WAVE INFLUENCED DELTAS

A survey of modern examples shows that wave-influenced deltas exhibit a variable degree of asymmetry in morphology and facies. An asymmetry index A, defined as the ratio between river discharge and longshore transport rate, was devised to explore the marine-fluvial interaction at the mouth of a deltaic distributary. A review of published data shows that asymmetry is favoured in deltas with an index over ~200. Between ~200 and ~1000 waves are typically not able to deflect the mouth (e.g., Danube-Sf. Gheorghe, Brazos, Sao Francisco), but for deltas with an index over 1000, the river may be periodically deflected downdrift (e.g., Mahanadi).

A facies model for asymmetric deltas predicts significant river-borne mudstones with potentially lower quality reservoir facies in prodelta and downdrift areas and better quality sand in updrift areas. If the model is confirmed in further tests, many ancient barrier-lagoon systems and “offshore bars” could be re-interpreted as components of large scale asymmetric wave-influenced deltaic systems. More generally, there is a complete spectrum from strandplain systems with minor deltaic promontories to river-dominated deltas with minor wave reworking. The definition for deltas should therefore be relaxed to include sediment delivered via longshore/onshore basinal transport.

Many deltas show river-, wave-, and tide- dominated facies both between and within lobes, but in variable proportions. With limited data, a common situation in petroleum exploration, simplified classification schemes based on relative importance of fluvial versus marine processes based on the present ternary classification of deltas can be misleading. This may lead to erroneous estimations of facies architecture, reservoir distribution, and reservoir quality. The asymmetry index is a significant step in quantifying the fluvial-basinal interaction in modern systems that could lead to better facies distribution predictions in ancient systems.