| Computer Code: |
ZSTK |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Guadalupian Epoch (PUG)
— Lopingian Epoch (PUL) |
| Lithological Description: |
The Zechsteinkalk Formation includes a wide range of carbonate lithofacies that accumulated in ramp, shelf, slope and basinal settings. In ramp, shelf, and slope sections, the Zechsteinkalk Formation is thickly developed and typically consists of two, broad facies. Firstly, pale grey-brown, hard, variably dolomitised, locally argillaceous and pelletoidal carbonate mudstones. These commonly pass up into carbonate grainstones, which are oolitic, pelletoidal and dolomitized. Several upward-decreasing gamma-ray sections are commonly present, reflecting upward-coarsening cycles (e.g. 48/30-1). Basinal sections typically consist of thin, dark grey to brown, argillaceous, pyritic, carbonaceous and locally sandy, laminated limestone and dolomite. |
| Definition of Lower Boundary: |
Typically, the base of the Zechsteinkalk Formation is taken at a sharp downward change from grey dolomitic limestone to dark grey or black, organic-rich, laminated mudstone of the Kupferschiefer Formation. This is marked on wireline logs by a sharp downward increase in gamma-ray values and a decrease in velocity (e.g. 49/26-4). In the Silverpit Basin a thin dark grey, bituminous mudstone unit with a high gamma-ray and low velocity log signature locally occurs within the condensed Zechsteinkalk Formation (e.g. 44/22-1). Locally, the Kupferschiefer Formation appears to be absent and the Zechsteinkalk Formation rests unconformably on pre-Zechstein rocks (e.g. 38/25-1, Upper Old Red Group). |
| Definition of Upper Boundary: |
In basinal sections, the top of the Zechsteinkalk Formation is normally defined by a downward change from white or yellow anhydrite of the Werraanhydrit Formation to dark grey to brown argillaceous, laminated or thinly bedded fine grained limestone. It is typically marked on wireline logs by a sharp downward increase in gamma-ray values and a decrease in velocity (e.g. 49/21-1, fig. 35 in Cameron et al., 1992), but the key reference section 49/26-4 has a thin high-velocity limestone at the top of the formation. Some sections in the slope facies also show a marked downward increase in gamma-ray values at this boundary (e.g. 48/30-5). In shelf sections, where the Werraanhydrit Formation is absent, the upper boundary of the Zechsteinkalk Formation is commonly defined by a downward change from red-brown mudstones and sandstones with thin anhydrites and dolomites of the Blakeney Formation (e.g. 53/1-2) or white to grey anhydrite with interbedded red mudstone of the Basalanhydrit Formation (e.g. 53/1-1), to pale greybrown dolomite. These boundaries are generally marked by a downward decrease in gamma-ray values and an increase in velocity. |
| Thickness: |
The Zechsteinkalk Formation shows considerable lateral thicknesss variation which reflects its depositional environment. In a 30-50 km-wide marginal belt around the southern rim of the basin, a thick, prograding wedge of dolomite accumulated in shelf and slope environments. Here it reaches about 120 m (e.g. 48/28-1). Relatively thick shelf sections are also locally present on the southern flank of the Mid North Sea High (e.g. 38/25-1). In contrast, the basinal fades which extends beyond the break of slope at the foot of the shelf and covers most of the Southern North Sea Basin, is very thin and typically in the range 2-6 m (e.g. 49/26-4). Locally, the basinal facies appears to be absent, with Werraanhydrit Formation resting directly on the Kupferschiefer Formation (e.g. 38/24-1). In the northern parts of Quadrants 38 and 39, strata equivalent to the Zechsteinkalk are included in the Halibut Bank Formation (e.g. 39/1-1). |
| Geographical Limits: |
The Zechsteinkalk Formation is present throughout most of the Southern North Sea Basin. It is, however, absent from the London-Brabant Platform (e.g. 53/16-1) and on parts of the Mid North Sea High (e.g. 38/16-1). |
| Parent Unit: |
Zechstein Group (ZG)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Reference Section |
North Sea well 38/25- 1: 2182.5-2213 m (7160-7260 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 48/30- 1: 1693.5-1795.5 m (5556-5890 ft) (Johnson et al., 1994). |
| Reference Section |
North Sea well 49/26- 4 (Rhys, 1974, table 2, fig. 3): 2007-2011.5 m (6584-6600 ft) (metric conversion revised) (Johnson et al., 1994). |
| Reference Section |
North Sea well 53/01- 2: 1626-1698 m (5335-5571 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. |
| 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.) |
| Clark, D N. 1986. The distribution of porosity in Zechstein carbonates. In: Brooks, J, Goff, J C, and Van Hoorn, B (eds.) Habitat of Palaeozoic gas in NW Europe. Geological Society, London, Special Publication No. 23, 121-149. |
| NAM and RGD (Nederlandse Ardolie Maatschappij and Rijks Geologische Dienst) 1980. Stratigraphic nomenclature of the Netherlands. Transactions of the Royal Dutch Geological and Mining Society of Delft. |
| 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. 1980. The evolution of the English Zechstein basin. In: Fuchtbauer, H, and Peryt, T M (eds.) The Zechstein basin with emphasis on carbonate sequences. Contributions to Sedimentology 9, 7-34. |
| 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. |
| Taylor, J C M. 1990. Upper Permian-Zechstein. In: Glennie, K W (ed.) Introduction to the petroleum geology of the North Sea, 153-190. Blackwell Scientific Publications, Oxford. |
| Taylor, J C M, and Coulter, V S. 1975. Zechstein of the English sector of the Southern North Sea. In: Woodland, A W (ed.) Petroleum and the continental shelf of North-West Europe, 249-263. Applied Science Publishers, London. |
| Smith, D B. 1986. The Trow Point Bed-a deposit of Upper Permian marine oncoids, peloids and columnar stromatolites in the Zechstein of NE England. In: Harwood, G M, and Smith, D B (eds.) The English Zechstein and related topics. Geological Society, London, Special Publication No.22, 113-125. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
