Stratigraphic architecture of extensional basins: Insights from a numerical model of sedimentation in evolving half grabens

Abstract

We present a numerical model for the tectonostratigraphic evolution of half grabens which duplicates many of the stratigraphic relationships observed in extensional terranes of the southwestern United States and elsewhere. The model also generates insights and predictions on the impact of fault geometry, rates of faulting, and rates of erosion and sediment transport on landscape evolution, stratal geometry, and facies distributions. Three processes control basin development in the model: faulting, erosion/deposition, and isostasy. Assumptions are thin plate flexure, normal fault hanging wall deformation by vertical simple shear, linear dependence of sediment flux on slope, and perfect sorting of sediment during deposition. Model limitations reflect oversimplification of the dynamics of sediment erosion and redistribution. During periods of fault motion, half grabens bounded by listric faults accumulate strongly asymmetric sedimentary units. The degree of under‐ or overfilling of the basin is controlled in the short term by basin geometry (fault dip) and in the longer term by extension rates, sediment supply, and out‐of‐plane sediment transport. After fault motion ceases, erosion and sedimentation redistribute the loads on the lithosphere, resulting in progressively more symmetric sedimentary units which overlap the original basin boundaries. Model facies architecture is affected by changing subsidence patterns, stratigraphic geometries, and sediment flux. Under most conditions, syntectonic gravels are trapped near the basin edge on the fault‐bounded side but prograde far out into the basin on the hinge (opposite the fault) side. When fault movement ceases, coarse‐grained sediment from the fault‐bounded side may initially be able to reach farther out into the basin, but overall retrogression is observed on both sides of the basin as topography is eroded and as posttectonic sedimentation buries former sediment sources. Changing fault dips and lithospheric rigidity affect facies patterns, whereas altering the ratio between rates of erosion and sediment transport affects landform development and depositional geometries.