Delta asymmetry: Concepts, characteristics, and depositional models

Delta asymmetry forms in wave-influenced settings where there is strong net longshore drift. Asymmetric deltas typically have two sediment sources and are characterized by morphology and facies asymmetry between the downdrift and the updrift sides. The downdrift, sourced primarily by the feeding river, are commonly influenced by mixed river and wave processes. Deposits on this side are muddy and consist of barrier, bar, lagoon, bay-fi ll, and bayhead delta facies with variable bioturbation intensity. At or near the river mouth, heterolithic river-dominated successions are more typical. Deposits are overall unburrowed (BI 0-2), but with high burrowing spikes. Ichnogenera are characterized by horizontal, morphologically simple, and facies-crossing structures. The updrift, in contrast, is sourced by a second sediment source and typically consists of laterally continuous sandy beach and shoreface facies. Bioturbation is overall high (BI 3-6) and ichnogenera are healthy and robust, attributable to the Cruziana and Skolithos Ichnofacies. Depending on shoreline trajectory and depositional history, facies characteristics of asymmetric deltas preserved in the ancient record, however, can be different from those predicted by the models.

Degree of asymmetry is indicated by the asymmetry index (A), defi ned as the ratio between the rate of longshore transport at the river mouth and river water discharge. The indexes in symmetric wave- infl uenced deltas is less than 200, whereas those in asymmetric and defl ected deltas are larger than 200. Overall the larger the index, the higher the degree of asymmetry.

Delta asymmetry concepts and models challenge the traditional definition of deltas and the delta classifi cation scheme. In a hydrocarbon exploration perspective, asymmetric deltas bear very different types of sands and, thus, reservoirs between the updrift and the downdrift. The updrift consists of well- sorted, mature, and laterally continuous homogeneous beach-shoreface reservoirs. The downdrift, in contrast, is muddier and consists of less continuous, less mature, heterolithic reservoirs.

Delta asymmetry; asymmetry index; ichnology; longshore drift; wave-influenced deltas