A calcite crisis unravelling Early Miocene (Ottnangian) stratigraphy in the North Alpine–Carpathian Foreland Basin: a litho- and chemostratigraphic marker for the Rzehakia Lake System

Abstract

Within the Lower Austrian part of the North Alpine Foreland Basin (NAFB), up to 1000 m of sediments were deposited throughout the Ottnangian (Early Miocene, Burdigalian). According to homogeneous compositions and sparse biostratigraphic resolution, a consistent stratigraphic concept from the basin margins into the foreland depocenter was still lacking. New investigations on several deep drill cores throughout the basin provide comprehensive sedimentological, mineralogical, chemical and micropaleontological data. A calcite poor, fossil- and pyrite-free, smectite-rich, up to 800 m thick interval was identified and correlated to the time interval of the late Ottnangian brackish Rzehakia Lake System. For this section, we introduce the term Calcite Minimum Interval (CMI). We define the onset of the CMI by a sharp decrease of calcite contents and the disappearance of autochthonous (and reworked) calcareous nannofossils. We define the termination of the CMI by the permanent increase of pyrite contents and the reappearance of calcareous nannofossils. The CMI as a litho- and chemostratigraphical marker for the Rzehakia Lake System constitutes a stratigraphic key horizon. Within the NAFB in Lower Austria, its onset corresponds to the middle/upper Ottnangian transition while its termination correlates roughly to the Ottnangian / Karpatian boundary. This allows a precise definition, identification and correlation of (upper) Ottnangian stratigraphic units of the NAFB. For the central basinal parts of the Rzehakia Lake System, we introduce the new lithostratigraphic term Wildendürnbach Formation which correlates to the marginal Traisen Formation.

Facies, palaeogeography and stratigraphy of the lower Miocene Traisen Formation and Wildendürnbach Formation (former “Oncophora Beds”) in the Molasse Zone of Lower Austria

“…His interpretation was mainly based on samples of OMV drill cores from wells around Wildendürnbach (Austrian/Czech border), situated 50 km north of the outcrops of the Traisen Formation. These sediments were deposited at about 1000 m depth with a thickness of several hundreds of metres and are dominated by deep-water turbiditic and massive sands, as described by Hamilton (1997), Palzer-Khomenko et al (2018) and internal OMV reports (e.g. Sauer and Kuffner, 1997;Kuffner, 2001;both unpublished).…”

“…Samples of calcite-cemented sands, gravels and concretions were excluded from the profile as they would produce outliers obscuring the CMI signal. They are a common feature within the Traisen Formation and typical for the CMI interval (Gebhardt et al, 2013;Palzer-Khomenko et al, 2018).…”

“…1B; Roetzel and Kurzweil, 1986;Krenmayr and Roetzel, 2000). The mica-rich sands and pelites of the lower Ottnangian "Robulus Schlier", the upper Ottnangian Traisen Formation and the Karpatian Laa Formation make up the major part of the sedimentary infill of the LAMB (Roegl et al, 1997;Krenmayr et al, 2002;Piller et al, 2004;Wessely, 2006;Gebhardt et al, 2013;Wimmer-Frey et al, 2013;Egger et al, 2017;Palzer-Khomenko et al, 2018). Characteristic for these sediments are the garnet-dominated heavy mineral spectra and the fluctuating sand/pelite ratio (Knierzinger et al, 2018).…”

“…In contrast to the wellknown and thoroughly investigated Molasse strata of Upper Austria and Bavaria (e.g. Robinson and Zimmer, 1989; Kowalke and Reichenbacher, 2005;Rupp et al, 2008;Heckeberg et al, 2010;Pippèrr and Reichenbacher, 2010;Grunert et al, 2012;Grunert et al, 2013;Liebl et al, 2013;Reichenbacher et al, 2013;Sant et al, 2017;Pippèrr et al, 2018), the easternmost Lower Austrian Molasse stratigraphy and facies are still partly underexplored (Wagner, 1998;Roetzel et al, 1999;Krenmayr et al, 2002;Piller et al, 2004;Roetzel et al, 2006;Gebhardt et al, 2013;Wimmer-Frey et al, 2013;Egger et al, 2017;Palzer-Khomenko et al, 2018). For instance, the depositional environment of the lower Miocene, upper Ottnangian (Piller et al, 2004;Piller et al, 2007) Traisen Formation (Gebhardt et al, 2013), predominantly massive mica-rich sands between St. Pölten and Tulln, has been a matter of debate over the past two decades.…”

“…It can be identified by various features such as reduced carbonate contents, increased smectite/illite ratios compared to under-and overlying sediments as well as the absence of (detectable amounts of ) pyrite and calcareous nannofossils. For the distal, turbiditic deposits of this interval in the central basinal parts of the LAMB, Palzer-Khomenko et al (2018) introduced and defined the Wildendürnbach Formation. The present study correlates the Traisen Formation with the Wildendürnbach Formation in the well of Wildendürnbach K4 and demonstrates that the (former) "Oncophora Beds" sensu OMV are overlying this interval and therefore correspond to the Karpatian Laa Formation.…”