SEDIMENTOLOGY AND STRATIGRAPHY OF DEEP-WATER MIOCENE DEPOSITS IN THE MOLASSE BASIN, UPPER AUSTRIA

The Miocene Hall Formation of the Molasse Basin, Upper Austria records the filling of the marine foreland trough. Architectural features and paleogeographic evolution were interpreted from drill-core analysis, integrated with an extensive high-resolution 3D seismic dataset, covering over 2000 km². The evolution of deep-water systems as well as the internal distribution of lithofacies and architectural elements is controlled by the complex interplay of numerous factors, including changes in relative sea level, sediment provenance, intrabasinal and extrabasinal tectonic activity, seabed topography and basin physiology.

The transition from deep- to shallow-water sedimentation took place during three major depositional phases: 1) the ~150 m thick Basal Hall Formation comprises fining upward successions of bioturbated siltstone and shale with relatively rare fine- to coarse-grained sandstone interbeds, representing the final stage of channel deposition, and chaotic units, interpreted as mass transport deposits; 2) the overlying, 150-300 m thick Lower Hall Formation records the transition from deep-marine channel, out-of-grade sedimentation to an inactive phase. Deposits are dominated by fine-grained turbidites and hemipelagite drapes, with sporadic rafted blocks; 3) the basal Middle Hall Formation records the transition to a graded slope system, and is characterized by sand-rich, upward coarsening sequences (fan lobe), fine-grained, low-energy sediments (distal fan), and poorly sorted mass transport deposits.

Accumulations of gas are stratigraphically and structurally trapped in reservoirs associated with three major depositional settings: axial channel belt, toe-of-slope fan, and lower clinoform (Gendorf sand).