Abstract

This study was carried out on a coal seam of the "Untere Brackwasser-Molasse" (Chattian), about 50 cm thick, interbedded with non-bituminous and bituminous limestones. Coals and associated beds were analysed megascopically (lithotype analysis), petrologically (maceral and microlithotype analysis) and geochemically (Rock-Eval pyrolysis). The vertical distribution of lithotypes and microlithotypes point to water table oscillations during the period of peat accumulation. The site of peat accumulation, however, was permanently covered with water. Maceral analysis reveals a dominance of detrovitrinite-subgroup macerals with variable liptinite-group and inertinite-group macerals. The amount of mineral matter is slight. Therefore, the coals are characterized by high gelification index (GI) values and low tissue preservation index (TPI) values. Plant remains found in the coals indicate that conifers were an important component of the peat-producing flora. The high degree of tissue degradation, typical of the coals, is the result of a high bacterial activity in the calcium-rich peat swamp and probably also of the occasionally brackish conditions. The limestones, which are rich in ostracods, bivalves and charophytes, have a variable TOC content (3-26%) and their organic matter is of terrestrial origin with a mixed liptinitic/vitrinitic character. The depositional environment is interpreted as a swamp forest with a variable density of herbaceous vegetation situated in a lower delta plain. Peat accumulation began subaquatically and probably influenced by brackish conditions. The main phase of peat accumulation represents autochthonous to hypautochthonous peat deposition under predominantly fresh-water conditions. A periodic rise in the water table due to the interaction of subsidence and compaction as well as eustatic sea-level rise, led to the deposition of the limestones under fresh- to brackish water conditions. Peat accumulation was terminated by a progressive drowning of the peat swamp accumulation and by a shift towards brackish conditions.

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