A group of clastic formations, forming a thick, monotonous succession of mostly grey to black, fine-grained siliciclastic sediments commonly containing intercalated coal seams in the middle and upper parts. Fossiliferous marine beds are frequently intercalated in the oldest parts, but these become scarce in the middle parts, and are absent from the youngest interval. The group also comprises light-coloured, massive sandstones, and primary red-bed intervals without coal seams. Volcanic beds (mostly mm-thin tuff layers) can be intercalated locally. The basal interval commonly consists of a black, bituminous shale, locally containing silicified limestone laminae. Secondary reddening is frequently observed beneath the top unconformity.
Various types of intrusive rocks have been encountered in this interval (Eigenfeld 1986), mostly much younger than the surrounding Pensylvanian to upper Mississippian (Silesian) sediments. Volcanic rocks that are intercalated in a stratigraphic sense, are rare and thin. These (a.o. ‘Tonstein’ bands) are part of the Limburg Group.
Overall regressive deltaic foreland-basin fill (van Wijhe 1974; Ramsbottom 1979; Ramsbottom 1987; Guion 1988). The oldest interval consists of marine and lacustrine basin-floor fines and turbiditic distal deltaic deposits. The middle part is dominated by an intercalation of deltaic and fluvial-plain deposits. The youngest interval is characterised by (locally coarse-grained) fluvial-plain deposits, some of which exhibit a primary red-bed facies. Marine influence gradually decreased in time.
Poorly known because only a few wells in widely separated areas reached it. Towards the London-Brabant Massif, the group rests on the carbonates (frequently silicified) of the Carboniferous Limestone Group (Dinantian). Around the Mid North Sea High, the group grades into an alternation of carbonates and clastics of the Yoredale Formation (Farne Group).
Unconformably overlain by the Lower Rotliegend Group (volcanics, volcaniclastics, red beds), Upper Rotliegend Group (red-bed clastics, evaporites), Zechstein Group (claystones, carbonates, evaporites), Rijnland Group (glauconitic sands, clays, marls), or Chalk Group (glauconitic sands, marl and chalk). In the specific unit definitions these truncated situations will not be mentioned separately.
