Effects of Tides on Deltaic Deposition: Causes and Responses

This paper explores the response of deltas to tidal reworking and discusses the conditions at which deltas may become strongly tide influenced or tide dominated. Four case studies are used, from the Cretaceous Western Interior, the Devonian Baltic Basin, the Eocene Central Basin of Spitsbergen and the Permian Karoo Basin in South Africa. All datasets contain extensive outcrop data, the Baltic Basin dataset also extensive core data. The presentation aims to show that beyond the commonly recognized morphological features and the recognizable tidal facies, the main effects of tidal reworking of deltas are associated with delta clinoform morphology, delta lobe switching rates, delta progradation rates, and the nature of the delta plain. Strong tidal influence is here documented to promote subaqueous, rapid progradation of deltas, by efficiently removing sediment from river mouth and thus reducing mouth bar aggradation and fluvial delta plain construction rates. Such subaqueous progradation of the delta front is decoupled from shoreline progradation. Delta plain of such tide-dominated deltas consists of a few distributary channels and tidal flats. The delta front clinoforms become gentler and longer, as tidal currents efficiently transport sediment to the basin. Tide-dominated deltas tend to maintain a funnel shape and show low lobe switching rates, compared to fluvial-dominated and tide-influenced deltas.

This paper emphasizes topographic restrictions or invaginations, caused by incision, delta-lobe deposition or tectonic uplift and subsidence, as significant controls on the occurrence of strongly tide-influenced deltas. The role of such topographic restrictions is twofold, by reducing wave energy and amplifying tidal energy. Significantly, such invaginations are not restricted to inner-shelf reaches as commonly assumed, but may also occur at the shelf edge, promoting tidal reworking of shelf-edge deltas. Moreover, in very shallow epicontinental seas, significant tidal reworking may occur throughout the basin evolution, independent on sea-level cycles or sediment supply.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California