Deep -sea foreland basin axial channels and associated sediment gravity flow deposits, Oligocene Molasse basin, Upper Austria, and Cretaceous Magallanes basin, Chile
The sedimentary units, morphology, and evolution of large-scale deep-water channel belt complexes that develop in the axial troughs of elongate foreland basins are the primary focus of this thesis. Two complimentary study areas were assessed, including the Magallanes basin of southern Chile, and the Molasse basin of Upper Austria. Upper Cretaceous strata of the Cerro Toro Formation (Chile), exposed in an outcrop belt > 100 km long, consist of coarse-grained facies deposited along the north-south trending axis of the Magallanes retro-arc foreland basin. The Austrian deposits, including the Oligocene-Miocene Puchkirchen and base Hall formations, accumulated in the east-west oriented foredeep trough of the eastern Molasse basin that was created as a result of Alpine tectonic loading. The units of interest remain buried 1.5--3 km in the subsurface; the dataset examined consists of drill cores, wireline logs, and an extensive 3-D seismic reflection volume.
The channel belts are 3--7 km wide and > 100 km long, composed largely of coarse-grained gravity flow deposits that derived from turbidity current, debris flow and slurry flow processes. Fine-grained material present in the upper portion of turbulent flows that passed through the channels spilled into overbank areas. Numerous mappable sedimentary bodies (depositional elements) have been interpreted in the strata examined, which collectively constitute the channel belt complexes that developed in the study areas. These include channel thalweg, levee, overbank lobe, overbank wedge, tributary channel, and mass transport complexes. Basin architecture imparted a significant influence on channel morphology, controlling the distribution and character of the various depositional elements.
Channels developed along confined axial foredeeps are distinguishable from submarine fan-channel complexes that form in unconfined basin settings. Complicating factors in forelands include the presence of multiple lateral sediment conduits along the length of channel belts, and fluctuations in basin architecture due to variable tectonic influences associated with active thrust fronts (e.g., changes in basin width and depositional slope gradients). Lateral migration and avulsion of basin axial channel belts is limited due to a lack of available space in foredeep troughs, resulting in substantial and distinctive vertical aggradation of coarse-grained channel deposits.