BOREHOLE-CORE FACIES ASSOCIATED WITH SEISMICALLY IMAGED DEEP-SEA CHANNEL DEPOSITS IN THE PUCHKIRCHEN FORMATION, MOLASSE BASIN, AUSTRIA

Deep-water deposits of the Puchkirchen Formation accumulated throughout the Late Oligocene to Early Miocene time periods in the eastern, Austrian part of the Molasse Foreland Basin. Recent seismic analysis (over an area encompassing almost 2000km²) has revealed that sedimentation took place primarily within the confines of a large (3-5 km wide by >100 km long), basin-axial channel belt. Also important were lateral fans, which supplied coarse sediment into the basin from the southern basin margin. Through detailed sedimentologic study of an extensive core database (>2000m), a series of recurring sedimentation units are recognized within the channel system including deposits of high- and low-density turbidity currents, debris flows, and slurry-flows.

A ‘fluvial-style’ architecture is evident within seismic data, an observation common from numerous deep-sea channel systems discussed in the recent literature. Facies in core are comprised exclusively of sediment gravity flow deposits, and therefore are inherently distinctive from facies commonly associated with fluvial channels. Similar to fluvial systems, an upwards-fining channel fill is prevalent where lateral migration was interpreted to be significant. An Alpine source is thought to have continuously generated sediment gravity flows into the deep-water complex resulting in a stratigraphic architecture dominated by laterally to upwards migrating channel deposits stacking for a combined thickness of greater than 1 km.

The various sediment-gravity flow deposits within the formation are each associated with distinctive, seismically imaged three-dimensional channel elements. Texturally, mudstone, sandstone and conglomeratic facies are all significant components of the channel system. Thick conglomeratic debris flow deposits, and locally important turbiditic sandstones characterize the channel thalweg fill. Lateral accretion deposits associated with channel meander-bends are predominantly muddy, and out of channel, overbank areas are characterized by thin-bedded low-density turbidity current deposits.