A sequence stratigraphic model for the Lower Coal Measures (Upper Carboniferous) of the Ruhr district, north-west Germany

Upper Carboniferous Coal Measures strata have been interpreted traditionall y in terms of cyclothems bounded by marine flooding surfaces (marine bands) and coal seams. Correlation of such cyclothems in an extensive grid of clo sely spaced coal exploration boreholes provides a robust stratigraphic fram ework in which to study the Lower Coal Measures (Namurian C-Westphalian A) of the Ruhr district, north-west Germany. Three distinct types of cyclothem are recognized, based on their bounding surfaces and internal facies archi tecture. (1) Type 1 cyclothems are bounded by marine bands. Each cyclothem comprises a thick (30-80 m), regionally extensive, coarsening-upward delta front succession of interbedded shales, siltstones and sandstones, which ma y be deeply incised by a major fluvial sandstone complex. The delta front s uccession is capped by a thin (<1 m), regionally extensive coal seam and an overlying marine band defining the top of the cyclothem. (2) Type 2 cyclot hems are bounded by thick (approximate to 1 m), regionally extensive coal s eams with few splits. The basal part of a typical cyclothem comprises a thi ck (15-50 m), widespread, coarsening-upward delta front or lake infill succ ession consisting of interbedded shales, siltstones and sandstones. Network s of major (>5 km wide, 20-40 m thick), steep-sided, multistorey fluvial sa ndstone complexes erode deeply into and, in some cases, through these succe ssions and are overlain by the coal seam defining the cyclothem top. (3) Ty pe 3 cyclothems are bounded by regionally extensive coal seam groups, chara cterized by numerous seam splits on a local (0.1-10 km) scale. Intervening strata vary in thickness (15-60 m) and are characterized by strong local fa cies variability. Root-penetrated, aggradational floodplain heteroliths pas s laterally into single-storey fluvial channel-fill sandstones and coarseni ng-upward, shallow lake infill successions of interbedded shales, siltstone s and sandstones over distances of several hundred metres to a few kilometr es. Narrow (<2 km) but thick (20-50 m) multistorey fluvial sandstone comple xes are rare, but occur in a few type 3 cyclothems. Several cyclothems are observed to change character from type 1 to type 2 and from type 2 to type 3 up the regional palaeoslope. Consequently, we envisage a model in which e ach cyclothem type represents a different palaeogeographic belt within the same, idealized delta system, subject to the same allogenic and autogenic c ontrols on facies architecture. Type 1 cyclothems are dominated by deltaic shorelines deposited during a falling stage and lowstand of sea level. Type 2 cyclothems represent the coeval lower delta plain, which was deeply erod ed by incised valleys that fed the falling stage and lowstand deltas. Type 3 cyclothems comprise mainly upper delta plain deposits in which the alloge nic sea-level control was secondary to autogenic controls on facies archite cture. The marine bands, widespread coals and coal seam groups that bound t hese three cyclothem types record abandonment of the delta system during pe riods of rapid sea-level rise. The model suggests that the extant cyclothem paradigm does not adequately describe the detailed facies architecture of Lower Coal Measures strata. Instead, these architectures may be better unde rstood within a high-resolution stratigraphic framework incorporating seque nce stratigraphic key surfaces, integrated with depositional models derived from analogous Pleistocene-Holocene fluvio-deltaic strata.