Biostratigraphy and paleoenvironments in a northwestern Tethyan Cenomanian-Turonian boundary section (Austria) based on palynology and calcareous nannofossils

The Ultrahelvetic Rehkogelgraben section of the Eastern Alps of Austria comprises a c. 5 m thick sequence of upper Cenomanian to lower Turonian pelagic sediments including black shales in a northwestern Tethyan paleogeographic setting. The following calcareous nannofossil events were recognized in stratigraphic order: The first occurrence (FO) of Quadrum inter-medium, the LOs of Lithraphidites acutus and Corollithion kennedyi, the FO of Eprolithus octopetalus, and the FO of Quadrum gartneri. Thus, nannofossil zones UC 4, UC 5c, UC 6 and UC 7 could be recognized. Palynological investigations and palynofacies analysis give additional information especially from the black shale layers and the carbonate-free interval in the section. Five out of seven samples from this interval have yielded some low diversity but distinctive palynological associations. Sporomorphs dominate in the associations, being represented mainly by age diagnostic Normapolles species as Atlantopollis microreticulatus, Atlantopollis reticulatus together with Complexiopollis praeatumescens, Complexiopollis christae and Complexiopollis funiculus. The concurrent presence of these pollen species is regarded as characteristic for latest Cenomanian and early Turonian assemblages. A low diversity dinocyst association is identified, dominated by the Cyclonephelium compactum - Cyclonephelium membraniphorum complex, together with Isabelidinium and Circulodinium species. The dinocyst association is encountered in palynofacies rich in marine organic matter of granular amorphous composition. The palynofacies points to deposition in a distal, low-energy, stressed anoxic environment with high continental input. The palaeoenvironmental significance of the low diversity Cyclonephelium/Eurydinium association is typical for Cenomanian-Turonian black shale sequences. Nannofossil indices and dinoflagellate associations indicate rather low-productivity, low-nutrient settings during at least the later part of OAE 2, and thus confirm interpretations based on foraminiferal assemblages. (C) 2012 Elsevier Ltd. All rights reserved.