| Computer Code: |
CORM |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Early Triassic Epoch (TE)
— Hettangian Age (JH) |
| Lithological Description: |
The Cormorant Formation is composed of white, pale grey, greenish grey, brown, and reddish brown sandstones, with red, reddish brown, greenish grey, and minor dark grey, purple, and white mudstones and rare thin beds of calcilutite and limestone. Sections at the centre of the East Shetland Basin and in parts of the Beryl Embayment are dominated by mudstone, whereas the formation is largely composed of sandstone near the East Shetland Platform and Crawford Spur. The coarsest and least sorted sandstones occur in the west, where they are locally underlain by conglomerate composed of pebbles of sandstone, shale, and of metamorphic and granitic basement rock types in a coarse sandy matrix. The sandstones locally contain grains of feldspar towards the base of the formation, and of glauconite or chlorite at higher levels. The sediments are commonly micaceous and sometimes calcareous. The limestones are white, pale grey, pale brown or pink, and are commonly argillaceous; most have a microcrystalline or cryptocrystalline texture.
Many sections display upward-coarsening cycles over part or all of the succession, with individual cycles ranging from about 50 m to hundreds of metres in thickness. |
| Definition of Lower Boundary: |
In central parts of the Beryl Embayment and the East Shetland Basin, the Cormorant Formation overlies Upper Permian clastics or carbonate-anhydrite sequences (e.g. 9/13a-22) or, locally, salts of the Shearwater Salt Formation. In such sections, the boundary is marked by a distinct downward increase in velocity. Over the Crawford Spur and along the western margin of the East Shetland Basin, the Cormorant Formation rests on Devonian strata or on metamorphic or granitic basement. |
| Definition of Upper Boundary: |
The Cormorant Formation is conformably overlain by basal Jurassic sediments of the Statfjord Formation (Banks Group) over large parts of the Beryl Embayment, and the East Shetland Basin close to the UK/Norway median line (Richards et al., 1993). Elsewhere, the formation subcrops younger Jurassic or Cretaceous strata (Lervik et al., 1989, fig.3), and the boundary is a sharp lithological transition and wireline-log break (e.g. 211/13- 1). In such areas, uppermost Triassic primary red beds are presumed to have undergone chemical reduction, as they are grey or greenish grey in colour in many sections. |
| Thickness: |
Thickness variations of the Cormorant Formation may locally exceed 2000 m from one fault block to another (Fisher and Mudge, 1990). The greatest recorded thickness of Triassic sediments in the UK sector is 1745 m in well 211/29-5, which terminated in the Ladinian. The Cormorant Formation is about 1500 m thick in well 3/3-4A, but is less than 250 m thick nearby in parts of block 3/12. Well 9/13a-22 recorded over 1000 m of the Cormorant Formation in a faulted section near the centre of the Beryl Embayment (Dean, 1993). |
| Geographical Limits: |
East Shetland Basin and south into the Beryl Embayment. |
| Parent Unit: |
Heron Group (HERO)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 211/21- 1A (Deegan and Scull, 1977, p.9): 2977-3443 m (9767-11295 ft) below KB. |
| Reference Section |
North Sea well 3/03- 4A: 2906-4408 m (9535-14463 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 9/13- A2: 3162-4119.5 m TD (10373-13515 ft TD) (Cameron, 1993). |
| Reference Section |
North Sea well 9/13a- 22: 3782-4800.5 m (12408-15750 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 211/13- 1: 2659-3922 m TD (8724-12868 ft TD) (Cameron, 1993). |
| Reference(s): |
| Ritchie, J D, Gatliff, R W and Riding, J, 1996. 1. Pre - Tertiary lithostratigraphy. In: Stratigraphic Nomenclature of the UK North West Margin. British Geological Survey, Nottingham. |
| Richards, P C, Lott, G K, Johnson, H, Knox, R W O'B. and Riding, J B. 1993. 3. Jurassic of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.). Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. |
| Lervik, K S, Spencer, A M, and Warrington, G. 1989. Outline of Triassic stratigraphy and structure in the central and northern North Sea. In: Collinson, J D (ed.) Correlation in hydrocarbon exploration, 173-189. Graham and Trotman, London. |
| Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) |
| Clemmensen, L B. 1987. Complex star dunes and associated aeolian bedforms, Hopeman Sandstone Formation (Permo-Triassic), Moray Firth Basin, Scotland. In: Frostick, L E and Reid, I (eds.) Desert sediments: ancient and modern. Geological Society, London, Special Publication No.35, 213-231. |
| Dean, K P. 1993. Sedimentation of Upper Triassic reservoirs in the Beryl Embayment: lacustrine sedimentation in a semi-arid environment (Abstract). In: Parker, J R (ed.) Petroleum geology of Northwest Europe: Proceedings of the 4th Conference, p.581. Geological Society, London. |
| Fisher, M J, and Mudge, D C. 1990. Triassic. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 191-218. Blackwell Scientific Publications, Oxford. |
| Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
