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The Early Evolution of the Molasse Basin, Austria: Depositional Environment and Stratigraphic Architecture of Deep-Water Deposits


The early development of the deep-water sedimentary system in the Tertiary Molasse foreland basin in Upper Austria is not well understood because the sediments have been partially incorporated into the structurally complex, syn-depositional Alpine fold-and-thrust belt. Previous studies focused on strata deposited in the later phase of deep-water deposition in the basin, including the late Oligocene upper Puchkirchen and early Miocene Hall formations, due to an interest in petroleum exploration. The strata have been interpreted to include deposits of a 4-6 km-wide axial-channel belt that shows large-scale meandering. However, it is unclear when the axial-channel belt originated or what were its initial characteristics. This research investigates the sedimentology, stratigraphy, and lithofacies distribution of the early stages of basin evolution, including coarse-grained gravity-flow deposits, the Zupfing and lower Puchkirchen formations based on cores, wireline logs, and 3D seismic interpretation. The results will improve our understanding of the early evolution of the Molasse basin and elucidate the influence of tectonic deformation and topography on hydrocarbon-reservoir quality and distribution.

Lithofacies interpreted from core descriptions are tied to well logs in order to determine rock physics properties that characterize each facies and understand the nature and distribution of associated facies. When possible, these interpretations are calibrated to the seismic dataset to understand the sediment deposits at the basin scale. Root mean square (RMS) amplitude and coherency are used to observe the depositional features, such as debris flow topography, distribution of sandstone bodies, and channel geometry and dimensions. These observations are combined with the architecture interpreted from the 3D seismic leading to interpretations of depositional environment of the systems, mass transport deposits and their origins, and internal architecture of the axial channel belt and its channel elements. The axial channel belt, previously interpreted in the upper Puchkirchen Formation, can be observed throughout the Zupfing and lower Puchkirchen formations and shows northward migration. The channel elements within the channel belt can also be observed in places within the lower Puchkirchen Formation, although this becomes increasingly difficult as older units are increasingly overridden by the fold-and-thrust-belt.