--> Abstract: Post-Tectonic Alluviation in a Continental Basin: Implications for Tectonic Controls on Fluvial Sedimentation, by G. A. Smith; #91012 (1992).

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ABSTRACT: Post-Tectonic Alluviation in a Continental Basin: Implications for Tectonic Controls on Fluvial Sedimentation

SMITH, GARY A., University of New Mexico, Albuquerque, NM

In most studies of fluvial sequences, it is assumed that sedimentation is sustained only by active subsidence and that the duration of a nonmarine sedimentary unit brackets the age of deformation in the enclosing basin. Extensional block faulting in southeastern Arizona virtually ceased in late Miocene time although sedimentation continues, in some basins, to the present day. Deposition continues in hydrologically closed basins after subsidence ceases and at least until headward erosion integrates the basin. The San Pedro Valley became hydrologically open at about 3.4 Ma but continued to fill with alluvium until about 0.6 Ma. Net sediment accumulation rates were low (2-10 cm/k.y.) and variations were coincident with climatic changes constrained by stable isotopic analyses of pedogenic carbonates. During relatively cool-wet times, sedimentation rate increased as a consequence of increased runoff and was possibly influenced by rising water tables, which could indirectly regulate local base level. High water tables are recognized by the occurrence of hydromorphic-soil carbonates. These carbonates might be easily confused with mature calcic horizons in vadose soils that would lead to erroneous interpretations of increased aridity and/or slower sedimentation

rates, which are inconsistent with decreased aridity recorded by isotopic data and higher sedimentation rates constrained by magnetostratigraphy. Channelized piedmonts were abruptly replaced by broad bajadas when the climate became very warm and, probably, monsoonal in the early Pleistocene. Not only sediment accumulation rates, but also channel-body geometries, paleosol types, and coarse-grained facies abundance correlate to relatively long-term (i.e., non-Milankovitch), nonperiodic climatic variations implied by the isotopic compositions of pedogenic carbonates. The relative contributions of tectonism and climate to control the temporally varying stratigraphy of tectonically and nontectonically accommodated fill in continental basins requires reexamination.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)