Abstract: Dynamics of the SE-German Molasse basin: eustatic versus tectonic controls in a classical foredeep

ZWEIGEL, JANINE, and TOM AIGNER

The North Alpine Molasse basin, as a classical foreland basin, was used as a natural laboratory to study the effects of eustasy versus tectonics in the basin history. The deposits are mainly derived from the raising Alps and form two classical transgressive-regressive units: the Upper and Lower Marine and Freshwater Molasse, respectively. The units are separated by an angular unconformity, formed during the Early Miocene. A detailed sequence stratigraphic interpretation, based on well and seismic data, showed that the formation of sequence boundaries was mainly controlled by eustatic sea-level falls which were able to outstrip the subsidence. A subsidence analysis revealed that the geohistory of the basin can be subdivided into four phases. Whereas, the interplay between subsidence and sedimentation rates controlled the transgressive-regressive cycles: 1) "flexural event" (Upper Eocene-uppermost Egerian, 38.6-22.3 Ma): increasing rates of subsidence and sedimentation related to the northward advance and internal stacking of the Alpine nappes: first transgressive-regressive cycle. 2) "visco-elastic reaction" (Uppermost Egerian - Eggenburgian, 22.3-19.5 Ma): decreased rates of subsidence at thrust front and uplift of distal parts due to a visco-elastic reaction of the lower plate after cessation of thrusting in the Eastern Alps - formation of angular unconformity; reduced sediment input due to sudden reduction of relief in the orogen: second transgression. 3) Ottnangian - end of Molasse sedimentation (19.5-8.5 Ma): decreasing rates of subsidence and increasing rates of sediment input since about 17.8 Ma: renewed regression. 4) "isostatic rebound" (Pannonian - recent, 8.5-0 Ma): uplift. Stratigraphic modeling employing two different modeling softwares was attempted to quantify the influence of subsidence and eustasy on the basin development.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria