A New Model for Development of River-Dominated Deltas

Wellner, Robert William¹, David Hoyal², Ben Sheets², Tao Sun³, John Van Wagoner², Anthony Sprague² (1) ExxonMobil Upstream Research, Houston, TX (2) ExxonMobil Upstream Research Company, Houston, TX (3) ExxonMobil Upstream Research Company, Houston

River-dominated deltas have been and remain a subject of great interest and controversy to geoscientists. These depositional features are easily recognized on many present day coastlines and have been frequently described in the ancient rock record. A simple model, however, for their genesis, evolution, and their ultimate abandonment and burial in the subsurface is lacking. Recent results from analysis of modern deltas, the ancient rock record, experimental tank data, and physics-based numeric models similarly indicate that these deltas are comprised of morphologically similar, scale invariant, depositional bodies and that the genesis of these bodies can consistently interpreted using a model based on turbulent jet flow at the mouths of distributary channels.

The morphology of individual bodies consists of an up dip scour pool that becomes conformable several 100 m seaward of the distributary channel. The transition from the confined scoured region to a conformable surface is accompanied by coarse-grained, bedload deposition. Outboard of this area, the body is dominated by a suspended load region that fines, thins, and forms an apron that is radial to the scour pool. This apron may exhibit partial or complete Bouma Sequences.

A model based on the compensational and progradational stacking of this fundamental body is proposed for sub-lobe to parasequence development in deltaic successions. This model has been successively applied to 2 ancient deltaic systems, the Panther Tongue Member and Ferron Sandstone, Cretaceous western interior, U.S.A. and in the modern Wax Lake Delta, Mississippi Delta Complex, Louisiana, U.S.A.