Distinguishing Shorefaces Versus Delta Fronts in the Rock Record

Janok P. Bhattacharya
UT Dallas, Richardson, TX

The shoreface is the seaward-dipping equilibrium profile that forms in response to the asymmetry of shoaling fair-weather waves causing landward or alongshore sand transport. The term shoreface has also been broadly applied to any shoaling-upward clastic facies succession that shows wave-formed sedimentary structures, and particularly hummocky cross stratification, which is produced by storms. This is unfortunate, as storm surges remove sediment from the shoreline, which is the opposite of shoaling fair-weather waves.

Many of these storm-dominated prograding facies successions are heterolithic, show low degrees of bioturbation, and contain, climbing wave-ripples, which indicate high-sedimentation rates. Prograding delta fronts also produce upward coarsening facies successions that resemble these so-called shoreface successions, but how can they be distinguished, especially in subsurface studies where core data may not be available? Application of new ichnofacies models suggests that many of these so-called “shorefaces” are fluvially-influenced and better interpreted as storm-flood dominated delta-fronts.

The general shoreface facies model predicts that shorefaces form homogenous, uniform reservoirs that require little effort to produce. These assumptions have not turned out to be valid in the production behavior of many “shoreface” type reservoirs. In fact, many wave-dominated shorefaces are actually delta front deposits. Wave-influenced deltaic coastlines commonly show a distinct facies asymmetry with homogenous beach and shoreface sands accumulating on the updrift side of the river mouth with significantly more-heterogeneous facies on the downdrift side. This asymmetric delta model has been applied to the re- interpretation of Cretaceous “shoreface” deposits in Wyoming, New Mexico, Utah, and Alberta.

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