Regional chemostratigraphic key horizons in the macrofossil-barren siliciclastic lower Miocene lacustrine sediments (Most Basin, Eger Graben, Czech Republic)

摘要

Six sediment cores from two of four depocentres in the Most Basin, mostly consisting of macrofossil barren fluviodeltaic and lacustrine sediments of Hole?ice and Libkovice members of the Most Formation (lower Miocene, Burdigalian) were subjected to chemostratigraphic correlation, based on CEC and EDXRF proxy element analyses. CEC-step, prominent K/Al minima and crandallite-bearing horizons in monotonous lacustrine mudstones of the Libkovice Member provide several local isochronous or nearly isochronous key horizons, which we propose for a basin-scale correlation of the upper Most Basin sediment fill. These key horizons prove a spatially uniform sedimentary environment in a single lake (original area ?1,000 km~(2)) during deposition of a considerable part of the siliciclastics overlying the main coal seam that tops the lower basin fill. We propose the CEC-step horizon as a novel boundary between Hole?ice and Libkovice members, i.e . the conversion of previously formal lithostratigraphic units to members with an isochronous boundary. That boundary together with recent sedimentological studies would assign B?e??any clay to the top of the Hole?ice Member. The upper boundary of the Libkovice Member could be the sediment coarsening related to a lake level decrease before deposition of Lom coal seam. The study will allow progress in palaeogeographic and palaeoenviromental reconstruction of the lower Miocene in the Most Basin. The proxy analyses (CEC vs Al/Si ratios) allow reliable numeric differentiation between kaolinite-rich, smectite-poor and smectite-rich clay assemblages in Hole?ice and Libkovice members. The plots of CEC vs Al/Si ratios should be applicable for fingerprinting any other monotonous lacustrine clastics with variable mineralogy of the clay assemblage.