Michael M. Adams1,
Janok P. Bhattacharya1
(1) University of Texas at Dallas, Richardson, TX
Abstract: Changes in fluvial style across a sequence boundary, Cretaceous Blackhawk and Castlegate Formations of Central Utah
Non-marine sequence stratigraphic models suggest that fluvial systems show dramatic changes in fluvial style across major sequence boundaries. Specifically, the transition between the Blackhawk and overlying Castlegate Formations has been interpreted as representing a transition from mudstone-dominated, isolated meander-belt deposits into amalgamated braided-stream terminal fan deposits. Our purpose was to test this model by comparing cross stratal thicknesses and the dimensions of associated bar and channel deposits between the Blackhawk and Castlegate along Salina Canyon in Utah. Assuming no major increase in basin discharge, Castlegate rivers should show evidence of shallower flow depths and higher width-to-depth ratios whereas the Blackhawk should display thicker dune-scale cross-stratification, thicker bars, and thicker channel storeys. We were also looking for facies characteristic of alluvial fans (sheetfloods and debris flows) in this more proximal part of the Castlegate basin.
Data compiled from 18 measured sections and 6 photomosaics show that the Blackhawk comprises isolated 5-8 m thick channel belt sandstones contained within thick floodplain mudstones, whereas the Castlegate consists of about 80 m of amalgamated 3-4 m thick sandy channel and bar deposits with only minor mudstone. Dune-scale cross strata reach 60 cm for the Castlegate but only 45 cm for the Blackhawk, although average Castlegate cross-sets are about 30% thinner. In contrast, maximum bar thicknesses are 1 m for the Castlegate and 1.4 m for the Blackhawk. While there is some indication that Castlegate rivers were smaller, we saw no evidence of sheetflood and debris flow deposits.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana