Abstract

Observations from southern White River Valley, southeastern Nevada, shed light on sediment dispersal and geomorphic development during and after extensional faulting that formed the valley. Fine-grained lacustrine and alluvial sediments of the late Neogene White River Narrows Formation were deposited in a subsiding half graben, while coeval coarse-grained deposits were confined to within 1 km of the active fault. The syntectonic deposits are overlain along an erosional unconformity by post-tectonic, uniformly coarse-grained, alluvial-plain and fluvial-terrace deposits. This sequence conforms to a recent proposal that fine-grained intervals in extensional basins are deposited syntectonically, whereas coarse-grained intervals are deposited during post-tectonic progradation of marginal clastic wedges.

The stratigraphy and geomorphology of the study area suggest that post-tectonic progradation of coarse clastic sediment resulted from incorporation of the White River Valley into the Colorado River drainage system. Meadow Valley Wash also links previously closed, young but presently quiescent, extensional basins to the Colorado River system. By contrast, most nearby basins that contain Quaternary scarps remain internally drained. Maintenance of internal drainage by active subsidence therefore may have been important in ponding of coarse-grained sediments next to the fault. After tectonic subsidence ceased, the basin drainage became southward. This action resulted in post-tectonic progradation of coarse-grained clastic deposits as sediment-dispersal systems responded to the new external base level. Regional comparisons, however, indicate that, depending on local rates of tectonic subsidence and erosion, a basin may become externally drained syntectonically, which can cause syntectonic deposits to be coarse grained. In addition, a basin may remain closed long after tectonic activity ceases, causing fine-grained deposits to continue to accumulate in a quiescent basin. We conclude that the interplay between internal and external drainage, which depends not only on the rate of local fault activity, but also regional tectonics, climate, and other factors, is a significant factor in the stratigraphic evolution of continental extensional basins.

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