to medium hard, smooth, white chalks with numerous marl seams and flint bands. Typically, the marls vary between 20 and 70 mm thick. They are much attenuated or absent locally, over positive synsedimentary features, where differentiation between the Seaford and Newhaven members is difficult. Channels with hardgrounds and phosphatic chalks have been recorded elsewhere within the member (Hopson, 1994; Mortimore, 1986b), but only one locality, east of Nore Down [SU 773 130], has been identified during this survey.

The brash is composed of smooth, angular flaggy fragments of white chalk very similar in appearance to that of the Seaford Chalk. The incoming of abundant Zoophycos flints near the base of the member serves as a useful marker for mapping the lower boundary.

Biostratigraphically the Newhaven Chalk covers the whole of the Uintacrinus socialis, Marsupites testudinarius and the Uintacrinus anglicus zones and the greater part of the Offaster pilula Zone. The Santonian/Campanian stage boundary lies within this member at the Friars Bay Marl (Mortimore, 1986a). The member is correlated with the strata from the base of Buckle Marl 1 to the base of the Castle Hill Marl of Mortimore (1986a), but recently Bristow et al. (1997) placed the top of the member slightly higher in thick basinal sequences at the Pepper Box Marls.

Individual thecal plates of the zonal index Marsupites testudinarius, can be found in numerous small pits and track-side exposures, but otherwise macrofossils are rare.

The largest exposure of the Newhaven Chalk is that at the huge Paulsgrove quarry (Figure 6), (Figure 7); (Plate 3), on the southern flank of Portsdown. Here about 50m of beds are exposed spanning the O. pilula and basal G. quadrata zones; the highest beds are inaccessible. A nearby exposure to the east, at Harleston Road (Figure 7) [SU 6478 0643], exposes the lower part of the Offaster pilula Zone, apparently stratigraphically below the level of the present Paulsgrove section.

Elsewhere, limited parts of the succession may be observed at Soberton [SU 6123 1630] (Figure 8), and in cuttings along the Meon valley railway south of Soberton [SU 608 160].

Tarrant Chalk (TCk)

This member crops out from north-west to south-east across the district forming the top of the first secondary scarp and the long dip slopes to the south, and on the south side of Ports Down. Along the main crop, the Tarrant Chalk is overlain by broad expanses of clay-with-flints. The maximum estimated thickness of the Tarrant Chalk in this district is 40 m.

The Tarrant Chalk is composed of soft white chalk without significant marl seams, but with some very strongly developed nodular and semitabular flints.

The member includes the topmost Offaster pilula Zone perhaps as low as the Telscombe/Meeching Marls of Mortimore (1986a) (see also Bristow et al., 1997), up to the lower part of the Gonioteuthis quadrata Zone. In the east of the district, the top of the member corresponds to the top of the Applinocrinus cretaceus Subzone of the G. quadrata Zone On Ports Down, the Tarrant Chalk is present in the upper inaccessible part of the Paulsgrove quarry (Figure 7), (Plate 3). Smaller exposures were noted east of Hambledon and north-east of Bishop’s Waltham where numerous sections show blocky soft white chalks with bryozoan and regular seams of large nodular and tabular flints.

Spetisbury Chalk (SpCk)

North of the Palaeogene outcrop, the Spetisbury Chalk crops out as a small outlier around Racton Monument [SU 776 094]. The main crop is on Ports Down where the member forms much of the crest and flanks. It is estimated to be about 40 m thick at its maximum.

The member consists of firm, white chalk with large flints, including tabular, paramoudra and potstone forms with Gonioteuthis and distinctive forms of Echinocorys. Its base in Sussex is inferred to be at about the level of the Whitecliff Flint of Mortimore (1986a), and the member covers the upper part of the Gonioteuthis quadrata Zone.

The principal exposures of the Spetisbury Chalk are at Warren Farm (Figure 9), (Plate 4), Cliffdale Caravan Park, Candy’s Pit, and the lower part of Down End (Figure 7).

Portsdown Chalk (PCk)

This member crops out on the northern and south-western flanks of Ports Down. An estimated maximum of 20 m of strata is preserved.

The Portsdown Chalk consists of relatively soft white chalk with common marl seams and some flints, and in its lower part several seams rich in inoceramid shell debris. The base is taken at the Portsdown Marl at Farlington Redoubt [SU 687 065] (Figure 9) east of Fort Purbrook on Portsdown, and thus the member spans the top of the G. quadrata and base of the Belemnitella mucronata zones. The best exposures are at Clapper Hill, Downend, Warren Farm (Plate 4) and Farlington Redoubt (Figure 9).

Strong strike-orientated near-vertical narrow fracture zones are a feature of the chalk members exposed on Ports Down (Plate 5). Some of the best examples can be seen in the Paulsgrove and Warren Farm pits but these structures have also been noted at Farlington Redoubt and the George Inn exposures. The zones are narrow usually between 0.5 and 1 m wide with parallel commonly slickensided faces. The chalk is highly fractured but the origin of the fractures is not known; the chalk was probably deformed in a plastic and in a brittle state during and immediately after deposition.

Chapter 6 Palaeogene

The Palaeogene strata are preserved in the asymmetrical Bere Forest Syncline, and in the ‘Solent’ Syncline (Figure 1b). The sequence consists predominantly of clay, silt and sand. Much of the early and late Palaeocene is not represented by strata in this district (Table 6), and the region formed part of a land area separating the Paris and North Sea basins.

In latest Thanetian times deposition was in a swampy, warm lowland traversed by braided rivers that deposited sands. These climatic and depositional environments gave rise to the characteristically brightly coloured sands and clays of the Reading Formation.

After a short hiatus a marine transgression spread from the north, and the district lay within a broad embayment which included the London, Hampshire, Belgium and Paris basins. Nummulitids attest to a marine connection to the west into the Tethyan Province at this time. The London Clay and the Wittering formations were deposited in this broad sea.

A hiatus separates the late Ypresian Wittering Formation from the overlying formations of the Bracklesham Group of Lutetian age, and corresponds with a eustatic sea-level lowstand. Renewed deposition was more restricted and confined to a narrow marine embayment (approximating to the present-day Solent) off the Channel Basin. Conditions within this embayment were slightly hyposaline due to fresh water runoff.

The Barton Clay indicates a return to a more normal marine environment.

There are few natural exposures in the generally unconsolidated deposits of the Palaeogene in this district and many of the quarries and pits worked in the past are now degraded and overgrown. Descriptions of older exposures are to be found in White (1910, 1915), Wrigley (1949), Curry and King (1965) and in the BGS Technical Reports for the area compiled in the 1980s (see Information Sources). General descriptions of the beds may also to be found in Edwards and Freshney (1987).

