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Avulsion-Generated Spatio-Temporal Arrangement of Fluvial Sandbodies, Blackhawk Formation, Wasatch Plateau, Utah

Sahoo, Hiranya; Gani, M. Royhan; Gani, Nahid D.; Hampson, Gary J.; Rittersbacher, Andreas; Ranson, Andrew; Howell, John A.; Buckley, Simon J.

Recent laboratory and subsurface studies have revealed clustering and compensational stacking of fluvial sandbodies in the apparent absence of allogenic forcing. However, the sandbody and floodplain architectures associated with these two arrangements have only rarely been evaluated in an outcrop dataset. We address this issue using an integrated outcrop, LIDAR, and core dataset from a high-quality outcrop "window" of fairly large spatial (~ 20 km2 area encompassing six contiguous, clean, and vertical cliff faces with their 3-D orientation) and temporal (~ 4 my) scales from the upper Cretaceous Blackhawk Formation in the Cottonwood Canyon area, eastern Wasatch Plateau, Utah. Fluvial architecture constrained from this integrated study reveals that deposition was predominantly guided by autogenic dynamics (i.e. fluvial avulsion) compared to allogenic controls.

Channelized sandbodies were mapped and populated on a 3-D outcrop model extracted from geo-referenced LIDAR data, ground-truthed with measured sections, and containing sandbody orientations corrected for paleoflow direction. The model shows that compensational stacking of sandbodies (i.e. lateral shifting of depocenter through time) is better developed in the lower Blackhawk Formation but gradually decreases stratigraphically upward. In contrast, sandbody vertical-clustering (i.e. depocenter fixed through time) is more apparent for the middle and upper Blackhawk Formation. Our study also reveals that single-storey sandbodies (a single bar-macroform combined with a laterally-adjacent channel-fill deposit) show affinity for vertical clustering, whereas multi-lateral sandbodies (discrete bar deposits that are laterally stacked together at the same stratigraphic level; i.e. channel-belt) are prone to compensational stacking. Floodplain facies diversity, assessed from measured sections and core description, is high for the lower Blackhawk Formation, within which sandbodies are prone to compensational stacking, but low for the middle and upper Blackhawk Formation, where sandbodies are more clustered.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013