Grain Size Controls on Planform Morphology and Stratigraphy of River-Dominated Deltas

Burpee, Alexander P.; Slingerland, Rudy L.; Edmonds, Douglas A.; Parsons, Daniel R.; Best, James; Caldwell, Rebecca; Nijhuis, Austin; Royce, Jordan; Cederberg, James; McGuffin, Andrew; Prozeller, Sarah

The proportions of sand and mud that make up a river-dominated delta strongly determine its topset morphology, which in turn controls its internal facies and clinoform stratigraphy. Here we show how these relationships allow one to predict the stratigraphy of a delta from the character of its topset or reconstruct its planform from measures of clinoform geometry. We used Delft3-D to simulate nine self-formed deltas having different sediment loads and critical shear stresses required for re-entrainment of mud. The deltas prograded into a shallow basin absent of waves, tides, Coriolis forcing, and buoyancy. Model results indicate that sand-dominated deltas are more fan-shaped and mud-dominated deltas are more bird's foot in planform because the sand-dominated deltas have more active distributaries and a smaller variance of topset elevations, and thereby experience more equitable distribution of sediment to their perimeters. This results in a larger proportion of channel facies and autogenic parasequences in sand-dominated deltas, and more uniformly-distributed clinoform dip directions, steeper dips, and greater clinoform concavity. These conclusions are consistent with data collected from the Goose River Delta, a coarse-grained fan delta prograding into Goose Bay, Labrador, Canada. These results allow a re-interpretation of the Kf-1 parasequence set of the Cretaceous Last Chance Delta, a unit of the Ferron Sandstone near Emery, UT. Inversion of Ferron grain size data, clinoform dips, clinoform concavity, and variance of dip directions suggests that the Kf-1 Last Chance Delta was more fan-delta than bird's foot, and therefore more consistent with Cotter (1976) and Thompson (1986) than Gardner (1995) and Anderson et al. (2004). It likely possessed numerous distributaries with at least five orders of bifurcation, and would be a high quality reservoir.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013