GEOCHEMICAL CORRELATION OF LIMNIC-MARINE UPPER CRETACEOUS SUCCESSIONS UNDERNEATH THE VIENNA BASIN (GOSAU GROUP, AUSTRIA)

The interplay of Late Cretaceous basin subsidence and sea-level oscillations produced a mixed freshwater-marine succession within the Gosau Group of the Northern Calcareous Alps (NCA). The Gosau Group comprises Upper Cretaceous to Paleogene strata unconformably overlying folded and faulted Permian to Lower Cretaceous units. These sediments that play an important role in the exploration of hydrocarbons from Alpine reservoirs within the basement of the Neogene Vienna Basin are exposed at the south-western (Austria) and the north-eastern (Slovakia) margin of the basin and continue as NE-SW-striking synclines underneath the Neogene fill of the basin where they can act as seals in the hydrocarbon system.

Cored sections (courtesy of OMV) from the boreholes Markgrafneusiedl T1, Glinzendorf T1 and Gänserndorf T3 of the Glinzendorf syncline and recently for boreholes of the Gießhübl syncline (Aderklaa 81, 84, 92) and the Brezová syncline (Závod 57, 68, Studienka 83) as well as sediments from the outcrop area of Grünbach have been investigated geochemically (bulk rock). The different borehole sections are characterised by alternating from limnic-freshwater dominated to marine intervals. First results show relatively lower carbon isotopies for non-marine parts (e.g. values between -3 and -8 VPDB in the boreholes Markgrafneusiedl T1 and Glinzendorf T1) in contrast to relatively higher values between 0 and -3 VPDB in marine sections. Boron contents which are used as a paleosalinity indicator rise from 88 ppm on average in limnic samples up to 142 ppm in the marine intervals.

Differences in the provenance of the marine and limnic parts can be observed by using concentrations and ratios of trace elements with the aim to correlate the drilling sections of the different boreholes. In general non-marine successions show higher chromium and nickel contents (up to 250 ppm / 400 ppm) while marine sections are characterised by lower concentrations (110 ppm / 60 ppm). In addition to that freshwater influenced samples are generally enriched in the Cr/V-ratio relative to the Y/Ni-ratio and therefore trend to an ultramafic source. This is also interpreted from the heavy mineral spectra that are characterised by a dominance of chrome spinel in non-marine intervals pointing to a provenance from a Tethys ophiolitic suture to the south of the NCA.