A numerical analysis of grain-size and colour variations of a fan-lake transition, Upper Carboniferous Bechtsrieth Formation (Southern Germany)

Abstract

The Upper Carboniferous Bechtsrieth Formation of the Weiden Basin (Southern Germany) shows a depositional evolution from an alluvial fan to a lake. The sedimentary record of the probably Westphalian GC-member reflects gravity slides and waterlaid deposition of braided streams; the younger GSC-member is dominated by floodplain and sheetflood deposition, locally hosting abundant freshwater limestone and clastic sediments deposited by meandering streams. They grade into the nearshore and offshore lacustrine sediments of the VC-member laid down during the Late Stephanian. These subfacies-types have been numerically coded as “fan ratios”, which normalized to a 10 m drill interval, resulted from the “sand-mud ratio” multiplied by the “degree of contrast” and “maximum clast size”. Supplemented by rock colour, which indicates oxidizing and reducing conditions of these strata, and carbonate content, a discrimination of these continental environments is possible and also makes the procedure amenable to cutting examinations. Because these terrigenous rocks have attracted the attention of exploration geologists for their coal, hydrocarbon, uranium and precious metal contents, the source and host environments have tentatively been described in terms of these facies codes. Low fan ratios between 0 and 0.75 are indicative of predominantly offshore and nearshore lacustrine environments and seem most prospective for hydrocarbons, uranium and gold. This approach is a rapid method to improve environmental analysis and facilitate well-log correlations.