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.