Lambeth Group

Reading Formation (Rea)

These beds rest unconformably on the eroded surface of the Chalk Group, and are between 30 and 35 m thick. They consist of brightly colour-mottled clays and silty clays; localised lenticular bodies of fine to medium-grained well-sorted sand occur at various levels particularly at the top and base. Only the most laterally persistent of these sand bodies are shown on the map. South of Ports Down, White (1915) recorded some lignites.

In places, glauconitic sand or interbedded sand and clay constitute a basal bed, analogous to the ‘Bottom Bed’ of the London Basin. This basal bed is up to 4 or 5 m thick in places.

The Reading Formation has been worked at numerous localities along the northern crop of the Bere Forest Syncline for both sand and for brick clay; only limited exposures remain. The narrow outcrops north of Ports Down, and crossing the natural harbours to the south, are for the most part drift covered and only limited exposures have been noted, for example in the low cliffs of Hayling Island.

Thames Group

London Clay Formation (LC)

Four members are identified within the formation together with the informal ‘Lingula Sands’. The thickness of the London Clay Formation varies between 77 and 120 m in the district, with the greatest thickness beneath Gosport and Portsmouth. This thickening may be due to the presence of the thicker sand bodies within the London Clay in that area, or to a general basinward thickening to the south-east.

This formation consists mostly of grey, pyritic, bioturbated silty and fine-grained sandy clay with interbedded seams of calcareous cementstone and rounded flint pebble beds; a glauconitic sandy bed occurs at the base (‘Basement Bed’). The sequence contains sheet-like and lenticular bodies of fine-grained sand which generally mark the top of coarsening-upward sedimentary rhythms (King, 1981). Each rhythm, when complete, has a basal pebble bed or richly glauconitic horizon that passes up into silty clay, which becomes progressively more silty and sandy upwards. The rhythm is completed by cross-bedded sand or interbedded channel-fills; some of these may be very shelly such as the ‘Lingula Sands’ (Meyer, 1871). The lithological change in each rhythm can be equated to an early marine transgression, followed by low-energy marine sedimentation and a final progradation of course sediment from the margins of the depositional basin.

Five rhythmic units, A to E, (Figure 10) have been identified in the Hampshire Basin (King, 1981) and are correlated with the London Basin. Divisions A to C and the lowest part of D are part of the London Clay; the upper part of D and all of E are part of the Bracklesham Group and are included in the Wittering Formation.

The top of the rhythmic units, A to D, are marked by the Bognor Sand (BoS), ‘Lingula Sands’, Portsmouth (Po) and Whitecliff (Whi) members (Figure 10). The Whitecliff Member includes a separate facet named the Durley Member (Du). These members represent the progradational or channel-fill tops of rhythms. They vary greatly in thickness; each is in the order of 4 to 6 m thick, but they die out laterally. The well-developed Bognor, Portsmouth and Whitecliff members may reach 10 m at their maximum.

Bracklesham Group (BrB)

This group contains a varied succession of interbedded clay, silty and sandy clay, silt and sand. Interbedded shell, lignite and pebble bed horizons reflect deposition in transgressive/regressive sedimentary cycles. The greater part of these beds were probably deposited in an offshore marine environment, but the presence of brackish water molluscs and abundant lignite suggests that a more restricted coastal marsh environment was established from time to time. The group is divided into four formations (Table 6); further subdivision into members is possible (see Curry et al., 1977; King and Kemp, 1982), but these are not generally mappable away from coastal sections or the more extensive artificial exposures.

The group is estimated to be about 81 to 137 m thick. The lowest Wittering Formation makes up about half of this thickness, and the other three formations are approximately of equal thickness. A correlation of the beds across the area is shown in (Figure 10).

Wittering Formation (Wtt)

The lower part of the formation comprises bluish grey clay and sandy clayey silt with two prominent flint pebble beds (‘Allbrook Formation’ of King and Kemp, 1982). The middle part consists of sand and silty sand with lignite and pyritised bivalves (‘Knowle Hill Formation’). The upper part comprises a sequence of shelly clay, silt and sand overlain by a complex series of lignitic silt and sand with pebble beds. The base of this upper part contains a vertebrate fauna (‘Cakeham Formation’).

Earnley Sand Formation (Ea)

The whole formation is seen south of the Portsdown Anticline, with eroded remnants preserved around Curdridge [SU 535 136] and Wickham [SU 565 115] in the west of the district. The formation is 16 to 25 m thick. It consists of clayey, silty, bioturbated glauconitic sand with a rich molluscan fauna, some of which give their names to marker beds. Near the top, sands containing abundant Nummulites laevigatus form a valuable marker horizon.

Marsh Farm Formation (MrF)

This formation is up to 22 m thick. It is composed predominantly of thinly bedded, shelly, clayey silt and silty sand. Marine molluscs occur in the lower part but towards the top there are brackish forms. Channel-form clean sands are common in the sequence around Gosport and these contain much plant debris.

Selsey Sand Formation (Slsy)

This formation is mapped in the south-west of the district around Lee-on-the-Solent. It comprises about 25 m of interbedded shelly sandy clay and clayey silt with a thin basal glauconitic sand bed.

Barton Group

Barton Clay (BaC)

This formation is the lowest division of the Barton Group; only the lowest 9 m or so are preserved in the district. It crops out on the foreshore at Browndown and Gilkicker points near Gosport but much of the sequence is obscured by drift. The beds are typically sandy clay with a thin basal glauconitic sand. Kemp et al. (1979) recorded a foreshore section including their ‘Coral Bed’ some 7 m from the base of the deposit.

Chapter 7 Quaternary

About 60 Ma is estimated to have elapsed between the deposition of the youngest preserved Palaeogene and the oldest Quaternary deposits in this district. During this time younger Palaeogene and Neogene strata were deposited across much of southern Britain, and subsequently removed following uplift along the Wealden axis. During the Quaternary, a further break in deposition occurred after the accumulation of the clay-with-flints and before the deposition of the younger Pleistocene drift.

During the Pleistocene, sea level rose and fell in relation to the quantity of water locked up in ice caps. At times of glacial maxima a periglacial environment was established with much subaerial erosion by solifluction and by an extensive river system flowing to much lower base levels. Three such glacial maxima affected southern England; the most severe was of Anglian age.

During the intervening warm stages, marine transgressions caused drowning of the lower courses of the river systems, principally the Solent River and its tributaries, and the breaching of the Straits of Dover. Beach and nearshore sediments were deposited along the margin of the English Channel. Two degraded cliff lines related to those marine transgressions are preserved on the Sussex coastal plain part of which lies in the south-eastern corner of this district. The southern flank of Ports Down forms the westward extension of those cliffs. A brief summary of the principal events of the Quaternary is shown in (Table 7). The Quaternary deposits are described in groups in relation to their mode of origin. Within each group the deposits are given in ascending stratigraphical order where that is possible.

Residual deposits

Clay-with-flints

This is primarily a remanié deposit created by the modification of the original Palaeogene cover and dissolution of the underlying chalk. The clay-with-flints caps the high ground underlain principally by the higher members of the Upper Chalk. Beyond the outcrop shown on the 1:50 000 Series Sheet 316 Fareham and Sheet 331 Portsmouth other small areas of clay-with-flints are known to exist. They are of limited extent and for the most part represent the eroded remnants of solution pipe fills. The thickness of the clay-with-flints is estimated at about 5 to 6 m at its general maximum but in limited areas, usually at sites where dissolution of the underlying chalk is most pronounced, this may rise to over 10 m.

Clay-with-flints is composed typically of orange-brown or reddish brown clays and sandy clays containing abundant flint nodules and pebbles. At the base of the deposit, the matrix becomes stiff, waxy and fissured (slickensided), and of a dark brown colour with relatively fresh nodular flints stained black and/or dark green by manganese compounds and glauconite. The deposit gives rise to a stiff, silty clay soil strewn with nodular and in some places, well-rounded flints. The soils have a characteristic red-brown hue.

The basal surface of the deposit approximates to the sub-Palaeogene unconformity, but it may be carried some distance below that level in solution pipes. Some areas of clay-with-flints, apparently in situ, are mapped well below this projected sub-Palaeogene surface, for example north of Windmill Down [SU 645 168]. Their preservation at this lower level may be the result of neotectonics. The margin of the clay-with-flints is sharply defined on the scarp edge but the boundary becomes diffuse on the chalk dip slope. In places, this down-slope feather edge is obscured by a lateral passage into a late stage solifluction deposit or head, distinguished with the prefix G on the map. These deposits have a more sandy matrix and a surface brash composed principally of gravel-sized broken angular flints.

Thin remnants, principally within solution pipes, of the clay-with-flints were seen in a number of chalk pits. None presented clear sections, or they were inaccessible at the top of steep faces. However, a trial pit in the clay-with-flints at Shere Copse, Soberton [SU 6275 1582] indicated that the deposit has a multi-phase developmental history over a considerable period of time. A full report of this pit can be found in Hopson (1995).

Older Head

A single outcrop of grey flinty, locally chalky clay is shown on the map near Petersfield. It is thought to have originated as solifluction lobes at the foot of the Upper Greensand scarp.

Head Gravel

The head gravel is generally regarded as the result of solifluction of chalk, Palaeogene deposits and clay-with-flints down the dip slope of the South Downs during cold phases of the Quaternary. Its principal occurrence is to the east on the Sussex coastal plain in the Chichester district (Sheet 317) where it overlies the older and younger raised beach deposits and it is largely obscured by aeolian deposits. The western end of the outcrop is mapped in the south-east of the district around Funtington.

The broad sheet-like deposit is composed mainly of an angular flint gravel set in a sandy stiff clay matrix. In some exposures the matrix contains chalk, but elsewhere this has been lost by decalcification. The deposit is thickest and coarsest along the older cliff line, generally between 5 and 7m thick, becoming thinner and finer grained southwards. Although mapped as a single deposit, interbedded fine grained sediments, perhaps aeolian in origin, suggest that it was formed in more than one Pleistocene episode.

Head

This includes a heterogeneous group of superficial deposits that have accumulated by solifluction, hillwash and hillcreep. Head is generally confined to the bottom of dry valleys on the chalk dip slope, at the foot of steep scarps and in minor valleys draining the Lower Cretaceous.

In general, head comprises yellow-brown, silty, sandy clay with variable proportions of coarser granular material, but all have an earthy texture. Clast composition varies depending on source materials, those deposits derived mainly from the chalk were formerly mapped as ‘dry valley deposits’ or ‘coombe deposits’.

Fluvial and organic deposits

River Terrace Deposits

Extensive tracts of fluviatile sandy gravels, up to 6 m thick, are present in the area which borders the Solent. Smaller areas inland are associated with the rivers Meon, Hamble (headwaters only) and Wallington, and in the extreme northeast of the district with the River Rother.

In the south, the deposits consist of gravels and sandy gravels, but the higher terraces are clayey. In the north the deposits are more sandy and many are graded as pebbly sands. In places, clayey and sandy silt and silty clay mask the underlying aggregate, perhaps indicating preservation of overbank or aeolian deposits at the top of each fluvial cycle. In the south, the gravel component is predominantly flint with subordinate quartz and rare ‘exotic’ clasts. In terrace deposits associated with the Rother, flint again predominates, together with chert, polished fine quartz and larger fragments of pebbly ‘carstone’ (sandstones with a ferruginous cement) derived from the Folkestone Beds.

In the south and west of the district, terraces relate to the ‘Solent River’ which developed in response to falling sea levels in Middle Pleistocene times. Up to seven terrace aggradations have been recognised in the district and they can be related to a more extensive system preserved in the Southampton district to the west (Edwards and Freshney, 1987).

Terrace formation, related to the ‘Solent River’, occurred over a considerable time span during the Pleistocene. The sixth terrace is thought to equate with the pre-Anglian raised beach at Portsdown (ApSimon et al., 1977); itself considered to be equivalent to the older raised beach at Boxgrove in the Chichester district to the east. There is little direct evidence of the age of the younger terraces, but most aggradations are probably periglacial in origin. Terrace deposits above the second all show cryoturbation structures indicating that they have suffered at least one periglacial event, and thus suggesting they are all pre-Devensian. Palaeolithic stone implements are common in the fourth and lower terraces, and the Acheulian tool industry affinities of some of these suggest an Ipswichian age.

There is no direct evidence for the age of the River Rother terraces in this district. They are directly related to the terrace sequence of the confluent River Arun to the east, where first to fourth terraces are considered to be of Devensian age and higher terraces of Hoxnian age (Thurrell et al., 1968).

Alluvium

Narrow ribbons of alluvium are mapped along the floor of the valleys with flowing streams. It represents the fine grained late-stage (mature) phase of river development with cut-off channels, overbank flood deposition and localised peat formation. In general, the deposit is thin, usually between 1 and 3 m in the upper reaches of rivers, but at major confluences and in the lower reaches of the rivers about 8 m have been proved. Exceptionally, 14.2 m of peat and peaty clays are present in the River Alver north of Browndown [SZ 585 995] in Gosport; this thickness is probably the result of accumulation behind the shingle bar at the mouth of the stream.

The alluvium comprises soft, organic, mottled silty and sandy clay which generally overlies a basal lag gravel. Thin stringers of gravel may occur within the sequence, indicating channel migration or periodic increases in the flow regime of the river over time. A common characteristic of streams flowing over chalk bedrock is the presence of calcareous tufa associated with peat accumulations at springs. No occurrences were sufficiently large to be mapped in the district, but thin concretionary carbonate deposits coating stream bed gravels were noted in places in the River Meon south of Warnford.

Alluvial Fan Deposits

A single narrow outcrop of the alluvial fan deposits is mapped in the east of the district, south of Funtington [SU 808 054]. By analogy with more extensive deposits in the Chichester district the deposits are a late-stage, possibly postglacial (about 6000 years BP), chalk-rich, fluvial outwash gravel, entrenched into and in some places through the head gravel and raised beach deposits of the Sussex coastal plain.

Peat

Small accumulations of peat and peaty material are associated with alluvium, river terrace deposits and the tidal river deposits, but are generally too limited in extent to map. An outcrop of dark, silty peat apparently still forming today was noted west of Ryefield [SU 772 226]. Peat is also mapped associated with the River Alver in Gosport. Three small areas of peat are identified in Southsea [SZ 647 987] associated with estuarine alluvium in the area known as the ‘Great Morass’; this is an area of peaty fen isolated from the sea by a shingle bar and now largely built over after being covered by fill in the 19th century.

Aeolian deposits

Aeolian Deposits (‘Brickearth’)

Large parts of the coastal area are mantled by aeolian deposits which give rise to characteristic brown loamy soils. Typically 2 to 4 m thick, boreholes indicate that they may reach 6m in places. They variously overlie head gravel, terrace and raised beach deposits.

The deposit consists of fairly homogenous, yellow brown, structureless, mainly non-calcareous, silt or clayey silt. It is commonly stoneless, but locally contains a few flint fragments, particularly near the base. Chalk detritus is common where the deposit rests on chalk outcrop.

In the Gosport area, these deposits are appreciably more clayey and darker brown in colour. Locally it interdigitates with the highest part of the underlying terrace gravels, and may be partly of fluviatile origin. Elsewhere, it is generally recognised that there is insufficient local source material for the silt-grade sediment represented by these deposits. Its overall grain size and sorting are similar to continental loess, suggesting that the silt is largely wind-blown. However the presence of flint and other pebbles throughout the material suggest that wholesale remobilisation occurred probably by solifluction.

Blown Sand

East of West Heath there is a tract of hummocky ground [SU 794 224] composed of coarse-grained sand, irregularly mantling the Folkestone Beds, Marehill Clay and Pulborough Sandrock. It is most extensive over the Marehill Clay where hollows between hummocks collect water giving rise to boggy, poorly drained ground. Overall, the deposit appears to be wind-blown, with transport being from west to east, but it does not seem to be accumulating at present. The blown sand is estimated to be only a few metres thick.

Blown sand is also associated with the storm gravel beach deposits along the present-day coastline. This occurrence forms a series of relatively stable dunes of medium-grained well-sorted sands with a small proportion of shell debris. The most extensive outcrop forms Sinah Common [SZ 695 994] at the entrance to Langstone Harbour. Two smaller areas occur at Black Point [SZ 750 990] and East Head [SZ 767 990] at the entrance to Chichester Harbour.

Marine and estuarine deposits

Marine Deposits, undifferentiated

This group of nearshore and intertidal deposits includes tidal flat and beach deposits below high water level. They comprise organic mud, channel sand, gravel and sand shoals, together with shell banks within the inlets of the coastal plain.

Raised Beach Deposits (Older)

These deposits are not exposed in the district, but are thought to be present on the older raised beach platform beneath head gravel in the east around Funtington, and at a similar height (about 35 m OD) beneath younger deposits at the western end of Ports Down (Apsimon et al., 1977).

The sediments of the older raised beach deposit are best seen to the east at Boxgrove where they are associated with a varied vertebrate fauna, Palaeolithic artifacts and Boxgrove Man. The association indicates a pre-Anglian probably Cromerian age for this beach deposit. Further discussion is contained in the sheet explanation for Chichester and Bognor.

Raised Storm Beach Deposits

The outcrop north of Westbourne [SU 755 088], at about 40 m above Ordnance Datum, is the only occurrence of this deposit in the district. It represents the westernmost end of a more extensive ridge of storm beach deposits stretching across the Chichester district. To the east, the geomorphological context becomes apparent where a low ridge is clearly associated with the older raised beach platform and is truncated to the south by the younger raised beach cliff line. The deposits contained in the low ridge are considered to be the remnants an offshore shingle bar.

These storm beach deposits comprise fine to coarse well-rounded flint gravel in a sparse silty sand matrix. In this district, they are little more than a metre thick, but are up to 7 m thick to the east.

Raised Beach Deposits (Younger)

Remnants of the younger raised beach deposits occur in the east around Southbourne and Chichester Harbour, and are present in places beneath head gravel and brickearth hereabouts. They rest on a wave-cut platform that shelves gently to the south from the degraded lower cliff line, the base of which lies at about 10 m above OD in the north to just below present sea level on Portsea Island. The platform is more extensive to the east between Chichester and the River Arun, and is broadly equated with that at Black Rock, Brighton.

These deposits are correlated with the high sea level of the Ipswichian interglacial but this stage is known to span three temperate intervals separated by two short cold phases. It is likely that the cutting of the platform and deposition on it may well be more complex than a single transgressive-regressive event. There is some evidence for an intermediate platform between those of the older and younger raised beach.

In the east of the district, the younger raised beach deposits comprise a complex sequence of sand, pebbly sand, and rarely sandy gravel and clean well-rounded shingle. The greater part of the deposit is of fine to medium-grained, thinly bedded, silty, calcareous sand containing a few fine flint and/or chalk pebbles. Generally, a thin basal gravel is present with scattered large erratic boulders. The deposit is usually oxidised and non-calcareous at the surface and a yellow-brown colour. Unoxidised material is pale yellow or greyish white depending on the amount of chalk detritus.

The thickness of the deposit is variable and may differ considerably over short distances. In general, the deposit ranges up to 3 or 4 m in thickness, but over the Chalk Group it may be thin or absent.

From Portsea Island towards the west, the younger raised beach deposits grade into fine to coarse gravel which also rests on a platform at about 10 m above OD; these are mapped as the second terrace of the ‘Solent River’.

Raised Marine Deposits

This term is used to differentiate the marine muds and sands protected by sea walls at the margins of the inlets.

Storm Gravel Beach Deposits

During postglacial times the eastern margins of the Solent have locally became accretionary shorelines. Long-shore drift, generally from east to west (to the west of Chichester Harbour), has built up, above mean high tide level, wide bars of fine to medium-grained gravel with some interstitial sand and shell debris. Re-curved spits of gravel flank the entrances to Langstone and Chichester harbours. Deposition is still active with historical relict shingle ridges being evident on Sinah Common. Since the primary six-inch geological survey the storm beach here has advanced by about 200 m to its present position since the last century. Ablation of these shingle ridges contributes to the blown sand deposits.

Tidal River Deposits (Estuarine Alluvium)

These deposits comprise brown and grey mottled soft silty clay and silt with a sparse shell fauna. They are thin except where they occupy former drainage channels. The main tracts of tidal river deposits occur around the natural harbours above mean high water; this distinguishes them from the marine deposits of the tidal flats.

Landslip

Landslips are a ubiquitous feature of the Upper Greensand/ Gault contact along much of the outcrop. They result from a combination of spring-head erosion and the physical properties of high moisture content and pore pressures. The resultant landforms, for the most part composites of successional rotational slips and slab slides, are quite striking, with fault-like backscarps up to 30 m high, ponds trapped by slip slices, and hummocky ground usually with a prominent toe separating the slips from the undisturbed Gault surface.

The age of the slips is uncertain, almost certainly they were initiated under periglacial conditions, but the landforms are still remarkably fresh suggesting that some movement is less ancient. Only one catastrophic landslip event has been recorded historically near Selborne to the north of this district, but the long-term stability cannot be judged. Inevitably much of the affected ground is wooded or under pasture; clearance of such areas may well effect stability.

Elsewhere in the district landslips are uncommon. Two small sites near Blendworth Common [SU 705 107] and Shirrell Heath [SU 580 144] are associated with the London Clay or sand bodies within that formation.

Worked and made ground

Extensive areas of made ground are shown around Gosport, Portsmouth Harbour and Langstone Harbour and associated with major routes. Much of this made ground is related to the development of the naval facilities of the port in historical times, and to the building of the major road network more recently. The nature of the fill it not known in detail, but much of the reclaimed land around the naval port must include the Quaternary and Palaeogene materials extracted during the excavation of the deep dockyard basins in the late 19th and early 20th centuries.

Only major areas of worked ground, generally associated with mineral extraction, are shown on the maps. Smaller areas of worked and made ground are not shown on the 1:50 000 maps but are shown on the larger-scale geological maps held in BGS archives.

Chapter 8 Applied geology

Hydrogeology

Chalk is the major aquifer in the district and has the largest storage capacity and catchment area. Water is also obtained in the north from the Upper Greensand, which is in hydraulic continuity with the chalk, and from the Lower Greensand. The latter is a separate aquifer beneath the aquiclude of the Gault.

Water is also obtained in small quantities from the Palaeogene and to a lesser extent from the drift, but supplies are variable in both quantity and quality. Supplies from the sand bodies within the Reading and London Clay formations are small and additionally there may be problems associated with high concentrations of sulphate and iron. In the Gosport area supplies of up to 6 l/s (litres per second) have been recorded from the Bracklesham Group but yields are much lower to the east and north. Small supplies have been obtained from the terrace deposits. However, in the coastal area they are vulnerable to sea water invasion and pollution from surface sources and in consequence are now little exploited.

Yields from the Chalk Group of the district (Hargreaves, 1982) vary between the formations with values of the order of 100 l/s in the Upper Chalk and values of 35 l/s and 5 l/s in the Middle and Lower Chalk respectively. The highest yields, up to 270 l/s, are obtained from large diameter shafts, boreholes and headings in the Upper Chalk. The Upper and Lower Greensands typically show maximum yields of 0.44 l/s and 0.43 l/s respectively (Hargreaves and Parker, 1980).

The chalk is microporous with low intrinsic permeability and derives much of its yield from interconnected fissures. The Upper and Lower Greensand and the Palaeogene sands are porous, essentially non-fissured aquifers, although the Upper Greensand is loosely indurated with some fissuring. Wells in the chalk are generally unlined whilst those in the sands require screening.

Perennial springs occur near the base of the Chalk Group, usually at the top of the West Melbury Marly Chalk. Typically, along this part of the South Downs these springs yield up to 3.79 l/s, but yields of up to 151.6 l/s have been recorded from such springs in the Chichester district. The aquifer also contributes to the baseflow of the rivers draining southwards across the district. With the exception of the Hamble, Ems and Meon rivers and the lowest reaches of minor streams in the district, valleys are normally devoid of surface water over the Chalk. However during the extreme weather conditions of the early spring of 1994, when torrential rain fell on a number of consecutive days, surface flow was noted in a number of these ‘dry’ valleys and the local water table was measured at over 20 m higher than previously recorded averages (for example in the public supply well near Brickkiln Farm (SU81SW/4), [SU 835 125] on the adjacent sheet to the east). Villages such as Hambledon and Stoughton were flooded for the first time in living memory.

Bulk minerals

Sand and gravel

Sand and gravel has been won from the Folkestone Beds, sand units in the Palaeogene, terrace deposits associated with the former ‘Solent River’ and the present-day rivers, raised beach deposits and head gravel.

Fine to medium-grained sands are extracted from the Folkestone Beds at West Heath in the north of the district. Similarly graded deposits have been taken from a large sand body in the Reading Formation at Bishop’s Waltham, and from the Whitecliff Member of the London Clay at North Boarhunt. Sand and gravel from the second terrace has been extensively worked east of Lee-on-Solent and the first terrace of the Meon at Mislingford. Elsewhere a number of small degraded pits have been noted in terrace deposits during this survey.

Principally flint gravel has been extracted from the head gravel in the east of the district around Hambrook [SU 785 080] and Southleigh Park [SU 738 085].

Clay

In the past, clay of the Gault, Reading Formation, London Clay, Wittering Formation and the ‘brickearth’ have been used in the manufacture of bricks and tiles. There are no working pits in the district at present and few of the former sites show clear sections. The Reading Formation was worked at Padnell and Rowlands Castle, the London Clay at Padnell, Cowplain and Swanwick, and the Wittering Formation at Lower Swanwick. Descriptions of the exposures are to be found in the references cited (p. 16 to 17) in the Palaeogene section. Outline geotechnical data on these deposits are given in Lake et al. (1985).

Chalk

There are many pits within the chalk sequence of the district attesting to the great historical use of the material for the liming of fields. The great majority are abandoned and in various stages of degradation. Agricultural lime is still produced from Butser Hill quarry and periodically from a pit east-south-east of Chalton [SU 748 156].

