The middle and Upper Ottnangian of the Simssee area (SE Germany): Micropalaeontology, biostratigraphy and chronostratigraphy

摘要

The Simssee area in the Molasse Basin of southeastern Germany exposes a complete Middle Ottnangian succession (approximately 100 in) and a major portion of the Upper Ottnangian (c. 45 in). The predominantly siliciclastic sequence consists of the Upper Marine Molasse and the Upper Brackish Molasse, of which we studied the lithofacies, microfossils and Sr-isotope composition of otoliths. The Upper Marine Molasse is dated as uppermost Early and Middle Ottnangian based on foraminifera and ostracoda. The basis of the Middle Ottnangian can be correlated with the mammal unit NIN 4a and an absolute age of 17.8 +/- 0.3 Ma (Sr-isotope data). The Upper Brackish Molasse is dated as Late Ottnangian based on the occurrence of otoliths characteristic of the Paratethys otolith zone OT-M4. One informal unit and two new formations are introduced for the Upper Marine Molasse. The informal unit, for which the name Simssee-Unit is proposed, correlates with the regressive part of the classical first cycle of the Upper Marine Molasse. Above positioned is the Achen Formation that corresponds to the Middle Ottnangian and the classical second cycle of the Upper Marine Molasse. Apart from a massive storm period (tempestites) at the basis, foraminifera and ostracoda of the Achen Formation are generally indicative of an inner to middle neritic environment and transgressive-regressive development. The overlying Ulperting Formation corresponds to the uppermost part of the Upper Marine Molasse and to the lowermost part of the Upper Brackish Molasse; it contains the transition from the Middle to the Upper Otmangian. The name Aschaholz Formation is introduced for the remaining portion of the Upper Brackish Molasse. The Upper Brackish Molasse is capped by a strong unconformity and overlain by the Upper Freshwater Molasse.