Computer-based mollusc stratigraphy – a case study from the Eggenburgian (Lower Miocene) type region (NE Austria)

The Eggenburg Group comprises the type deposits of the Lower Miocene Central Paratethys regional stage Eggenburgian. In NE Austria these marine to estuarine deposits transgress in northwestern direction either on Palaeozoic rocks of the Bohemian Massif or on Oligocene to Lower Miocene fluviomarine sediments. They are known from three (palaeo)geographic units situated on the southeastern margin of the Bohemian Massif. These are: (1) the southern Marginal Zone, (2) the northwestern, N–S striking Horn Basin and (3) the northeastern crystalline plateau, termed the Eggenburg Bay. The present study investigates distribution patterns of the exceptionally diverse mollusc fauna composed of 321 taxa (species and subspecies) within and between those units. Hierarchical Cluster Analysis is employed, using Ward’s method and squared Euclidean distance calculation. A comprehensive database with 1900 sets based on field, museum and literature investigations allows compilation of 50 taxonomic lists (samples) for 30 different localities and six lithostratigraphic units (four marine and two brackish formations). The resulting grouping pattern implies the existence of four groups – three corresponding with (palaeo)geography and one including low-diversity, taxon-poor samples. The analysis of the diversity distribution supports the interpretation that the lower diversity of samples from marine formations stems from secondary distortions such as diagenesis or insufficient sampling. Hence, the samples characterised by a species-richness value lower than half of the maximum species-richness value of the corresponding formation were excluded from analysis. The resulting grouping pattern verified the secondary distortion premise: the pattern remained the same except that the low-diversity cluster became restricted to samples of brackish origin. Among the marine samples, palaeoecological differences between two fully marine formations from the Eggenburg Bay were of minor importance, as they could not be clearly separated. In contrast, the individual palaeogeographic regions were shown to be taxonomically coherent, but inconsistent with one other. As palaeobiogeographic processes were implausible in such a small geographic area, the explanation of these differences is based on the time factor, including speciation, extinction and faunal migration processes. The relative stratigraphic position of the Horn Basin between the Marginal Zone and the Eggenburg Bay is implied by hierarchically ordered similarities. In order to also indicate their chronological ordinance, the index fossil distribution was analysed. The chronological pattern is unequivocally provided by the abundant presence of the Lower Eggenburgian pectinid Chlamys gigas (Schlotheim), which is restricted to the Marginal Zone and the Horn Basin, and by the abundant Upper Eggenburgian to Lower Ottnangian pectinids Flexopecten palmatus (Lamarck) and Chlamys holgeri (Geinitz), which are restricted to the Eggenburg Bay. This refinement of the regional stratigraphic pattern shed light on the Eggenburgian marine flooding progression. Early in the transgression, the southern Marginal Zone was first marine flooded. At the same time the Horn Basin developed an estuarine environment. The next phase installed the fully marine environment in the Horn Basin. At the same time the Eggenburg Bay, still a mainland, represented a southward-pointing cape. It first became flooded at the beginning of the Upper Eggenburgian.