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Standard geomorphic and sedimentologic assessments of extensional nonmarine basins commonly represent an axial fluvial or playa/lake depositional tract flanked by piedmont settings - the latter invariably characterized as either alluvial fans or pediments. In reality, non-fan, non-pediment piedmonts comprised of parallel, moderately high gradient streams and adjacent interfluve floodplains (with or without significant eolian processes) are common in the Basin and Range region of the western United States. We apply the term "alluvial slope" (first used by the venerable Kirk Bryan) to describe these piedmonts. Geomorphologically and sedimentologically, alluvial-slopes are distinct from alluvial fans, with which they have been regrettably lumped in many cases. Alluvial-slopes, rather than alluvial fans, form in response to contrasting geomorphic expressions of basin-margin mountain fronts and are, therefore, sensitive to the tectonic history of the basin. Alluvial-slope depositional sequences are inadequately characterized by existing alluvial-fan or fluvial facies models, the latter strongly biased toward the deposits of large, low-gradient rivers. Better understanding of alluvial-slope sedimentology will, therefore, be significant for evaluating characteristics of aquifers and reservoirs in nonmarine extensional basins. Field study of modern alluvial slopes and Neogene alluvial-slope facies is underway at several localities in New Mexico and southern Arizona.
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Publications:
Hydrostratigraphic attributes of hanging-wall derived piedmont facies in asymmetric graben
G.A. Smith
SEPM Congress Program and Abstracts, v. 1, p. 114-115, 1995.
Sedimentology of Miocene alluvial-slope deposits, Española Basin, Rio Grande rift: An outcrop analogue for subsurface heterogeneity
Andrika Kuhle
University of New Mexico, M.S. thesis, 1997.
Recognition and significance of streamflow-dominated piedmont facies in extensional basins
G.A. Smith
Basin Research, v. 12, p. 399-412, 2000.
Sedimentology of alluvial-slope deposits in the Miocene Skull Ridge Member of the Tesuque Formation, Española basin, New Mexico
A.J. Kuhle, G.A. Smith
New Mexico Geology, 2001
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