| Computer Code: |
KTCH |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Bolsovian Substage (CC)
— Bolsovian Substage (CC) |
| Lithological Description: |
The Ketch Member is composed of reddish brown to purple mudstones and silty mudstones, and grey, reddish-brown, or occasionally white, fine to coarse grained, sometimes pebbly sandstones. The sandstone beds occur as single and stacked channel-fill units that are mostly less than 10 m, but are exceptionally up to 20 m thick. Such sandstones are respectively associated with blocky and complex gamma-log profiles, and they constitute up to 40 per cent of the member. They are moderately or poorly sorted, and variably micaceous. Where the sandstones are pebbly, intraformational clasts of red mudstone are common, but the predominant clast type comprises well rounded quartzite, presumably derived from Caledonian massif sources. The sandstones are cemented mainly by silica and hematite, but halite and anhydrite cements have been reported locally near the top of the member.
Red mudstones and siltstones occur in units that are mostly less than 40 m thick. In cores, such units are seen to include mature palaeosols, each comprising a grey, leached upper zone overlying red and grey, vertically veined and mottled beds. Units of interlaminated fine-grained sandstone and mudstone also occur and are associated with ratty gamma-log profiles. The mudstones contain root systems, oxidized remains of plant fragments, and hematite concretions that replaced early diagenetic siderite (Besly et al. 1993). Desiccation cracks occur towards the top of the red-bed section in well 44/28-1.
Besly et al. (1993) reported grey, coal-bearing intercalations and prominent caliche horizons near the top of the primary red beds in well 44/21-3. Sandstone bed thickness is significantly reduced in the coal-bearing section, and gamma-ray response of the mudstones is higher than in the underlying red beds. This lithofacies is absent from other wells in the Silver Pit area, presumably due to pre-Permian erosion. |
| Definition of Lower Boundary: |
Westphalian red beds display complex, laterally diachronous and locally unconformable relationships with underlying coal measures in central England (Besly and Turner 1983). According to Besly et al. (1993), the base of primary red beds in the Silver Pit area and in adjacent parts of the Dutch sector seems to occur at an approximately constant height above a regionally persistent, near-top Westphalian B wireline log marker, implying a conformable facies transition. Secondarily reddened horizons occur in underlying coal-measure facies locally (e.g. in 44/28-2) but, unlike in the Ketch Member, the red pigmentation is concentrated around the sandstone beds. The base of the Ketch Member is therefore defined by the lowest horizon of primary red clays. As this boundary is generally not marked by a distinctive wireline log break, it must be distinguished by careful analysis of cuttings, or of core where available. The boundary is commonly close above the highest coal seam of the Schooner Formation. |
| Definition of Upper Boundary: |
The Ketch Member is unconformably overlain by Lower Permian desert sediments in all sections. In the north of the Silver Pit area, the overlying sediments are mainly reddish brown, anhydritic lacustrine mudstones and evaporites (Silverpit Formation). These have a brighter hue in cuttings than the red mudstones of the Ketch Member and the boundary is commonly marked by a sharp downhole increase in gamma-ray response (e.g. 44/27-1). Farther south, thin sandstones also occur at the base of the Permian, but the boundary is generally marked by a sharp downhole decrease in velocity (e.g. 49/1-3). |
| Thickness: |
357 m + on seismic evidence. |
| Geographical Limits: |
Preservation of the Ketch Member from pre-Permian erosion has been limited to axial regions of deep, mainly NW-SE trending Variscan synclines. The most significant areas of preservation are in southeastern Quadrant 42 and in southern Quadrant 44 / northern Quadrant 49 (the Silver Pit area). |
| Parent Unit: |
Schooner Formation (SCNR)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 44/28- 1: 3786.5-4008 m (12423-13150 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 44/21- 3: 3828-4185 m (12558-13730ft) (Cameron, 1993). |
| Reference Section |
North Sea well 49/01- 3: 3927-4262 m (12883-13983 ft) (Cameron, 1993). |
| Reference(s): |
| Cameron, T D J. 1993. 5. Carboniferous and Devonian 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. |
| Besly, B M. 1990. Carboniferous. 90-119 in Introduction to the Petroleum Geology of the North Sea. Glennie, K W (editor). (Blackwell Scientific Publications, Oxford.) |
| Besly, B M, Burley, S D, and Turner, P. 1993. The Late Carboniferous red bed play of the Southern North Sea. 727-740 in Petroleum geology of Northwest Europe: Proceedings of the 4th Conference. Parker, J R (editor). (Geological Society, London.) |
| Cameron, T D J, Crosby, A, Balson, P S, Jeffery, D H, Lott, G K, Bulat, J, and Harrison, D J. 1992. United Kingdom offshore regional report: the geology of the southern North Sea. (London: HMSO for the British Geological Survey.) |
| Leeder, M R and Hardman, M. 1990. Carboniferous of the Southern North Sea Basin and controls on hydrocarbon prospectivity. In: Hardman, R F P and Brooks, J (eds.) Tectonic events responsible for Britain's oil and gas reserves. Geological Society, London, Special Publication No.55, 87-105. |
| Mykura, W. 1960. The replacement of coal by limestone and the reddening of Coal Measures in the Ayrshire Coalfield. Bulletin of the Geological Survey of Great Britain 16, 69-109. |
| 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. |
| Trotter, P M. 1953. Reddened beds of Carboniferous age in north-west England and their origin. Proceedings of the Yorkshire Geological Society 29, 1-20. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
