The Upper Marine Molasse (Burdigalian, Ottnangian) in Southwest Germany - facies interpretation and a new lithostratigraphic terminology

摘要

The geology of the lower Miocene Upper Marine Molasse (OMM) of the North Alpine Foreland Basin (Molasse Basin) has been studied intensively. However, one problem that remains concerns the different informal names that have been assigned to the OMM units by local workers. Moreover, there is still no general consensus with regard to the precise lithostratigraphic content of these units. In this study, three outcrops in the Lake Constance area (SW Germany) displaying OMM sediments are investigated with regard to lithofacies, sedimentology and microfossils (benthic foraminiferal assemblages). The Kalkofen Formation is introduced as a formal lithostratigraphic name for the so-called Sandschiefer, which represents a mud-dominated succession of an inner to middle neritic environment. The overlying newly defined Baltringen Formation includes the former Bodmansande or Baltringer Schichten, which are sand-dominated deposits indicating a subtidal environment with strong tidal currents. The lower boundary of the Baltringen Fm is marked by a basal erosion surface and a pebbly basal layer (Baltringer Horizont). The Steinhofe Formation is introduced as a formal lithostratigraphic term for the so-called Deckschichten or Feinsandserie. It overlies the Baltringen Fm and is marked by an unconformity at base. The Steinhofe Fm comprises finer-grained sand/silt alternations indicative of a less high-energetic but still tidally influenced marginal marine setting. The abrupt shift between the mud-dominated Kalkofen Fm and sand-dominated Baltringen Fm is a result of the wellknown shallowing-upward event that separates the two classical OMM sedimentation cycles (sensu Lemcke et al. 1953). The unconformity at base of the Steinhofe Fm adds support to the hypothesis of a third sedimentation cycle.