Preservation of Autogenic Processes and Allogenic Forcings in Set-Scale Aeolian Architecture II: The Scour-and-Fill Dominated Jurassic Page Sandstone, Arizona, U.S.A.

Abstract

AbstractThe stratigraphic architecture of aeolian sandstones is thought to record signals originating from both autogenic dune behavior and allogenic environmental boundary conditions within which the dune field evolves. Mapping of outcrop-scale surfaces and sets of cross-strata between these surfaces for the Jurassic Page Sandstone near Page, Arizona, USA, demonstrates that the stratigraphic signature of autogenic behavior is captured by variable scour depths and subsequent fillings, whereas the dominant signatures of allogenic boundary conditions are associated with antecedent surface topography and variable water-table elevations. At the study area, the Page Sandstone ranges from 55 to 65 m thick and is separated from the underlying Navajo Sandstone by the J-2 regional unconformity with meters of relief. Thin, climbing sets of cross-strata of the basal Page representing early dune-field accumulations fill J-2 depressions. In contrast, the overlying lower and middle Page consist of cross-strata ranging from less than 1 to 15 meters thick (average 2.44 m), and packaged between outcrop-scale bounding surfaces, though parts of the lower Page are bounded from beneath by the J-2. These bounding surfaces have been previously correlated to highstand deposits of the adjacent Carmel sea and at this site possess up to 13 meters of erosional relief produced by dune scour. Notably absent in packages of cross-strata bounded by these outcrop-scale surfaces are strata of early dune-field accumulations, any interdune deposits, and climbing-dune strata. Instead, these packages preserve a scour-and-fill architecture created by large dunes migrating in a dry, mature, dune field undergoing negligible bed aggradation. Any record of early phases of dune-field construction for the lower and middle Page are interpreted to have been cannibalized by the deepest scours of later, large dunes. Interpretations are independently supported by the relatively large coefficients of variation (cv) in middle Page set thicknesses (cv = 0.90), which are consistent with set production by successive deepest trough scours, the relatively low coefficient of variation for the depression-filling basal Page and lower Page sets consistent with a significant component of bed aggradation in J-2 depressions (cv = 0.64 and 0.49), and the fit of set thickness distributions to established theory. Numerical modeling presented here and more completely in the companion paper demonstrates how this cannibalization of early-phase stratigraphy is an expected outcome of autogenic dune-growth processes, and that early-phase strata can be preserved within antecedent depressions. Relative rise of the inland water table from basin subsidence and changing Carmel sea level forced preservation of 5–6 stacked packages composed of scour-and-fill architecture. Without these allogenic forcings, the Page would be little more than an erosional surface.