Alluvial Fans and Fan Deltas: Depositional Models for Some Terrigenous Clastic Wedges: ABSTRACT

Alluvial fans and fan deltas are constructed by similar processes; both require a highland and adjacent lowland for development. Alluvial fans are associated with interior basins, whereas fan deltas develop along coastlines. A fan delta is an alluvial fan which progrades into a marine body of water. Modern alluvial fans are present in both arid and humid regions throughout the world, ranging from Arctic to lower latitudes. Geometry and facies are controlled by rate of basin subsidence, source material, and frequency and magnitude of floods. In arid regions, where fans are most common, principal processes include debris-flows, sieve deposition, and fluvial deposition. Processes are intermittent and commonly one is dominant. Debris-flows and sieve deposits are major contributors to the upper 1/3 to ½ of a fan. Sieve deposits are generally confined to the fan apex. Debris-flows, characterized by a heterogeneous mixture of clay- to boulder-size material, extend for considerable distances downfan where they grade to mudflows containing few large clasts. Debris-flows reflect a fin -grained source area. Fluvial processes are dominant on the distal fan. Humid-region fans, e.g., Kosi River fan of India, are constructed entirely by fluvial processes during large annual floods. Compared to arid-region fans, humid-region fans have a low slope from apex to toe, are large in areal extent, and thin in cross section. Humid-region fans also contain smaller clasts, less fine material in the upper fan, and sediment is better sorted. Source-area vegetation aids in breakdown of rock material into smaller particles than under arid conditions. Erosion is great in humid areas because of intense flooding. Alluvial fans in closed drainage basins commonly are associated with lakes. Where alluvial fans build into basins with through-flowing streams, braided-stream deposits of the distal fan are interbedded with floodplain deposits. Fan deltas may be distinguished from alluvial fans only by the nature of related facies. Modern fan deltas develop in areas of high or low rainfall, from deserts to tropical rain forests, and are associated with a wide range of marine depositional environments, e.g., reef-lagoonal to submarine fan association. Types of depositional environments associated with fan deltas are determined by such factors as tidal range, shelf width, and climate. Fan deltas differ considerably from modern oceanic (high-constructive) deltas which are constructed by continuously flowing, large rivers characterized by a large suspension-load/bed-load ratio. Deltaic plains of oceanic deltas generally are covered by dense vegetation, whereas subaerial parts of fan deltas are virtually barren. Oceanic deltas have ragged lobate or digitate margins indented by interdistributary bays; fan deltas commonly have a smoothly arcuate distal end with no interdistributary bays. Prodelta deposits associated with oceanic deltas are commonly the thickest delta facies; equivalent facies of fan deltas are comparatively thin. Fan-delta deposits are continually reworked by marine processes. Deposition is sporadic, therefore marine processes are effective in redistributing prodelta sediment. Marine currents redeposit sediment along the distal fan as beaches and associated berms, and within adjacent shallow marine areas as thin sand sheets and local fan-margin islands or spits. Many ancient clastic-wedge deposits from Precambrian to Pleistocene ages are alluvial-fan systems. Deposits composing these systems become finer in the direction of transport. Lacustrine or fluvial deposits commonly are associated with the finer grained alluvial-fan deposits. Ancient alluvial fans are known from the (1) Precambrian of Texas, (2) Devonian of Norway, (3) Carboniferous of Canada, (4) Permian-Triassic of England, (5) Triassic of the Connecticut Valley, and (6) Pleistocene of California. Ancient fan deltas have been described as fanglomerates, continental deposits, and tectonic deltas. Subaerial facies have the same character as ancient alluvial fans but are associated with marine facies ranging from turbidites to tidal-flat deposits. Ancient fan deltas occur in the (1) Devonian of New York and Northwest Territories, (2) Pennsylvanian-Permian terrigenous clastics shed off the Ancestral Rockies, Amarillo Mountains, Wichita Mountains, and Arbuckle Mountains, (3) Miocene of Texas and California, and (4) the Pleistocene of Baja California. Fan deltas and possibly high-destructive deltas prograded shorelines and filled basins during early geologic periods, prior to evolution of terrestrial vegetation. High-destructive deltas are produced by marine reworking of river-borne sediment. Streams associated with high-destructive deltas are characterized by short duration peak discharge which allows sediment deposited at the mouths to be immediately reworked into spits and beach ridges. Lag time between precipitation and runoff was short and the fluvial systems which developed these 2 delta types were either braided streams or coarse-grained meander belts. End_of_Article - Last_Page 155------------