COAL FORMATION, SEQUENCE STRATIGRAPHY AND TECTONICS OF THE GERMAN VARISCIAN FORELAND (U.CARBONIFEROUS, GERMANY)

In Late Carboniferous, a chain of foreland-basins developed in response to faulting and thrusting along the orogenic front of the evolving Variscan mountain belt in Central Europe. The Ruhr Basin was part of this system.

In the Ruhr Basin, industrial deep wells and exposures from the southern rim of the basin gave insight into the development of the sedimentary facies and the tectonic development of the basin. Sedimentological interpretation of cored sections, litho-logs and supplementary electro-logs faciliated extensive sequence stratigraphic analyses of the sedimentary basin. High subsidence in Late Carboniferous led to the deposition of more then 3500 m molassoid sediments, which are subdivided by marine incursions. Starting in the Namurian, cyclic sequences in marine to deltaic to fluvio-lacustrine sedimentary environments developed.

Sequence stratigraphy is based on the description of space- and time-dependent distribution of sedimentary enviroments. Parasequences and high order sequences are the building blocks of a sequence stratigraphic scheme. Van Wagoner (1985) defined parasequences as a laterally consistent sedimentary succession, that is bounded by marine flooding surfaces or their equivalents. In the Ruhr Basin such equivalent facies systems be can easily detected even in the absence of a diagnostic marine flooding surface, based on a detailed lateral facies analysis.

Spatially subsidence rates changed remarkably in the Ruhr Basin during the Westphalian, during which subsidence rates decreased from 80m to 20 m/100ka. A shift of a relative subsidence minimum zone through the Ruhr Basin during Westaphalian A is documented in detail.

Paralic facies systems, such as the Upper Carboniferous Ruhr Basin, require a spatial multidimensional dynamic model to define sequence stratigraphic units. In the Westphalian of the Ruhr Basin, where marine flooding events dominated, a detailed analysis of parasequences reveals the influence of the critical allogenic controls. Many of the observed facies patterns can be explained by taking into account  the pronounced variation in subsidence rates from one part of Ruhr Basin to another.

A detailed facies classification, based on lithological, sedimentological and faunal information, is necessary to understand sedimentary environments in paralic systems, such as the Ruhr Basin. For sequence stratigraphic analysis, the crucial distinction of upper and lower delta plain environments can benefit from the understanding of the nature of coals and occurrence of flood basin and flood plain facies.

High interseam connectivity (splitting and merging) and the development of local seams are the rule in most parts of the discussed succession. It was shown that the amount of coal seam splitting and the cumulative interval coal thickness can be positively correlated with the subsidence rate within the basin. This indicates that coal seam generation and preservation is not only related to sea level rise, but also to the regional distribution of environments resulting from accommodation space. Accordingly coals must be considered as another sedimentary facies type within the paralic environment, consisting of a wide variety of subfacies types. Their stratigraphic value must be seen in the context of the occurrence of other facies types.