Step Graben Formation DCHP

 

Premise Unit, defined by Van Adrichem Boogaert and Kouwe (1995). The Schooner Formation of Cameron (1993) embraces the UK offshore equivalents of the Step Graben Formation and the Hospital Ground Formation. The sedimentology and diagenesis of that formation is described extensively in Besly (1988) and Besly (1993)
Derivatio nominis Named after the Step Graben, a fault block situated between the Elbow Spit High and the Central Graben. The formation has also been found in the vicinity of this area.
Type section Location map See figure (pdf)
  Well K07-08 (pdf)
  Location N 53°39’52.8
E 03°10’31.3
  Depth 3462 to 3641 m
  Length 179 m along hole
Additional section Location map See figure (pdf)
  Well E18-02 (pdf)
  Location N 54°09’05.1
E 03°46’40.7
  Depth 3983 to 4058 m
  Length 75 m along hole
Definition Red- to orange-brown, grey, or variegated, silty to fine-grained sandy mudstones. Intercalated sandstone beds consist of 2 to 10 m thick sheets. The sands are white, grey or red-brown, subangular to rounded, moderately- to well-sorted, fine- to medium-grained, with coarse-grained, pebbly intercalations. The mudstone intervals are 5 to 100 m thick, averaging around 40 m. Carbonaceous matter is common in some grey intervals, specifically in the lower section. Coal seams are restricted to a few localities and specific stratigraphic horizons. Some thin, light-grey, dolomitic limestone beds can be encountered.
Upper Boundary The top of the formation is constituted by the Saalian erosion surface. In general, the formation is covered unconformably by the Upper Rotliegend Group, consisting of a succession of orange-red, evaporitic mudstones ( Silverpit Formation) with or without a thin basal sandstone ( Slochteren Formation).
Lower Boundary The massive sandstones of the Hospital Ground Formation grade laterally into the mudstones of the Step Graben Formation. The base of the Step Graben Formation has been placed at the top of the uppermost massive sandstone bed of the Hospital Ground Formation, which is overlain by a mudstone interval of more than 60 m thick. Locally, the Step Graben Formation may rest directly on the Maurits Formation. This contact can be conformable or unconformable. In well G18-01, the Maurits Formation is characterised by light-grey mudstones, usually with abundant coal seams. In the vicinity of the Mid North Sea High and Schill GrundHigh, the formation might rest unconformably on older formations, analogous to the Brig Formation in the UK offshore (Cameron (1992) ).
Distribution Restricted to the northwestern offshore (Cleaver Bank High/Silverpit Basin area). Erosional remnants have been encountered in blocks K07-K08 and around E17-E18. An extension into the (Proto-)Central Graben has been inferred (Figure C.4 (see pdf) ).
Age Late Westphalian C to Westphalian D (- Stephanian?). The base of the formation has been age-dated as latest Westphalian C in the northwestern offshore Besly (1993) .
Depositional Setting Deposited on a well-drained flood plain. During certain periods of elevated base level, swamp areas with poorly-drained soils formed. The thick mudstone intervals with thin sheet sandstones point to a distal depositional setting. Sandstones were deposited by sheet floods or small-scale fluvial channel systems (Leeder (1990) ; Besly (1988) ; Besly (1993) ).
References See References Upper Carboniferous

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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