Palaeotopography related plant succession stages in a coal forming deltaic succession in early Jurassic in Hungary

Abstract

An integrated palaeoecological study was performed regarding the Early Jurassic coal bearing succession (Mecsek Coal Formation) of the Mecsek Basin (S Hungary) focusing on boreholes from Máza-South and Rücker areas representing the periphery and the central part of the coastal plain of a delta respectively. Comparative investigations were based on a previous palaeobotanical ecogroup model and on the integrated interpretation of sedimentological and palaeobotanical data from Máza-South.Sedimentary isopach and coal seam ash content data were applied to reconstruct palaeotopography and coal seam thickness data to detect plant growing intensity at Máza-South (10km2). Plant fossils identified from the corresponding strata were used as sources for the identification of four topography-related plant succession stages determined by sequential facies changes. Channel banks and small islands are characterised by underdeveloped associations composed of colonist elements. Exposed slopes or levees, or local elevations in overbank conditions are covered with assemblages of small herbaceous plants. Crevasse splay complexes are characterised by more developed plant cover and swamps in floodplain depressions are characterised by typical swampy successions. Based on these succession stages, plant remains from Rücker were also interpreted, however, mainly swampy and elevated overbank habitats alternated at Rücker reflecting continuous floods.According to the results, Máza-South was a small area located very close to the bayline between the alluvial plain and the coastal plain of a delta, less favourable for intensive plant growing where dry land was restricted to levees and crevasse splays. This resulted in more dynamic plant changes, typical for deltaic peripheral territories. In contrast, Rücker represents the central part of the delta plain environment dominated by woody forms of large growth. In the uppermost sequence plant composition at Máza-South became similar to that of Rücker indicating that, with transgression, the central delta plain environment, permanent at Rücker, advanced towards the peripheral zone represented by Máza-South.