William E. Galloway, 1985. "Ancient Alluvial Fans and Fan Deltas", Recognition of Fluvial Depositional Systems and their Resource Potential, Romeo M. Flores, Frank G. Ethridge, Andrew D. Miall, William E. Galloway, Thomas D. Fouch
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Although much has been learned about modern alluvial-fan and fluvial systems, the step from active systems to their ancient geologic counterparts is a big one. The following notes attempt, by means of selected examples, to illustrate the facies characteristics of a broad spectrum of fluvial and fan deposits.
Alluvial fans are primarily geomorphic features most readily distinguished by their conical, lobate, or arcuate morphology. As morphology is not readily reconstructed in the stratigraphic record, indirect criteria are necessary. In addition to morphology, alluvial fans display several features suggestive of their origin (Fig. G-1):
Radial or arcuate sediment dispersal pattern reflecting the point source. Note sand distribution on Kern fan (Fig. G-1).
Highly compressed down-flow textural and compositional gradients. The Kern fan grades from dominantly gravel to less than 50 percent sand along a transverse section.
A uniquely predictable ground-water flow system that may produce distinctive geochemical facies or alteration patterns.
Well-defined areal relationships with structura! features or known uplands.
Distinctive associations with bounding facies, such as lacustrine depos-its or major through-flowing trunk stream sequences.
Fan deltas are special situations in which alluvial fans build directly into a standing body of water. Although it might be argued that only small features qualify as true fan deltas, the juxtaposition of marine deposits with coarse fan-like sediments is worthy of note, and the term “fan delta” has been widely applied (see Ethridge, this volume).
Pennsylvanian strata now exposed in the Sangre de Cristo Mountains record deposition of a