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Fan Deltas and Braid Deltas: Conceptual Problems

John G. McPherson, Ganapathy Shanmugam, Richard J. Moiola

The concept of fan deltas has been widely misinterpreted in the geologic literature. A true fan delta is defined as an alluvial fan deposited into a standing body of water. Such sequences are of limited areal extent and are, as expected, uncommon in the rock record. By contrast, braid deltas (herein defined), formed by progradation of a braided fluvial system into a standing body of water, are a common geomorphic feature in many modern settings, and their deposits are common in the geologic record. Braid-delta sequences are often identified as fan deltas, on the false premise that coarse-grained deposits in a deltaic setting are always part of an alluvial fan complex. We find that most published examples of so called fan deltas contain no direct evidence for the presence f an alluvial fan. Even in examples where an alluvial fan could be documented, we found that, in many cases, the alluvial fan complex was far removed from the shoreline, separated by an extensive braid plain. We suggest that such systems are better classified as braid deltas. We consider that it is essential to distinguish the environmental setting of true fan deltas from that of braid deltas. Misclassification will lead to incorrect interpretations of expected facies, sandstone geometry, reservoir quality, and tectonic settings. Criteria based on geometry, vertical and lateral lithofacies associations, and paleocurrent patterns should be used to correctly identify and distinguish these depositional systems.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.