| Computer Code: |
SCNR |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Duckmantian Substage (CB)
— Bolsovian Substage (CC) |
| Lithological Description: |
The Schooner Formation consists of sandy coal measures overlain by primary red beds.
The coal measures are composed of mudstones and silty mudstones, with beds of grey, pale brown or white sandstone and occasional seat earths and coal seams. The mudstones are mainly grey, but reddish brown and purple beds also occur locally near the top of the unit. Sporadic beds of highly radioactive, possibly sapropelic, mudstone have been encountered in some wells (e.g. 49/1-3). Coal seams are mostly less than 2 m thick and constitute between 1 and 5 per cent of the coal measures. Cyclicity of facies is generally less clearly developed in the Schooner Formation than in the underlying Westphalian formations.
Both the basal coal-measures unit and the overlying red beds contain between 10 and 40 per cent of sandstone, occurring as single and stacked channel-fill units mostly less than 10 m, but exceptionally up to 20 m thick. Such sandstones are respectively associated with blocky and complex gamma-log profiles. Grain size of the sandstones is mainly very fine or fine, but the primary red beds also contain coarser, sometimes pebbly, poorly sorted beds. The sandstones are notably clean or only slightly argillaceous. The principal clast types in the pebbly sandstones are red mudstone and well rounded quartzite; the latter were presumably derived from Caledonian massif sources.
It is possible to recognize a lower red bed unit in which the sandstones are interbedded with reddish brown or purple mudstones and siltstones. Cored sections reveal the presence of 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, oxidised remains of plant fragments, and hematite concretions that replaced early diagenetic siderite (Besly et al. 1993). |
| Definition of Lower Boundary: |
The Schooner Formation rests, apparently conformably, on argillaceous coal measures of the Westoe Coal Formation. The boundary is marked by a sharp downward decrease in abundance and bed thickness of sandstones, and is hence defined along the base of the lowest clean sandstone unit of the Schooner Formation. This boundary is likely to be regionally diachronous. |
| Definition of Upper Boundary: |
The Schooner Formation 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 red mudstones of the Schooner Formation, and the boundary is commonly marked by a sharp downhole increase in gamma-ray response (e.g. 44/28-2). Farther south, thin sandstones also occur at the base of the Permian, but the boundary is generally marked by a downhole decrease in velocity (e.g. 49/1-3). |
| Thickness: |
Lower Schooner unit 40-300 m; Upper unit, Ketch Member 357 m. |
| Geographical Limits: |
Preservation of the Schooner Formation from pre-Permian erosion has been limited to the Silver Pit area and to the axial regions of other deep, mainly NW-SE trending Variscan synclines in the northern offshore area. |
| Parent Unit: |
Conybeare Group (CNYB)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 44/27- 1: 4038.5-4438 m (13250-14561 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 44/21- 2: 3868-4203 m (12690-13789 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 44/28- 2: 3675-4060 m (12057-13329 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 49/01- 3: 3927-4541 m (12883-14899 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, 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.) |
| 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 |
