The BGS Lexicon of Named Rock Units — Result Details

Forest Marble Formation

Computer Code: FMB Preferred Map Code: FMb
Status Code: Full
Age range: Bathonian Age (JN) — Bathonian Age (JN)
Lithological Description: Silicate-mudstone, greenish grey, variably calcareous and in the south notably sandy, with lenticular typically cross-bedded limestone units that form banks and channel-fills, especially in lower part. A variety of limestone types occur, of which grey, weathering brown and flaggy, variably sandy medium to coarsely bioclastic grainstone or less commonly packstone predominates, especially at the base, which is increasingly ooidal north from Bath (termed the Acton Turville Beds from Biddestone to Didmarton). Other types include fissile sandy limestone, grading to calcareous sandstone, and oyster-limestone. South of the Mendip Hills, a silici-muddy, fossiliferous lime-mudstone (Boueti Bed) lies at the base. Bivalves and brachiopods dominate the fauna, and lignite debris and fish scales and teeth are common, but infauna and signs of bioturbation are rare. The formation consists of interbedded mudstone and limestone in the Weald and English Channel basins, but in St George's Channel Basin it comprises rhythmically bedded mudstone, siltstone and fine sandstone.
Definition of Lower Boundary: South Midlands and Cotswold region: base of silicate-mudstone, greenish grey, or limestone, typically grey to brown variably sandy medium to coarsely bioclastic grainstone or packstone, resting with erosive, commonly channelled, disconformity on white to yellow, peloidal, ooidal or lime mud-rich, less silici-muddy limestone of the White Limestone Formation, or on a bored and oyster-encrusted hardground surface of the ooidal limestone of the Athelstan Oolite Formation or the Chalfield Oolite Formation, or on ooidal and shell detrital limestone with coralliferous lenses of Corsham Limestone Formation. South of the Mendip Hills: marked by the Boueti Bed, a fossiliferous lime-mudstone, resting non-sequentially on olive-grey bioturbated mudstone of the Frome Clay Formation (formerly Upper Fuller's Earth).
Definition of Upper Boundary: Generally mudstone in the upper part of the Formation, overlain sharply and non-sequentially by peloidal shelly wackestone/packstone of the Cornbrash Formation.
Thickness: Up to 5m thick in Buckinghamshire, 10 to 30m in Oxfordshire and Gloucestershire, 30 to about 50m in north Dorset, 30 to 75m in south Dorset and the English Channel (Hamblin et al., 1992; Barton et al., 2008), St George's Channel 242m proved (Tappin et al., 1994).
Geographical Limits: Onshore: Weymouth area, Dorset coast to Olney area [SP 88 51], Buckinghamshire, where the formation is either overstepped by the Cornbrash Formation or passes into the Blisworth Clay Formation. Present at depth in Weald (Wyatt, 2011). Offshore: English Channel, Bristol Channel, St George's Channel.
Parent Unit: Great Oolite Group (GOG)
Previous Name(s): Wychwood Beds [Obsolete Name And Code: Use FMB] (WYWB)
Bradford Beds (-3407)
Forest Marble (-2804)
Kemble Beds [Obsolete Name And Code: See BLAD And SI] (KEB)
Alternative Name(s): none recorded or not applicable
Type Area  Wychwood Forest, Oxfordshire (i.e. the area approximately from Burford to Bladon, and northwards to the River Evenlode (Smith, 1812, unpublished stratal tables; see Arkell, 1933). 
Reference Section  British Geological Survey North Leigh Borehole (SP31SE/9) (in type area). 38.46m - 45.75m depth (manuscript log and registered specimens). 
Reference Section  Shipton-on-Cherwell Cement Works Quarry (immediately east of type area), exposes the full thickness of the formation, underlain by the Bladon Member of the White Limestone Formation and overlain in places by the Cornbrash Formation. The Forest Marble comprises between 5 and 8m of beds clearly displaying the lateral passage between the low energy mudstone facies and the high energy carbonate-sand shoal and channel-fills (Allen and Kaye, 1973; Wyatt, in Cox, B M, and Sumbler, M G. 2002). 
Reference Section  Watton Cliff, West Bay, Dorset, permanently but variably exposes over 25m of beds, including 0.35m-thick Boueti Bed at base (overlying the Frome Clay Formation), overlain by mudstone with lenses and beds of limestone: top not seen (Callomon and Cope, 1995). 
