| Computer Code: |
AMTH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Early Triassic Epoch (TE)
— Early Triassic Epoch (TE) |
| Lithological Description: |
Red and red-brown silty mudstones with beds of sandstone. The proportion of sandstone increases, southwestwards and westwards, towards the London-Brabant Platform and the eastern UK onshore area. Sandstone beds are commonly 5 to 10 m thick but some thicker units occur towards the basin margins. The member typically displays more varied gamma-ray values and velocities than the underlying Bunter Shale Formation mudstones and the overlying Bunter Sandstone Formation. |
| Definition of Lower Boundary: |
The base of the Amethyst Member is placed at the base of the lowest significant sandstone bed in the upper part of the Bunter Shale Formation; this level is marked by a downward change to higher gamma-ray values and lower velocities, and to more even log signatures. When traced eastwards from the UK onshore area and northwards from the flank of the London-Brabant Platform the base of the member occurs at progressively higher levels within the Bunter Shale Formation. |
| Definition of Upper Boundary: |
The top of the Amethyst Member corresponds to the top of the Bunter Shale Formation which occurs at an abrupt downward increase in the proportion of mudstone below the dominantly arenaceous Bunter Sandstone Formation. The boundary is typically marked by an abrupt downward increase in gamma-ray values and a slight decrease in velocity. |
| Thickness: |
The member thins distally, away from the coast, as a consequence of a decrease in the thickness and number of sandstone beds and the lateral passage of those in the lower part of the member into finer sediments; it is about 170 m thick in the type section area but over 220 m thick in the area of the more proximally situated reference section. |
| Geographical Limits: |
The Amethyst Member occurs on the southern and western margins of the basin, adjacent to the London- Brabant Platform and the eastern UK onshore area, and extends to between 15 and 100 km from the English coast. |
| Parent Unit: |
Bunter Shale Formation (BTSH)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 47/14a- 3: 1780.5-1952.5 m (5842-6405 ft) below KB (Johnson et al., 1994). |
| Reference Section |
North Sea well 47/18- 1: 1677-1899 m (5502-6230 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, Crosby, A, Balson, P S, Jeffery, D H, Lott, G K, Bulat, J, and Harrison, D J. 1992. The geology of the southern North Sea. United Kingdom offshore regional report. (London: HMSO for the British Geological Survey.) |
| 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: |
| none recorded or not applicable |
