Autori: Janok P. Bhattacharya and Liviu Giosan

Editorial: Blackwell, Sedimentology, 50,1, 2003.

Rezumat:

A process-based facies model for asymmetric wave-influenced deltas predicts significant river-borne muds with potentially lower quality reservoir facies in prodelta and downdrift areas, and better quality sand in updrift areas. Many ancient barrier-lagoon systems and „offshore bars” may be better re-interpreted as components of large scale asymmetric wave-influenced deltaic systems.

The proposed model is based on a re-evaluation of several modern examples. An asymmetry index A is defined as the ratio between river discharge (in 10 million m3/month) and longshore transport rate at the mouth (in m3/year). Symmetry is favoured in deltas with an index below ~200 (e.g., Tiber, lobes of Godavari delta, Rosetta lobe of the Nile, Ebro) while deltas with a higher index are asymmetric (e.g., Danube – Sf. Gheorghe lobe, Brazos, Damietta lobe of the Nile delta). Periodic deflection of the river mouth for significant distances in the downdrift direction occurs for extreme cases of littoral drift dominance (e.g., Mahanadi), resulting in a series of randomly distributed, quasi-parallel series of sand spits and channel fills.

Asymmetric deltas show variable proportions of river-, wave-, and tide- dominated facies both among and within their lobes. Bayhead deltas, lagoons, and barrier islands naturally form in prograding asymmetric deltas and are not necessarily associated with transgressive systems. This complexity underlines the necessity of interpreting ancient depositional systems in a larger palaeogeographic context.

Cuvinte cheie: deltaic sedimentation; littoral drift; barrier islands; tidal inlets; sand bodies; reservoir properties. // deltaic sedimentation, littoral drift, barrier islands, tidal inlets, sand bodies, reservoir properties