ALLUVIAL FANS AND THEIR NATURAL DISTINCTION FROM RIVERS BASED ON MORPHOLOGY, HYDRAULIC PROCESSES, SEDIMENTARY PROCESSES, AND FACIES ASSEMBLAGES

Contrary to common contemporary usage, alluvial fans are a naturally u nique phenomenon readily distinguishable from other sedimentary enviro nments, including gravel-bed rivers, on the basis of morphology, hydra ulic processes, sedimentologic processes, and facies assemblages. The piedmont setting of alluvial fans where the feeder channel of an uplan d drainage basin intersects the mountain front assures that catastroph ic fluid gravity flows and sediment gravity flows, including sheetfloo ds, rock falls, rock slides, rock avalanches, and debris flows, are ma jor constructional processes, regardless of climate. The unconfinement of these flows at the mountain front gives rise to the high-sloping, semiconical form that typifies fans. The plano-convex cross-profile ge ometry inherent in this form is the inverse of the troughlike cross-se ctional form of river systems, and precludes the development of floodp lains that characterize rivers. The relatively high slope of alluvial fans creates unique hydraulic conditions where passing fluid gravity f lows attain high capacity, high competency, and upper flow regime, res ulting in sheetfloods that deposit low-angle antidune or surface-paral lel planar-stratified sequences. These waterlaid facies contrast with the typically lower-flow-regime thick-bedded, cross-bedded, and lentic ular channel facies, and associated floodplain sequences, of rivers. T he unconfinement of flows on fans causes a swift decrease in velocity, competency, and capacity as they attenuate, inducing rapid deposition that leads to the angular, poorly sorted textures and short radii typ ical of fans. This condition is markedly different than for rivers, wh ere sediment gravity flows are rare and water flows remain confined by channel walls or spill into floodplains, and increase in depth downst ream. The distinctive processes that construct alluvial fans, coupled with the secondary surficial reworking of their deposits, yield unique facies assemblages that permit the easy differentiation of fan sequen ces even where the geomorphic context has been lost, including in the rock record. The fault-proximal piedmont setting critical for their pr eservation makes properly identified alluvial-fan deposits in the rock record an invaluable tool for reconstructing and interpreting the tec tonic and stratigraphic evolution of ancient sedimentary basins and th eir contained register of Earth history.