Changes in Fluvial Style Across a Sequence Boundary, Cretaceous Blackhawk and Castlegate Formations, Utah

M. M. Adams and J. P. Bhattacharya
University of Texas at Dallas, Richardson, TX

Non-marine sequence stratigraphic models suggest dramatic changes of fluvial style across sequence boundaries. Specifically, the transition between the Cretaceous Blackhawk and overlying Castlegate Formations of central Utah has been interpreted as representing a transition from mudstone-dominated isolated meander-belt deposits into amalgamated braided-stream deposits. The lack of detailed studies of fluvial facies architecture within the Blackhawk represents an opportunity to test this model by comparing dimensions and styles of channel deposits, bars, and cross strata between the Blackhawk and Castlegate Formations along Salina Canyon in Utah. Assuming no major increase in basin discharge, braided Castlegate channels should show evidence of shallower flow depths and higher width-to-depth ratios than the “meandering” Blackhawk channels, which should display thicker dune-scale cross-stratification, thicker bars, and thicker channel storeys.

Our data show the Blackhawk comprises isolated 5–8 m thick channel belts within thick floodplain fines, whereas the Castlegate consists of 80 m amalgamated 3–4 m thick sandy channel storeys with minor mudstone. Maximum dune-scale cross-set thickness is 38 cm for the Castlegate and 45 cm for the Blackhawk. Average crossset thickness is 12 cm for the Castlegate and 16 cm for the Blackhawk. Maximum bar thicknesses are 3 m for the Blackhawk. Blackhawk bedding-diagrams based on photomosaics show overlapping lens-shaped bar and channel-fill deposits. The bars show evidence of active channels on either side, indicating that they formed as midchannel braid bars. The Blackhawk and Castlegate rivers show a similar fluvial style in that they are both braided, although Castlegate rivers were about 15–25% smaller.

AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas