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Controls on Architecture and Stacking Patterns of Fluvial Channel Belts in a Rapidly Subsiding Basin: Ferris and Hanna Formations, Hanna Basin, Wyoming, USA


Current interpretations of fluvial avulsion style and architecture are largely model-driven and based on experimental tanks and computer simulations. Cretaceous and Paleocene strata of the Hanna Basin have been used to support these models and comprise a large data set (∼4,000 m thick and ∼60 km of outcrop belt around the basin margins). Climate, sea level, and tectonics have been ignored in recent studies of these units in favor of autogenic responses and steady sedimentation rates as observed in flume experiments. The Cretaceous-Paleocene Ferris Formation in the western Hanna Basin is 2463 m thick and 458 m (18.6% the thickness) in the eastern basin. This difference reflects uplift of a Laramide anticline during deposition and no evidence of faulting or stratigraphic truncation exists. Biostratigraphic data demonstrate that sediment accumulation rate (SAR) in the early Paleocene was 60% of the Cretaceous (0.55 mm/yr vs. 0.89 mm/yr). SAR in the middle Paleocene was 0.22 mm/yr, then 0.16 mm/yr. and 0.24 mm/yr into the earliest Eocene. Compaction of coal in the Paleocene section contributes to the decreased thickness, but even when decompacted, the Paleocene section shows decreased sediment accumulation rate. Clastic sediment supply decreased as did accommodation. Channel belts range from 4-40 m thick and 200-4,690 m wide with single and multistorey architectures. Stratigraphically abrupt contact between channel and floodplain deposits has been proposed, in contrast to other ancient deposits that show a transitional relationship (ie cutting into crevasse splay deposits). In fact, channel belts in the Ferris and overlying Hanna Formation incise heterolithics that represent lacustrine or brackish water deltas in the central part of the basin. Ferris “floodplain” deposits are predominantly lacustrine claystone so splay deposits are not present, but this has nothing to do with avulsion style. Crevasse splay deposits are more common in paleosol-bearing portions of the Paleocene Hanna Formation. Vertical stacking patterns in both units are the same as described in the Williams Fork, Blackhawk, Skagerrak, and Morrison Formations, with increased amalgamation and better drained paleosols towards the top of each cycle. Autogenic signals are clearly present in the basin fill, but direct comparison to experimental data must be tempered by the understanding that external controls were active and influential as well.