--> Abstract: The Missing Mudbelts of the Ancient Record: Implications for Sequence Stratigraphy, by Boyan K. Vakarelov, Charles D. Winker, and Janok P. Bhattacharya; #90039 (2005)

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The Missing Mudbelts of the Ancient Record: Implications for Sequence Stratigraphy

Boyan K. Vakarelov1, Charles D. Winker2, and Janok P. Bhattacharya1
1 University of Texas at Dallas, Richardson, TX
2 Shell International E&P Inc, Houston, TX

Numerous modern prodeltas have a major along-shore growth component driven by geostrophic circulation. Such mudbelts may extend for hundreds of kilometers from a river mouth and show internal clinoform geometries that transition from oblique in the delta region to sigmoidal further downdrift. While the oblique portion of such systems is represented in sequence stratigraphical models, the sigmoidal portion, often laterally more persistent, remains largely unincorporated. Under existing models, a mudbelt showing an onlapping relationship with pre-existing topography can only be interpreted as either a transgressive healing phase deposit requiring a rise in sea level, or as a lowstand deposit requiring a relative sea level drop. Since their formation is not restricted to only these conditions, we advocate that “mudbelts” be recognized as distinct elements in sequence stratigraphical nomenclature.

Mudbelts are characetrized by 1) a transition from oblique to sigmoidal clinoform geometries within the same stratigraphic or seismic unit, 2) offlapping sigmoidal relationships within a mud dominated interval. Examples of individual seismic units displaying transition from oblique to sigmoidal geometries are demonstrated in Pleistocene shelf-edge deltaic successions in the Gulf of Mexico, offshore Alabama. Oblique portions of such units are interpreted as delta lobe deposits, which pass laterally into sigmoidal and coeval mudbelts. A Cretaceous Western Interior example of a mudbelt is indicated by offlapping sigmoidal geometries of bentonite-bounded, mudstone- and siltstone-dominated units within the Cenomanian-aged Lower Belle Fourche Member of the Frontier Formation, Powder River Basin, Wyoming.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005