The large and numerous chalk pits on Portsdown are all essentially closed for the extraction of chalk although small quantities are still produced at Warren Farm pit near Fort Nelson. Agricultural lime, burnt lime, fill and cement were produced during the acme of the industry in the 1950s and 1960s.

Building stone

Building stone is not produced commercially in this district, but in the past locally derived materials have been used in construction. Only the Upper Greensand ‘malmstone’ or ‘bluestone’ (Plate 6a), (Plate 6b) has been used extensively, and much of that use is restricted to the outcrop.

The best-quality building stone comes from indurated calcareous siltstones (‘bluestone’) found as discrete beds within the malmstone. The ‘bluestone’ weathers to a pale greenish white, but still retains its blue-grey core. Excellent examples of houses built of this stone can be seen in all of the villages along the base of the primary escarpment. The most notable of these villages are South Harting and Buriton where a majority of the older buildings and many boundary walls are made of trimmed rectangular blocks.

Limited use has been made of the harder beds within the chalk sequence (Melbourn Rock, nodular beds within the Lewes Chalk). The phosphatic chalks used in the construction of Boxgrove Priory were obtained from within the Newhaven Chalk near Stoke Clump [SU 835 097] (Lavant Stone) in the Chichester district. A similar phosphatic chalk, also quarried historically, was noted to the east of Nore Down [SU 774 130]. It is not known whether material from here contributed to the construction of the Priory.

Extensive use is made of flint nodules within the Chalk Group for building, both as dressed squared-flint and single-faced trimmed nodules (Plate 6a), (Plate 6b), particularly in churches, vicarages and the larger estate houses and farms.

Flint, as a ‘waste’ product of chalk extraction and from ‘field picking’, has also been used to maintain farm tracks on the chalk dip slope.

Hydrocarbons

The district was first explored in the 1930s and, more successfully, in the 1970s with the discovery of the Horndean Oilfield. The reservoir rocks are in the Great Oolite Formation of the Jurassic.

The processes of hydrocarbon formation, migration and entrapment were controlled by east-west, pre-Albian extensional faults. In the basins to the south of the major faults, and particularly in the centre of the Weald Basin, the Lias source rocks, and possibly the Kimmeridge Clay were buried sufficiently deep to generate hydrocarbons. The hydrocarbons migrated south from the centre of the Weald Basin into the Great Oolite rocks of the palaeo-highs, where antithetic faults provided traps. Migration probably began in Early Cretaceous times and may have continued until uplift in mid-Tertiary times (Penn, et al., 1987). Tertiary compression caused inversion of the Weald Basin, but did not destroy all the traps. Many Tertiary anticlines in the Wessex Basin, for example the Portsdown and Littlehampton anticlines, do not contain oil and gas, suggesting that primary oil migration ceased before they were formed.

Geotechnical considerations

In addition to the incidence of flooding mentioned in the hydrogeology section there are four potential hazards inherent in the strata of this district. The following statements should be taken only as a guide to likely or possible problems and should not replace site-specific studies.

The relatively loose sand of the Folkestone Beds provides unreliable foundations on steep slopes. Freshly ploughed fields or exposed ground can become gullied during heavy rainfall.

The Gault and parts of the Palaeogene(mainly the London Clay Formation) are composed primarily of a highly shrinkable clay with a high smectite content and may crack and move during extreme drought conditions. Suitable precautions should be taken during construction.

Landslip and foundering of strata along the spring line between the Upper Greensand and the Gault is a known hazard along stretches of the outcrop. Elsewhere only limited landslip has been noted in the London Clay sequence. Most other natural slopes are thought to be stable in the district but this can be strongly influenced by human activity, particularly where oversteepened slopes are created during engineering.

The chalk may be affected by solution phenomena which result in small surface depressions (dolines) that range in size up to 50 m across and up to 6 m deep. As a consequence of this dissolution, fractures naturally occurring in the chalk are enlarged. The resultant pipes that may be filled with clay-with-flints, continue to provide sumps for excess surface water, and may be liable to further subsidence. Solution features are likely to be common on the outcrop of the higher members of the Upper Chalk, particularly where a thin clay-with-flints or Palaeogene cover occurs nearby.

In addition to the naturally occurring hazards, man has had considerable influence on the landscape. Large parts of the ground adjacent to the natural harbours have been reclaimed, usually by filling and the construction of sea walls. In many cases the nature of the fill, and hence its geotechnical properties are unknown. In some cases the fill was the product of the excavation and expansion of the dockyard in the 19th century, when both thin Quaternary and Palaeogene (principally London Clay) materials were extracted. Large amounts of fill were also derived from the site clearance of parts of Portsmouth and Gosport following bomb damage in the Second World War.

Many of the abandoned sand and gravel, chalk and clay pits in the area have been used as landfill sites particularly adjacent to the urban areas. Records are held by the local authorities but old areas of fill are often poorly documented. Outline information accrued from local authorities is given in the BGS Open File Reports for Fareham and Havant (p. 26).

Cuttings and embankments for major road and rail links are commonplace in the district. For the most part they are not shown on the published 1:50 000 Series map.

Information sources

Further geological information held by the British Geological Survey relevant to the Fareham and Portsmouth district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Regional Geologist, Southern and Eastern England, BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in British Geological Survey libraries. This is a developing computer-based system which carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographical information system linked to a relational database management system. Results of the searches are displayed on maps on the screen. At the present time (1998) the data sets are limited and not all are complete. The indexes which are available are listed below.

• Index of boreholes
• Topographical backdrop based on 1:250 000 scale maps
• Outlines of BGS maps at 1:50 000, 1:10 000, 1:10 560 and County Series maps
• Chronostratigraphical boundaries and areas from British Geological Survey 1:250 000 maps
• Geochemical sample locations on land
• Aeromagnetic and gravity data recording stations
• Land survey records

Maps

Geology

• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• Solid geology of the United Kingdom (south) 1979 Quaternary map of the United Kingdom (south) 1977
• 1:250 000
• 51N 02W Chilterns, Solid geology, 1991 50N 02W Wight, Solid geology, 1995
• 1:50 000
• Listed by sheet number Solid and Drift
• 299 Winchester, 1976^‡1 
• 300 Alresford (in press) ^‡2 
• 301 Haslemere, 1981
• 315 Southampton, 1987
• 316 Fareham, 1997
• 317/332 Chichester and Bognor, 1996
• 330 Lymington, 1997^‡3 
• 331 Portsmouth, 1994
• 1:10 000 and 1:10 560
• The 1:10 000 scale revision survey of the district was carried out in 1978 to 1984 by N G Berridge, F G Berry, C R Bristow, E C Freshney, R D Lake, S J Mathers, R C Scrivener, E R Shephard-Thorn, R J Wyatt and J A Zalasiewicz, and in 1993 to 1996 by D T Aldiss, P M Hopson, A Pedley and R K Westhead (Figure 11). These maps and the accompanying technical reports give a detailed description of the area and form the background against which this concise account is written.
• Copies of the fair drawn maps are deposited for public reference in the library of the British Geological Survey at Keyworth. Copies of the completely surveyed sheets may be purchased from BGS either as uncoloured dyeline sheets, photographic copies, or as published sheets, subject to availability.

Geochemistry maps

• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain - north and south, 1995.
• Radon potential based on solid geology, Great Britain - north and south, 1995.
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain — north and south, 1995.

Geophysical maps

• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:250 000
• Aeromagnetic anomaly, Wight (1978), Chilterns (1980) Bouguer gravity anomaly, Wight (1978), Chilterns (1983)
• 1:50 000
• A geophysical information map (GIM) at a scale of 1:50 000 is available for this district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.

Hydrogeological maps

• 1:100 000
• Hydrogeological map, South Downs and part of the Weald, 1978 Hydrogeological map, Hampshire and Isle of Wight, 1979

Minerals

• 1:1 000 000
• Industrial minerals resources map of Britain, 1996
• 1:25 000
• Mineral Assessment Report 138

Books and reports

Books

• British Regional Geology The Hampshire Basin and adjoining areas, 4th edition, 1982
• British Regional Geology The Wealden district, 4th edition, (1965) 4th impression 1992

Memoirs

• Listed by sheet number
• 299 Winchester, 1912, out of print
• 300 Alresford, 1910, out of print
• 301 Haslemere, 1968
• 315 Southampton, 1987
• 316 Fareham, 1913, out of print
• 317 Chichester, 1903, out of print
• 330/331 Lymington and Portsmouth, 1915, out of print
• 332 Bognor, 1897, out of print

BGS Technical Reports and other reports

Geology

• The Technical Reports and Open-file Reports listed below are detailed accounts of the constituent 1:10 000 scale maps of the Fareham and Portsmouth 1:50 000 Series Sheet. Copies of the reports may be ordered from the British Geological Survey. Keyworth, Nottingham, NG12 5GG.
• West Sussex Coastal Plain between Chichester and Littlehampton, WA/VG/82/02
• West Sussex Coastal Plain between Selsey and Bognor, WA/VG/82/2 The south-east Hampshire district: Havant and surrounding areas, WA/VG/84/06
• The south-east Hampshire district: Fareham and surrounding areas, WA/VG/85/4
• The Petersfield district, WA/91/24
• The Treyford, Cocking and Chilgrove district, WA/94/48 The Bishop’s Waltham district, WA/96/55
• The Petersfield to Rowlands Castle district, WA/96/65
• The South Harting, Compton and West Marden district, WA/96/67 The Warnford and Droxford district, WA/96/70
• The East Meon and Clanfield district, WA/97/10

Biostratigraphical reports and palaeontological collections

• There are 43 biostratigraphical reports covering the Fareham and Portsmouth district. These are held as restricted open file reports by the Biostratigraphy Group of BGS under the prefixes WH and PDL. Readers are recommended to contact the Group Manager, Biostratigraphy and Basin Analysis Group at BGS, Keyworth for access to these reports and to the palaeontological collections.

Other data sources

NGRC Database

At the time of going to press a total number of 3340 borehole records are held in the collections of the National Geological

Records Centre at BGS Keyworth. These can be consulted by contacting the Manager NGRC at BGS Keyworth.

The British Geological Survey also holds extensive Geophysical, Geochemical, and Lexicon data records and Photograph collection. Contact can be made through the BGS Website at http://www.bgs.ac.uk

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheet 316 and Sheet 331 are held in the Lexicon database. This is available on Web Site http://www.bgs.ac.uk.Further information on the database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS Petmin database

Thin sections and hand specimens of rocks from the district are held in the England and Wales Sliced Rocks and Museum Reserve collections at BGS Keyworth. Enquiries concerning all petrogical material should be directed to the Manager, Petrological Collections, BGS Keyworth.

BGS (Geological Survey) photographs

Copies of the photographs that appear in this report are deposited for reference in the British Geological Survey library, Keyworth, Nottingham NG12 5GG. Colour or black and white prints and transparencies can be supplied at a fixed tariff.

Fossils

Macrofossils and micropalaeontological residues for samples collected from the district are held at BGS Keyworth. Enquiries concerning all macrofossil material should be directed to the Curator, Biostratigraphy Collections, BGS Keyworth.

Minerals

Directory of Mines and Quarries, 1998 United Kingdom Minerals Yearbook, 1997

MINGOL is a GIS based minerals information system, from which hard-copy and digital products tailored to individual clients' requirements can be obtained.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references may be purchased from the Library subject to the current copyright legislation.

ALLEN, J R L and NARAYAN, J. 1964. Cross-stratified units, some with silt bands, in the Folkestone Beds (Lower Greensand) of southeast England. Geologie en Mijnbouw, Vol. 43, 451–461.

ALLEN, P. 1975. Wealden of the Weald: a new model. Proceedings of the Geologists’ Association, Vol. 86, 389–437.

ANDERSON, I D. 1986. The Gault Clay—Folkestone Beds junction in West Sussex, southeast England. Proceedings of the Geologists’ Association, Vol. 97, No. 1, 45–58.

APSIMON, A, GAMBLE, C, and SHACKLEY, M. 1977. Pleistocene raised beaches on Ports Down, Hampshire. Proceedings of the Hampshire Field Club and Archaeological Society. Vol. 33, 17–32.

BERRY, F G, and SHEPHARD-THORN, E R. 1983. Geological notes and local details for 1:10 000 sheets SZ89NW, NE, SW and SE, SZ99NW and NE (West Sussex Coastal Plain between Selsey and Bognor). Keyworth: British Geological Survey, WA/VG/83/2.

BRISTOW, C R. 1991. Geology of the Petersfield district, Hampshire. British Geological Survey Technical Report, WA/91/24.

BRISTOW, C R, BARTON, C M, FRESHNEY, E C, WOOD, C J, EVANS, D J, COX, B M, IVIMEY-COOK, H I, and TAYLOR, R T. 1994. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313, (England and Wales).

BRISTOW, C R, MORTIMORE, R N, and WOOD, C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 108, 293–316.

BROMLEY, R G, and GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273–306.

BRYDONE, R M. 1912. The stratigraphy of the Chalk of Hants. (London: Dulau.)

CALKIN, J B. 1934. Implements from the higher raised beaches of Sussex. Proceedings of the Prehistory Society of East Anglia, Vol. 7, 333–347.

CASEY, R. 1961. The stratigraphical palaeontology of the Lower Greensand. Palaeontology, Vol. 3, 487–621.

CASEY, R. 1963. The dawn of the Cretaceous period in Britain. Bulletin of the South-Eastern Union of Scientific Societies, No. 117.

CAYEUX, L. 1897. Contribution à l'étude micrographique des terrains sédimentaires. I Études de quelques dépôts siliceux secondaries et tertiaires du Bassin de Paris et de la Belgique. II Craie du Bassin de Paris. Mémoires de la Société Géologique du Nord, Vol.4, No. 2, 589pp. [In French.]

CHADWICK, R A. 1985. End Jurassic - early Cretaceous sedimentation and subsidence (late Portlandian to Barremian), and the late-Cimmerian unconformity. 52–56 in Atlas of onshore sedimentary basins in England and Wales. Post Carboniferous tectonics and stratigraphy. WHITTAKER, A. (editor). (Glasgow: Blackie.)

CHADWICK, R A. 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London, Vol. 143, 465–488.

CLAYTON, C J. 1986. The chemical environment of flint formation in the Upper Cretaceous. 43–54 in The scientific study of flint and chert: Proceedings of the Fourth International Flint Symposium held at Brighton Polytechnic 10–15 April 1983. SIEVEKING, G de G, and HART, M B (editors). (Cambridge: Cambridge University Press.)

CLEMENTS, R G. 1993. Type-section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of the Dorset Natural History and Archaeological Society, Vol. 114, 181–206.

COOPER, J. 1976. British Tertiary stratigraphy and rock terms formal and informal, additional to Curry 1958. Lexique Stratigraphique International; with a stratigraphical table. Tertiary Research Special Paper, No. 1.

CURRY, D, and KING, C. 1965. The Eocene succession at Lower Swanwick Brickyard, Hampshire. Proceedings of the Geologists’ Association, Vol. 76, 29–35.

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Figures, plates and tables

Figures

(Figure 1a) The topography and main physiographical features of the district.

(Figure 1b) The main geomorphological elements in the landscape of the district.

(Figure 2) The structure of the Wessex Basin.

(Figure 3) The Glauconitic Marl/Upper Greensand contact along Greenway Lane, East Meon.

(Figure 4) A composite section of the Zig Zag and basal Holywell Chalk exposed in the Butser Hill Quarry [SU 726 203]. Bed numbers in the Plenus Marls those of Jefferies, 1963.

(Figure 5) Lithological logs illustrating the boundary between the New Pit and Lewes Chalk members at Teglease Down, and the Turonian/Coniacian boundary in the upper Lewes Chalk Member at Downley Hanger.

(Figure 6) Composite section through the Newhaven Chalk Member, Paulsgrove Quarry, Portsdown (after Mortimore 1986a).

(Figure 7) The lithostratigraphical and biostratigraphical ranges of the principal Chalk exposures on Ports Down.

(Figure 8) Lithological log showing beds high within the Newhaven Chalk sequence, Soberton Pit [SU 6123 1630].

(Figure 9) Lithological logs and correlation of the Spetisbury Chalk and Portsdown Chalk Members exposed at Warren Farm Pit and Farlington Redoubt.

(Figure 10) Lithological logs and correlation of the Palaeogene Beds in the district.

(Figure 11) Plan of 1:10 000 scale maps of the district and details of surveyors.

Plates

(Front cover) HMS Victory, the flagship of the Royal Navy, looking east from the harbour. The natural harbour at Portsmouth created by the drowning of the lower reaches of southward flowing streams provided a safe haven for ships. This, together with a hinterland of oak, yew and beech forests growing on the Palaeogene strata of the Bere Forest and the chalk of the Downs encouraged the growth of Portsmouth as a ship building centre and the home of the navy.

(Plate 1) East Meon valley looking east from Old Winchester Hill towards Butser Hill and the closure of the Weald. The valley is founded on chalk folded into a pericline with the steeper limb to the north (left in this view). (GS474).

(Plate 2) Upper Greensand and Glauconitic Marl contact along Greenway Lane, East Meon. Hammer 30cm. (GS471).

(Plate 3) Part of the face at Paulsgrove demonstrating the upward change from massive chalks with marl seams, characteristic of the Newhaven Chalk, to bedded chalks with regular large flint seams characteristic of the highest Newhaven and basal Tarrant Chalk. Face 30 m high. (GS370).

(Plate 4) The western face in the Warren Farm Pit showing the regularly spaced flint seams in blocky chalk of the topmost Spetisbury and lower Portsdown Chalks. The prominent flint pair, which are 0.8 m apart, (marked) are about 5 m above the position of the Portsdown Marl in this exposure. (GS334).

(Plate 5) Paulsgrove upper bench. One of the best examples of the high-angle fracture zones common on Portsdown. Scale: yellow board is 0.5 m high. (GS386).

(Plate 6a) The Upper Greensand ‘Malmstone’ or ‘Bluestone’ here used in a barn at Church Farm, South Harting. The dark fragments pressed into the mortar for decoration are ironstone fragments from the Iron Grit at the top of the Folkestone Beds. (GS472).

(Plate 6b) Dressed flint nodules used in the Church Cottages, Compton. Flint shards created during the dressing are pressed into the wet mortar for decoration, a process known as ‘galletting’. Hammer 25 cm. (GS473).

(Back cover)

Tables

(Table 1) Geological succession of the Fareham and Portsmouth district. *timescale from Gradstein and Ogg 1996.

(Table 2) Bed thickness in five boreholes within the district. ¹The Potwell borehole was deviated, thicknesses quoted are adjusted to show true vertical thickness.

(Table 3) Jurassic succession of the district. *timescale follows Gradstein and Ogg 1996. †not found in this district.

(Table 4) Lower Cretaceous sequence of the district. *timescale follows Gradstein and Ogg 1996.

(Table 5) The sequence of Upper Cretaceous rocks in the district and their correlation to the biozonal and earlier chalk nomenclature schemes. The nomenclature used on the map (Sheet 331 Portsmouth), is based on Bristow et al. (1997). ¹ With the exception of the Studland Chalk, all these members occur in the district; only the major divisions of the chalk group - Lower, Middle and Upper Chalk - are shown on the map.

(Table 6) Palaeogene strata outcropping within the district.

(Table 7) Principal Quaternary deposits of the district.

Tables

(Table 2) Thickness of beds in five boreholes within the district