Lithofacies analysis and paleogeography of Bochumer Formation (Westfal A 2), Rufargebiet

Abstract

Regional patterns of sandstone distribution were examined in the coal-bearing Bochumer Formation with the help of fence diagrams and isolith (thickness) maps. In addition, isolith maps and cross-sections were drawn for some well defined sandstone bodies, like Dickebank sandstone (Lower), Rottgersbank sandstone (Middle), and Matthias sandstone (Upper), and sand-mud ratio maps were prepared, on which isolith contours showing the total thickness of coal were superimposed. Most sandstone bodies are oriented west-southwest; they are elongate, straight to sinuous, show bifurcation and branching, and closely resemble sand-bodies designated as belt and dendroid. The area with greater thickness (50 m or more) of sandstone and poorer in mudstone (sand-mud ratio > 1.0) is linearly distributed to the north in the Lower unit, but shifts toward the south in the Middle and Upper units. Interrelationship between certain stratigraphic variables was investigated statistically, and it is concluded that the total thickness of strata or net subsidence of the basin exhibits direct relationship with the number and thickness of sandstone, rooty bed, and coal. A synthesis of integrated results reveals that the sediments of the Bochumer Formation were transported to and across the basin down the regional paleoslope toward west-southwest, and partly toward northwest; that it was a longitudinal marginal basin; and that the depositional environment consisted mainly of a high-constructive delta plain, in which distributary and tributary channels and their subenvironments including natural levees and coal-forming swamps developed and migrated constantly across the plain to give rise to characteristic interbedded (cyclic) sequence.