| Computer Code: |
TTHY |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Norian Age (TN)
— Rhaetian Age (TR) |
| Lithological Description: |
The Triton Formation consists largely of mudstones but a significant amount of anhydrite is present in the lower and middle parts, and some thin beds of buff or green dolomite occur in the upper part. The mudstones are mainly red and red-brown, but some grey-green beds are present and the top of the formation is predominantly grey-green. The anhydrite is white, grey or colourless and occurs in thin beds, a concentration of which occurs in the middle of the formation and is distinguished as the Keuper Anhydrite Member. |
| Definition of Lower Boundary: |
Where the Keuper Halite Member marks the top of the underlying Dudgeon Formation, the base of the Triton Formation is defined by a downward change from red-brown mudstones, with some grey-green beds and thin anhydrites, to halite. This boundary corresponds to an abrupt downhole decrease in gammaray values and a marked increase in velocity. Where the Keuper Halite Member is absent, as in much of quadrants 41, 42, 43, 44 and 47 and parts of quadrants 48, 49, 52 and 53, the boundary is marked by a downward change from rather uniform, red-brown mudstones, with thin anhydrites, of the Triton Formation, to more varied red, red-brown, orange-red, and green mudstones, with some silty beds, thin anhydrites and dolomites, of the Dudgeon Formation; this boundary corresponds to a downhole change in the gamma-ray and velocity values to levels that are generally lower and higher respectively than in the overlying Triton Formation and which form the top of a 'waisted' log profile. |
| Definition of Upper Boundary: |
The top of the Triton Formation is defined by a sharp downward change from dark grey shales, and white to grey, fine to medium grained sandstones of the Penarth Group, to grey-green, dolomitic mudstones overlying red-brown mudstones with some grey-green beds and thin dolomites. This boundary typically corresponds to an abrupt downhole increase in gamma-ray values and a sharp decrease in velocity which reflects a downward change from sandstone to mudstone (Lott and Warrington, 1988); where sandstone is not present at the base of the Penarth Group (e.g. 43/25-1) the boundary corresponds to a decrease in gamma-ray values and a marked increase in velocity. |
| Thickness: |
The formation is thickest in the Sole Pit Basin where it exceeds 200 m; 250 m occur at the southern end of that basin. Westwards from the Sole Pit Basin the thickness decreases abruptly across the Dowsing Fault Zone to less than 100 m, indicating that syndepositional movement occurred in that zone. Northwards from the Sole Pit Basin the thickness decreases less abruptly (Cameron et al., 1992). |
| Geographical Limits: |
The Triton Formation is present throughout much of quadrants 41, 42, 43, 47 and 48 and parts of quadrants 44, 49, 52 and 53. |
| Parent Unit: |
Haisborough Group (HAI)
|
| Previous Name(s): |
Triton Anhydritic Formation [Obsolete Name and Code: Use TTHY]
(-3097)
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 49/21- 2 (Rhys, 1974): 660.5-882 m (2167-2893 ft) below KB (metric conversion revised; revised top). The top of the formation in this section is here revised from 668 m (2287 ft) (Rhys, 1974) to the level of a regionally consistent log-marker (Lott and Warrington, 1988); the upper boundary is poorly characterized on the velocity log of this section and is better represented in the reference sections. |
| Reference Section |
North Sea well 43/25- 1: 1680-1880 m (5512-6168 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 47/05a- 3: 1123-1294 m (3685-4246 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 49/21- 5: 567-744.5 m (1861-2442 ft) below KB, drilled depths, 567-743.5 m (1860-2440 ft) true vertical depths (Johnson et al., 1994). |
| Reference Section |
North Sea well 53/02- 4: 652.5-817.5 m (2140-2682 ft) (Johnson et al., 1994). |
| Reference(s): |
| 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.) |
| Lott, G K, and Warrington, G. 1988. A review of the latest Triassic succession in the UK sector of the Southern North Sea Basin. Proceedings of the Yorkshire Geological Society 47, 139-147. |
| 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. |
| Taylor, S R. 1983. A stable isotope study of the Mercia Mudstones (Keuper Marl) and associated sulphate horizons in the English Midlands. Sedimentology, Vol. 30, 11-31. |
| Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. |
| Geiger, M E, and Hopping, C A. 1968. Triassic stratigraphy of the southern North Sea Basin. Philosophical Transactions of the Royal Society of London B254, 1-36. |
| Morbey, S J. 1978. Late Triassic and Early Jurassic subsurface palynostratigraphy in northwestern Europe. Palinologia, num. extraord. 1, 355-365. |
| Warrington, G, and Ivimey-Cook, H C. 1992. Triassic. In: Cope, J C W, Ingham, J K, and Rawson, P F (eds.) Atlas of palaeogeography and lithofacies. Geological Society, London, Memoir No. 13, 97-106. |
| 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. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
