Effect of relative lake-level changes in mire-lake system on the petrographic and floristic compositions of a coal seam, in the Most Basin (Miocene), Czech Republic
Abstract
Exposures in the Bilina opencast mine provide a direct evidence that the 3.5 m thick upper bench of the Main Seam in the Most Basin passes over a distance of about a kilometre into a hundred metres thick equivalent of clastic deltaic sediments with Gilbert-type mouth bars and thus clearly corroborates the co-existence of lacustrine delta-mire system. Deposition of the delta and the upper bench of the Main Seam were affected by five short-term lacustrine incursions, possibly related to tectonic subsidence before final lacustrine transgression took place.
Between the temporary lacustrine incursions, delta mouth bars prograded into an extensive mire being separated only by a shallow compaction lake along mouth bars foreland. However, most clastics, introduced from delta were caught by dense vegetation in a narrow proximal zone of mire and did not affect more distal parts of relatively dry woodland mire with water table fluctuating around the surface.
It is therefore characterised by low ash yield and increased resinite, fusinite and inertodetrinite contents. Palynological data indicate that the peat-forming vegetation of the upper bench of the Main Seam was dominated by Taxodiaceae (Glyptostrobus, Athrotaxis) and Calamus palm accompanied by paludal species of the family Pinaceae and rich undergrowth of ferns and Vaccinium-type Ericaceae.
Progradation of delta mouth bars to the proximity of the study section is documented by increasing proportion of allochthonous elements produced by plants colonizing the flood plain and delta plain areas (Nyssa-Taxodium, Parrotia-Ulmus pyramidalis and Comptonia-Pinus oviformis associations). In contrast, during periods of temporary lacustrine transgressions the mire was replaced by open water environment, which favoured deposition of clay-rich coals or partings (>30% of ash yield) of the flooding horizons in spite of fact that delta body retreated to the basin margin and the clastics were introduced from a larger distance.