Carbon, oxygen and strontium isotopes as a tool to decipher marine and non-marine environments: Implications from a case study of cyclic Upper Cretaceous sediments

The interplay of Late Cretaceous basin subsidence and oscillations in sea level produced a mixed freshwater–marine succession within the Upper Cretaceous Gosau Group of the Northern Calcareous Alps. Cored sections from wells of the Glinzendorf and Gießhübl Syncline, as well as sediments from the outcrop area of Grünbach–Neue Welt and Slovakian equivalents have been investigated for their stable isotopic composition. Bulk carbonate dC and dO values of 116 fine-grained samples (shales, siltstones, marls) and Sr/Sr values of 10 samples from the borehole Markgrafneusiedl T1 were analysed in order to distinguish between nonmarine and marine deposits and to compare and correlate isotope characteristics of the different Gosau synclines and basins. Non-marine samples have significantly lower mean dC values compared to the mean of marine samples. The discrimination between a marine and non-marine group using dO is also highly significant statistically, even though the difference between the average non-marine and marine values is small. Strontium isotope values of marine intervals are near the range of values of normal Upper Cretaceous sea water but show a trend towards higher ratios in marginal marine and non-marine deposits. Although diagenesis and the detrital carbonate admixture partly influence the isotopic composition, the original environmental signal can still be reliably identified. Stable carbon and oxygen isotope data are widely used in sedimentology as an indicator for depositional conditions and facies (palaeoenvironment, palaeosalinity, palaeotemperature and so on) because they primarily reflect the composition of the water from which the carbonate precipitated (Hudson 1977; Anderson & Arthur 1983; Talbot 1990; Reinhardt et al. 2003). The primary goals of our study were twofold: (i) to test the applicability of stable isotopes to distinguish between freshwater–brackish and marine deposits, and (ii) to evaluate how and to what extent this tool can be used in a complex basinal setting affected by diagenesis and the presence of detrital carbonate. The data used for this study stem from an applied petroleum geology project aimed at comparing the isotope (C, O, Sr) characteristics of different Upper Cretaceous to Palaeocene basins in eastern Austria and western Slovakia, including industrial well data from below the Neogene Vienna Basin. In addition to isotope data, other geochemical as well as micropalaeontological data (Hofer et al. 2011) are available from these samples. These data are used to unravel palaeogeographical relationships and stratal correlations in this area where the differentiation of marine and non-marine intervals is challenging. An area of special interest is the Upper Cretaceous Glinzendorf Syncline, where classical facies analysis using thin sections and micropalaeontological methods yielded ambiguous results, partly because the fossil content is extremely low (Wessely 2006). These sediments have previously been considered as mainly limnic due to the lack of fossils, and correlated to outcrops of the Campanian Grünbach Formation of the Grünbach Syncline (Wessely 1992, 2006). Recent petrographical and geochemical analyses (Hofer 2009; Hofer et al. 2011, in press), however, indicate a strong marine influence in the Glinzendorf Syncline. Correlations of these Upper Cretaceous to Palaeogene strata are of great significance for hydrocarbon exploration in the Vienna Basin, because these successions act as seals for gas (e.g. Zimmer & Wessely 1996). Carbon and oxygen isotope compositions of carbonate depend in principle on the isotopic composition of the water (related to salinity; Veizer 2003) From: Bojar, A.-V., Melinte-Dobrinescu, M. C. & Smit, J. (eds) 2013. Isotopic Studies in Cretaceous Research. Geological Society, London, Special Publications, 382, 123–140. First published online May 14, 2013, http://dx.doi.org/10.1144/SP382.5 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on December 5, 2013 http://sp.lyellcollection.org/ Downloaded from