Allen, J R L and Kaye, P. 1963. Sedimentary facies of the Forest Marble (Bathonian) Shipton-on-Cherwell Quarry, Oxfordshire. Geological Magazine, Vol.110, 153-163. 
Holloway, S. 1983. The shell-detrital calcirudites of the Forest Marble Formation (Bathonian) of south-west England. Proceedings of the Geologists' Association, Vol.94, 259-266. 
McKerrow, W S and Kennedy, W J. 1973. The Oxford district. Geologists' Association Guide No.3. 
Richardson, L. 1933. The Country around Cirencester. Memoir of the Geological Survey of Great Britain, Sheet 235 (England and Wales). 
Barron, A J M, Lott, G K, and Riding, J B. 2012. Stratigraphical framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf. British Geological Survey Research Report RR/11/06. 
Callomon, J H and Cope, J C W. 1995. The Jurassic geology of Dorset. 51-103 in Taylor, P D (Editor), Field Geology of the British Jurassic. (Bath: The Geological Society.) 
Hamblin, R J O, Crosby, A, Balson, P S, Jones, S M, Chadwick, R A, Penn, I E, and Arthur, M J. 1992. The geology of the English Channel. British Geological Survey United Kingdom Offshore Regional Report. 
Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. 
Barton, C M, Woods, M A, Bristow, C R, Newell, A J, Westhead, R K, Evans, D J, Kirby, G A, and Warrington, G. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey, Sheets 328, 341/342, 342/343 and parts of 326/340, 327, 329 and 339. 
Arkell, W J. 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
Woodward, H B, 1894. The Jurassic Rocks of Britain, Vol.4. The Lower Oolitic Rocks of England (Yorkshire excepted). Memoir of the Geological Survey of the United Kingdom. 
Arkell, W J. 1947. The Geology of Oxford. 267pp. [Oxford: Clarendon Press.] 
Tappin, D R, Chadwick, R A, Jackson, A A, Wingfield, R T R and Smith, N J P. 1994. United Kingdom offshore regional report; the geology of Cardigan Bay and Bristol Channel. (London: HMSO for the British Geological Survey.) 
Cox, B M, and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series No.26. [Peterborough: Joint Nature Conservation Committee.] 
Cope, J C W (Editor) 1980b. A correlation of Jurassic rocks in the British Isles. Part Two: Middle and Upper Jurassic. Geological Society of London Special Report, No.15 
Sumbler, M G. 1999. Correlation of the Bathonian (Middle Jurassic) succession in the Minchinhampton-Burford district: a critique of Wyatt (1996). Proceedings of the Geologists' Association, Vol. 110, 53-64. 
Sumbler, M G, 1984. The stratigraphy of the Bathonian White Limestone and Forest Marble formations of Oxfordshire. Proceedings of the Geologists' Association, Vol.95, 51-64. 
Buckman, S S, 1922. Jurassic Chronology: II - Preliminary studies. Certain Jurassic strata near Eypesmouth (Dorset): the Junction-Bed of Watton Cliff and associated rocks. Quarterly Journal of the Geological Society of London, Vol.78, 378-436. 
Cave R. 1977. Geology of the Malmesbury District. Memoir of the Geological Survey of Great Britain, Sheet 251 (England and Wales). 
Warner, R. 1811. A new guide through Bath and its environs. (Bath.) 
Wyatt, R J. 2011. A gamma-ray correlation of boreholes and oil wells in the Bathonian Stage succession (Middle Jurassic) of the Wealden Shelf subcrop. British Geological Survey Open Report, OR/11/048. 
Sumbler, M G, 1991. The Fairford Coral Bed: new data on the White Limestone Formation (Bathonian) of the Gloucestershire Cotswolds. Proceedings of the Geologists' Association, Vol.102, 55-62. 
Donovan, D T, and Hemingway, J E. 1963. Europe; Fasc.3a England, Wales and Scotland; Pt.3a X Jurassic. Lexique stratigraphique international. No. 1. (Paris, France: Centre national de la recherche scientifique.) 
Wyatt, R J. 1996. A correlation of the Bathonian (Middle Jurassic) succession between Bath and Burford, and its relation to that near Oxford. Proceedings of the Geologists' Association, Vol. 107, 299-322. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E217 E218 E201 E219 E234 E235 E236 E237 E251 E252 E253 E265 E281 E297 E312 E313 E327 E328 E341 E342 E343