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Abstract

Energy conditions at the seaward edge of deltas allow their division into fluvial-dominated, wave-dominated, and tide-dominated deltas. Each kind of delta has a distinct framework orientation and depositional pattern which results in a characteristic seismic reflection pattern.

Fluvial-dominated deltas are characterized by clinoform seismic reflection patterns which include oblique (tangential), complex oblique (tangential), sigmoid, and complex sigmoid-oblique. Seismic facies analysis can be used to define those facies which should contain sand.

Wave-dominated deltas are characterized by shingled seismic reflection patterns. Seismic fades analysis of this delta is not effective in identifying those facies which should be sand prone. Shingled reflections may be used in determining the possible location and depositional attitude of strandline sands.

Tide-dominated deltas have not yet been identified using seismic stratigraphic methods and therefore are not covered in this paper.

Turbidite fans are sequences of sands and shales deposited in conjunction with and basinward of deltas or submarine canyons. Turbidite sands can be classified generally into channel and suprafan sands.

Certain seismic events and reflection patterns occurring in various combinations may suggest the presence of turbidites. These indicators include troughs, submarine canyons, mounds, prograded fluvial-dominated delta reflection patterns which vary in thickness, and onlap-offlap patterns on depositional slopes.

Regional studies provide the best means of identifying and mapping depositional sequences. Examples from the North Sea, Gulf Coast, and Sacramento Valley illustrate the geologic and geophysical expression of delta and turbidite sequences and their interrelations.

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