AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Stratal Signature of Backwater Hydraulic Conditions in Campanian Lower Castlegate Sandstone Paleo-Rivers, Book Cliffs, USA
(1) St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN.
(2) Jackson School of Geosciences, University of Texas at Austin, Austin, TX.
(3) University of Illinois at Urbana-Champaign, Urbana-Champaign, IL.
The Campanian Lower Castlegate Sandstone provides an exceptional record of downstream transitions in fluvial deposition from the thick proximal, sand-rich type section dominated by amalgamated channel-filling barforms to the distal pinchout with isolated mud-filled channels and thin overbank deposits. Paleo-hydraulic analyses along a downstream-oriented outcrop transect in concert with observations of facies and architectures have revealed the stratal signature of backwater hydraulic conditions in the distal reaches of Lower Castlegate paleo-rivers. Backwater hydraulic conditions result from the influence of the receiving basin on river flow, and occur where the elevation of the channel bed is below mean sea level. The backwater reach exerted a significant influence on downstream fining and sorting within Lower Castlegate paleo-rivers, and consequently, backwater hydraulics conditions are shown to have a profound impact on the morphodynamics of Lower Castlegate paleo-rivers by controlling the downstream distribution of bed material (sand). This study marks the first recognition of the effects of backwater conditions on the rock record.
The backwater reach of the Lower Castlegate paleo-river system is recognized by downstream increase in paleo-flow depths which indicates divergence of the channel bed and water surface. The upstream end of the backwater reach coincides with the downstream limit of the pure bedload component of sediment transport as shown by grain-size distributions of bar material within the Lower Castlegate. As volumes of bed material decreased downstream into the backwater reach, the paleo-river style changed from braided to meandering and avulsion frequencies and rates of lateral channel migration decreased. The channel bed was incompletely covered with bed material in the distal backwater reach, and experienced local erosion and incision as a result. Near-complete deposition of sand within the river system led to disconnect between the distal pinchout of the fluvial sandstone and the co-eval shoreline deposits. The basal surface of the Lower Castlegate is scoured, but does not reflect a significant unconformity or sequence boundary. It is instead interpreted as a composite scour surface created by erosion at the base of an avulsing and laterally migrating river system. The main factor controlling the spatial variability of facies was downstream loss of bedload from transport by channel aggradation influenced by backwater conditions.