A sequence stratigraphic investigation of a Miocene formation supported by coal seam quality parameters – Central Paratethys, N-Hungary

Abstract

A stratigraphic investigation of the coal-bearing Miocene siliciclastic succession in structurally dissected basins of Northern Hungary was performed based on log correlations and is supported by coal petrology and sulphur data. The succession represents the Karpatian lower order global eustatic cycle of the Paratethys and was dissected into six higher order sedimentary sequences (sq-1 – sq-6). The coal-bearing strata in the Nógrád Basin (sq-1 – sq3) represent a lowstand systems tract (LST) prior to the Karpatian relative sea level rise. The transgressive systems tract (TST) starts with the ‘Oncophora Beds’ (sq-4) accumulated above a regional unconformity and containing coal beds in the Borsod Basin. The TST is superimposed by tectonically-forced higher order lowstand systems tracts (sq-5, sq-6) containing coal beds only in the relatively elevated East Borsod Basin.To support the sequence stratigraphic model, the average maceral and sulphur content of coal seams was also interpreted. The low sulphur (0.47%) content in sq.-1 reflects the lowstand environment determined by continental derived freshwater input. Transitional sulphur contents (0.66%–1.02%) in the late lowstand (sq-2 and sq.-3) indicate occasional saline invasions related to relative sea level rises. High (2.3%–2.6%) sulphur contents in the TST (sq-4) were caused by regular seawater inundations into the lagoon, while decreasing sulphur contents in sq-5 and sq-6 (2.28%–2.09%) reflect the increasing rate of freshwater income in association with the stepwise basinward shifts of the shoreline and the lagoonal facies. Within the Lower to Middle Miocene Salgótarján Lignite Formation, the analysis of coal seams contributed to the interpretation of relative sea level changes, and the sequence stratigraphic context facilitated both the interpretation of sulphur content in coals and the reliable correlation of coal seams.