| Computer Code: |
BTSH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Early Triassic Epoch (TE)
— Early Triassic Epoch (TE) |
| Lithological Description: |
The Bunter Shale Formation consists predominantly of red, red-brown, and some grey mudstones and shales. In basin margin areas, sandstones interbedded with silty mudstones in the upper part of the formation form the Amethyst Member; this passes distally into the Rogenstein Member which comprises mudstones with siltstone and some dolomite interbeds and traces of anhydrite, and is characterized by the presence of calcareous oolitic beds (rogenstein). The lowest beds of the formation comprise redbrown, silty, dolomitic or anhydritic, mudstones interbedded with siltstones and some fine sandstones, and constitute the Brockelschiefer Member which includes the Hewett Sandstone. |
| Definition of Lower Boundary: |
The base of the Bunter Shale Formation is defined by a downward change from red-brown, silty, dolomitic or anhydritic mudstones, interbedded with siltstones and some fine sandstones, to anhydrites and halites or, in rare instances (e.g. 54/11-1), carbonates of the Zechstein Group. This boundary corresponds to an abrupt downhole decrease in gamma-ray values and a marked increase in velocity. Throughout most of the UK sector the Bunter Shale Formation overlies the Grenzanhydrit of the Z5 cycle. In the southeast of the sector, however, the Bunter Shale Formation may be interpreted as resting upon Zechstein carbonates (e.g. 54/11-1), indicating the presence of an unconformity with overstep. |
| Definition of Upper Boundary: |
The top of the Bunter Shale Formation is defined by a downward change from the dominantly arenaceous sequence of the Bunter Sandstone Formation to a predominantly argillaceous succession. The boundary is usually clear and is marked by a downward increase in gamma-ray values, but may be less distinct towards the London-Brabant Platform and the eastern UK onshore area where the Amethyst Member, which includes sandstones up to 10 m thick, forms the upper part of the Bunter Shale Formation. |
| Thickness: |
The formation is over 200 m thick throughout most of the UK sector and exceeds 300 m in quadrants 42, 43 and 44, and adjacent parts of quadrants 41, 47, 48 and 49; it is over 400 m thick on the northwest side of the Cleaver Bank High (Cameron, 1993). |
| Geographical Limits: |
The Bunter Shale Formation is the most widely occurring of the formations assigned to the Triassic in the Southern North Sea region. It thickens progessively eastwards and northeastwards from the UK onshore area and the London-Brabant Platform. |
| Parent Unit: |
Bacton Group (BACT)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 49/21- 2 (Rhys, 1974): 1683-2011 m (5522-6597 ft) below KB (revised top). The top of the formation in this section is here revised from 1695.5 m (5563 ft, metric conversion revised) (Rhys, 1974) to the level of a regionally consistent log-marker. |
| Reference Section |
North Sea well 41/24a- 1: 695.5-1002 m (2281-3288 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 48/07b- 3: 1885-2263.5 m (6184-7426 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 53/14- 1: 766-997.5 m (2513-3273 ft) (Johnson et al., 1994). |
| Reference(s): |
| Johnson, H, Warrington, G and Stoker, S J. 1994. 6. Permian and Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. |
| 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. |
| Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. |
| Smith, D B, Harwood, G M, Pattison, J and Pettigrew, T H. 1986. A revised nomenclature for Upper Permian strata in eastern England. 9-17 in Harwood, G M and Smith, D B (editors). The English Zechstein and Related Topics. Geological Society Special Publication No.22. |
| Van der Zwan, C J, and Spaak, P. 1992. Lower to Middle Triassic sequence stratigraphy and climatology of the Netherlands, a model. Palaeogeography, Palaeoclimatology, Palaeoecology 91, 277-290. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E162
|
