Michael M. Adams1, Janok P. Bhattacharya1
(1) University of Texas at Dallas, Richardson, TX
ABSTRACT: Braided Rivers of the Cretaceous Blackhawk Formation, Utah: Implications for Sequence Stratigraphy of Non-marine Depositional Systems
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 meander-belt deposits into amalgamated braided-stream deposits. This interpretation is based largely on the observation that the Blackhawk consists of isolated channel-belt sandstones encased in floodplain mudstones versus the Castlegate, which is dominantly sandstone.
Our study, along Salina Canyon, Utah, confirms that 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. Blackhawk bedding-diagrams show overlapping lens-shaped bar and channel-fill deposits. The bars show evidence of active channels on either side, indicating that they formed as mid-channel braid bars. The Blackhawk rivers are interpreted as braided, not meandering, and are thus similar to the Castlegate rivers.
Our results imply that regionally-mappable changes in proportion of channel belt sandstones versus floodplain mudstones may reflect changes in basin accommodation, rather than a major change in fluvial discharge or plan-form channel geometry. Major changes in proportion of preserved floodplains fines do not appear to correlate with a major change in fluvial style in the area studied.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado