The occurrence of cyclic patterns of sedimentation on a large scale, or abrupt changes in lithology or facies patterns in foreland basins, are most commonly attributed to tectonism. Climatic controls are invoked much less often, and geomorphic controls are rarely considered except for small-scale features. Tectonism is the first-order control on sedimentation at mountain fronts by providing accommodation space for sediment accumulation, and the requisite energy for the system to operate. However, geomorphic controls on sediment yield from source areas, transformation of sediment yield in transfer systems, and feedback mechanisms between source areas and depositional basins may be the secondary controls on sediment dispersal and accumulation near mountain fronts. Drainage basins pass through a series of stages during their evolution. Sediment flux is large during initial stages of basin evolution, allowing fans to prograde rapidly. But as drainage nets in the source area expand, and valleys increase their capacity to store sediment eroded from interfluves, the quantity and caliber of the sediment load at the outlet diminishes, and fan sequences begin to fine upward. Eventually the source area drainage network will expand to its maximum size. Relief is greatest in the source area at this time, and the quantity and calibre of the sediment eroded from valley walls reaches a maximum. Dynamic equilibrium between fan and source area is attained and a period during which spontaneous incision of source area valleys, fanhead entrenchment, and depositional lobe progradation occurs The amount and size of sediment supplied to the fan reaches a maximum at this time, and fan deposits coarsen upward through this period. However, feedback relationships between fan and depositional basin limit the ability of fans to prograde basinward. Fans begin to retrograde as relief in the source area declines and storage capacity increases. Fining-upward sequences arc deposited and basinal facies encroach on the fan during this final phase. Ancient alluvial fans commonly consist of coarsening-then fining-upward stratigraphic sequences consistent with this evolutionary model, and cross-sections of alluvial fan deposits normally show that fan facies stack vertically near their upland sources. A typical first-order fan deposit ranges in thickness between 100 m and 250 m, but those in foreland basins near thrust margins are considerably thicker, possibly in response to increased accommodation space provided by basin subsidence, extended periods of downcutting caused by continued movement on the thrust, and to basinward translation of the source area.
|Original language||English (US)|
|Number of pages||20|
|State||Published - Sep 1 1992|
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