Abstract

A delta is a partially subaerial, contiguous mass of sediment deposited around the point where a river enters a standing body of water. A deltaic system is a three-dimensional rock-stratigraphic unit composed of many adjacent delta lobes deposited as a part of a major cycle of terrigenous sediment influx. Delta morphology and internal stratigraphy are primarily the product of an interplay between fluvial sediment input and reworking of sediment by marine or lacustrine processes. Although sources of marine energy include oceanic and wind-generated currents, density currents, gravitational potential, tidal currents, storm surge, and wave surge, deltaic progradation is modified primarily by tidal currents and wave surge. Marine deltas can thus be characterized in terms of three end-member types: (1) fluvial-dominated deltas, (2) wave-dominated deltas, and (3) tide-dominated deltas. Modern fluvial-dominated deltas include the birdfoot lobe of the Holocene Mississippi Delta system and the Po and Danube deltas. The Rhone and Sao Francisco are typical wave-dominated deltas. The Ganges-Brahmaputra, Fly, and Colorado deltas are of the tide-dominated type. Gravity induced sediment transport tends to remove sediment basinward from the delta system into slope, submarine fan, and basin floor environments which are best considered separate depositional systems.

Within deltaic depositional systems, longterm evolutionary trends can be recognized and interpreted in terms of response to changing process intensity. Pennsylvanian deltas of north-central Texas changed from fluvial-dominated elongate to wave-influenced or even wave-dominated lobate types as they prograded across a shallow platform into deeper, open marine water. Early Eocene (Wilcox) and Miocene clastic cycles of the Gulf Coast Tertiary basin evolved from fluvial-dominated elongate and lobate deltas of the regressive phase to wave-dominated deltas of the transgressive phase of the cycle.