Fluvial sedimentation in continental half-graben basins
The stratigraphic architecture of intracontinental rift basins is defined by a dynamic relationship between depositional belts associated with the basin floor and flanking tributary streams on the piedmont. Spatiotemporal distributions of these deposits are sensitive to basin geometry, subsidence rate, and discharge. Understanding how these depositional belts respond to allogenic forcing is examined using experimental and field approaches. Physical experiments focused on the geomorphic evolution of drainage and the resulting stratigraphic architecture in an asymmetrically subsiding basin based on the form of a simple half graben with four interacting supply points of sediment and water that produced an axial fan and longitudinal channel flanked by transverse fans. Imposition of various combinations of lateral and axial sediment flux showed that the locations and widths of the deposits were controlled by relative sediment discharges and not by the location of the subsidence maximum. Except during the highest of axial sediment discharges in the experiment, the axial drainage was dominated by transverse sources through toe cutting. Footwall fans persisted under conditions of high axial-sediment discharge, aided by topographic inheritance and steeper deposit slopes. The hanging-wall fan responded to changes in sediment discharge more slowly than the footwall. Field comparisons focused on a study of Plio-Pleistocene deposits in the tectonically active Albuquerque Basin of New Mexico. Deposits on the distal hanging-wall ramp onlapped a widespread Miocene erosion surface, burying it by 3.0 Ma. These deposits coarsened after 2.6 Ma, forming a broad sheet of amalgamated channel deposits that prograded into the basin until 1.8 Ma. Axial-river deposition focused near the eastern master-fault system until piedmont deposits prograded away from the basin border after 1.8 Ma. Basin-fill deposition ceased when the axial river began incising shortly after 0.8 Ma. The asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of half-graben basin shape that promoted extensive bypass of sediment. Integration of the axial drainage and development of this Plio-Pleistocene sequence likely formed as a result of increased discharge due to late Pliocene and early Pleistocene climatic changes.
Neogene tectonics, nonmarine stratigraphy, basin analysis, Rio Grande Rift, magnetostratigraphy, geomorphology, experimental stratigraphy, Sedimentary basins
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