Abstract

On the basis of topographic and stratigraphic relations between pediments and terraces along the Moapa Valley, Clark County, Nevada, a temporal-genetic model is proposed for the origin of these surfaces. Pedimentation is a continuous process below the 1,000-m contour and is essentially unaffected by transitions from pluvial to interpluvial periods because climatic conditions below this elevation remain arid. Incision of the trunk drainage is initiated during pluvial periods owing to the increase in runoff from the higher, upper reaches of the Muddy River watershed in east-central Nevada. During pluvial periods, pediment formation proceeds without aggradation of the trunk drainage by local tributaries because the competence of the main stream is held at a high level. During a long pluvial period, pedimentation may extend to the marginal limits of the weak basin fill. This buffers the trunk drainage against aggradation by local tributaries during the subsequent interpluvial period, when the competence of the main stream is greatly reduced because the bedrock mountain ranges erode very slowly as compared with the basin fill, and because waste from the mountain ranges can be stored in alluvial fans which spread out onto the pediments. Aggradation of the trunk drainage by local tributaries occurs during interpluvial periods following short pluvial periods, because under these conditions there is little opportunity to destroy the older, higher surfaces while the competence of the main stream is at a high level.

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