Fluvial Channel Stacking Patterns are Not a Foolproof Indicator of Changes in Accommodation

Wroblewski, Anton¹ (1) ConocoPhillips, Houston, TX

Amalgamation of channel sandstone bodies can be caused by decreases in sediment supply, resulting in less delivery of fine-grained overbank deposits and/or decreases in accommodation creation. In foreland basins, a tendency has developed to place sequence boundaries at the bases of zones of increased amalgamation and to make inferences about accommodation based on the degree of interconnectedness of channels. Latest Cretaceous and Paleocene clastic basin-fill deposits of southern Wyoming's Hanna Basin preserve a variety of fluvial channel deposits that are well-dated on the basis of vertebrates, invertebrates, and pollen. This allows comparison of channel style and architecture to calculated rates of subsidence (accommodation creation). Single storey and multistorey channels were investigated to determine what, if any, differences existed between them, especially with regard to channel size and flow depth. Average maximum flow depths of single storey channels were 4-5 meters. In contrast, maximum flow depths for channels comprising multistorey bodies are calculated to have been between 8 and 14 meters on the basis of barform and bedform heights. Tidal facies are more abundantly preserved in the multistorey sandstones and sand is coarser-grained, but both single and multistorey bodies preserve IHS. In at least some examples, multistorey sandstones can be shown to truncate older deposits, resulting in reworking of age-indicative fossils. Since this truncation is a function of increased channel depth, but is not laterally traceable into surrounding overbank deposits in all cases, not all examples can be considered sequence boundaries or correlated with times of decreased accommodation.