Confined channel-levee complex development in an elongate depo-center: Deep-water Tertiary strata of the Austrian Molasse basin

Abstract

A basin axial-channel belt was largely responsible for the observed distribution of coarse-grained gravity-flow deposits in the Tertiary Puchkirchen and basal Hall formations of the Molasse foreland basin in Upper Austria. Elements of this depositional system, mapped in three-dimensional (3D) seismic-reflection data, include channel-belt thalweg, mass-transport complexes, overbank wedge, overbank lobe, and tributary channel. The primary objective of this paper is to develop a comprehensive understanding of the sedimentary processes that were prevalent in the channel-belt complex through the analysis of well data, including drill cores and wireline logs, in conjunction with 3D seismic interpretations. Sedimentation in the 3–6 km wide channel belt was commonly focused within smaller channel elements, 1–2 km in width. These elements consist of upward fining and thinning gravity-flow deposit successions, which record the waning of flows in the channel setting. Chaotically bedded fine-grained units, including debris-flow deposits, slumped material, and rafted sediment blocks are common in the channel belt locally. Overbank sedimentation, from flows that spilled over the margins of the smaller channel elements, was common within the confines of the axial channel belt (inner levees). A significant amount of fine-grained material overtopped the margin of the axial channel belt, lapping onto the confining basin slopes and ultimately accumulating in upward fining sediment wedges up to 200–300 m high. As a result of the piling up of overbank wedges onto basin slopes, they too are characterized by a morphologic expression comparable to that of inner levees. Overbank wedges are incised by tributary channels oriented perpendicular to the axial channel belt, formed through slumping and the focusing of low-density currents that originated from flow-stripping upstream. They were backfilled by thin-bedded turbidities sourced directly from the axial channel belt. Tabular silt beds were deposited from flow-stripping at outer channel meander bends locally, resulting in the deposition of overbank lobes; breaching of the channel belt levee also resulted in the deposition of upward fining and thinning turbidites in overbank settings periodically. Confinement of the depositional system within the narrow foredeep had a profound effect on sediment distribution. Lateral migration of the axial channel belt was limited, and as a result, coarse-grained material stacked nearly vertically for approximately 8 myr. Channel-belt avulsion was not possible because of a lack of available space within the foredeep trough. The architecture of overbank units was influenced by marginal basin slopes; accommodation space for sand accumulation in overbank areas was limited and thus, overbank lobes only formed in limited locations. Sedimentation in the basin was also influenced by active Alpine fold-thrust belt structuring.