ABSTRACT

Modern terrigenous clastic deposits that are lobate in plan, wedge-shaped in cross-section, and which prograde into a body of marine water, or bay, from an adjacent area of high relief are termed fan deltas. Periods of construction are of short duration and sediment is dispersed across the subaerial fan by shallow braided streams. Both the subaerial segments and marine extensions of these features are relatively coarse grained. Because of the brief and sporadic progradational pulses, marine reworking of distal fan deposits is operative most of the time.

A study of depositional processes and resulting facies of a modern fan delta along the mainland shore of Nueces Bay has provided data for constructing a model with which ancient terrigenous clastic deposits may be compared. Recognizable facies of the modern fan delta are (1) the fan plain which extends from the apex downfan to a point where surficial features of braided streams are no longer identifiable, (2) the distal fan, a relatively featureless part of the subaerial fan, which lies in the realm of fluvial, wind and tidal processes, (3) the prodelta, the marine extension of the distal fan, bears a fluvial imprint but has been modified, and (4), facies which include depositional features modified in the bay by physical and biological processes, and abandoned parts of the subaerial fan modified chiefly through fluvial processes.

1. Publication authorized by the Director, Bureau of Economic Geology, The University of Texas at Austin.

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