Geology and Thermal Maturity of Autochthonous Upper Cretacous Deposits on the Southeastern Margin of the Bohemian Massif

Abstract

Knowledge of deposits of the Klement Formation (Upper Cretaceous) has been deriven solely from subsurface data in the southern Moravian territory of the Czech Republic. These deposits constituted a passive margin of the Neo-Tethys Ocean and a seaway continuation into the Bohemian Cretaceous Basin towards the north and northwest. These poorly studied deposits of the Klement Formation have been revisited with the aim of better describing their provenance and depositional environment and thermal maturity.Three recognised facies associations provide evidence of lower to middle shoreface to offshore depositional environments with a role of storm events. Palynological studies of the deposits indicate a Late Albian age, which connect them with the initial Cretaceous transgression (Albian-Lower Cenomanian) onto the Bohemian Massif. The provenance from the eastern margin of the Bohemian Massif is supposed and some potential source areas of the Cretaceous deposits of the Klement Formation are evaluated. The principal and proximal source can be located in the geological unit of the Moravo-Silezian Zone (Brunovistulicum), which compose the direct crystalline basement of the Mesozoic deposits. Slightly more distant sources can be traced to the increasingly western geological units of the Bohemian Massif, ie. the Moravian and Moldanubian units or to even more distant sources such as the Teplá-Barrandian or Lugicum units. However, the redeposition from the syn-Variscan or post-Variscan sedimentary formations from units adjacent to the study area, i.e. the rocks of the Moravo-Silesian Paleozoic deposits (Culmian), are supposed as the material input.The direct basement of the formation is composed of Upper Jurassic deposits which form a more or less sub-horizontal surface before the deposition of the Cretaceous sediments. The base of the Klement Formation represents a composite, polyhistory surface and subaerial unconformity. A remarkable difference in provenance has been recognised when compared with the clastic Jurassic deposits of the Gresten and Nikolčice Formations known from the area under study. Significantly higher evidence of input from ultramafic rocks, paired with lower evidence for sources from intermediate to felsitic igneous rocks and gneisses metamorphosed under amphibolite facies conditions, have been recognised for the deposits of the Klement Formation; this is explained by the different palaeodrainage system versus that of the Jurassic period. The thicknesses of Cretaceous Klement Formation deposits is generally increasing teastwardly pointing to the general increase in basin depth in this direction.All the studied samples of the Klement Formation have a comparable degree of thermal maturity; however, the current depth is significantly different, which is interpreted as evidence of sediment burial to a maximum depth before the West Carpathians´ overthrust.

Keywords: Klement Formation, Upper Cretaceous, borehole cores, provenance, depositional environment, thermal maturity