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Modern Rivers: Geomorphology and Sedimentation
Distinction of Aggradational and Degradational Fluvial Regimes in Valley-Fill Alluvium, Tapia Canyon, New Mexico
An ephemeral tributary of the Rio Puerco northwest of Albuquerque, the present channel of Tapia Canyon is highly sinuous and deeply entrenched in the valley-fill alluvium. More than half of the drainage area is basalt. It produces gray or black gravel that constitutes almost all the bed-load of the stream. In contrast, the bedrock of the remaining area is almost all Cretaceous sandstone. It produces buff-colored sand that is transported principally as suspended load. The segregation of detritus in response to two source-rock types and modes of sediment transport enhances the distinction of different regimes of the past that are preserved in the entrenched channel walls. Furthermore, the geometry, distribution, and either lateral or vertical continuity of sedimentation units permit the identification of process-structure relations that distinguish episodes of aggradation and degradation.
Two basic types of deposits exist. Aggradational-regime deposits consist of sets of black or gray laminae of basalt gravel that are vertically persistent, comparatively uniform in grain size, and within which the individual laminae grade laterally but abruptly into sand laminae. Degradational-regime deposits are laterally persistent, they have sharply bounded, interfingering basalt and buff-colored sand units, and they overlap like cards thickening outward in a fanned-out deck. In comparison to degradational deposits, those resulting from aggradation suggest flow that was perennial and more uniformily distributed seasonally. Under these conditions, the deposition of basalt gravel effectively stablized the locus of thalweg deposition while the bed aggraded vertically. During degradational regimes, such as that at present, deposits reflect the seasonal, flashy nature of the flow. Lateral incision predominates. At one exposure of valley-fill alluvium in Tapia Canyon, radiocarbon and archeological dates temporally bracket episodes of vertical aggradation, lateral incision, and a neck cutoff.
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