Recognition and significance of streamflow‐dominated piedmont facies in extensional basins

Alluvial slopes are piedmonts characterized by parallel steam channels rather than alluvial fans. They are common landforms in extensional basins of the south‐western United States but have received little attention from geomorphologists or sedimentologists. Persistence of channellized flow across piedmonts, as opposed to sheetflooding due to loss of flow confinement on alluvial‐fan surfaces, distinguishes alluvial‐slope and alluvial‐fan facies. Miocene strata of the Tesuque Formation (Española basin, New Mexico) and Pliocene strata of the St. David Formation (San Pedro Valley, Arizona) provide examples of extensional basin–piedmont successions constructed by discrete gravel and sand bedload channels and aggrading interfluve floodplains and aeolian sand sheets. Distinction of alluvial‐fan and alluvial‐slope piedmont deposits has several important implications. The contrasting facies geometries associated with the two landforms produce distinctly different aquifer and reservoir properties. It is hypothesized that alluvial slopes are more likely to form than alluvial fans where mountain fronts lack abrupt structural and topographic definition. This circumstance will most likely be met (a) along tectonically inactive and embayed mountain fronts and (b) on the hangingwall ramp side of half grabens.