Autogenic Processes and Environmental Forcings Recorded in Aeolian Stratigraphy I: the Jurassic Page Sandstone, Arizona, U.S.A.
The stratigraphic architecture of aeolian sandstones records signals originating from autogenic dune behavior and allogenic environmental boundary conditions within which the dune field evolves. Mapping of outcrop-scale surfaces and sets of cross-strata of the Jurassic Page Sandstone near Page, Arizona, demonstrates that the stratigraphic signature of dune autogenic behavior is captured by variable dune scour depth, whereas the dominant signatures of allogenic boundary conditions were produced by antecedent surface topography and water-table elevation. At the study area, the Page Sandstone is ~ 60 m thick and is separated from the underlying Navajo Sandstone by the J-2 regional unconformity, which displays 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 consists of packages of one to a few m-scale sets of cross-strata between the outcrop-scale bounding surfaces. These surfaces were previously correlated to high stands of the adjacent Carmel sea. At the outcrop scale, these surfaces show meters of erosional relief resulting from dune scour. Notably absent within packages of cross-strata bounded by outcrop-scale surfaces are early dune-field accumulations, interdune deposits, and climbing-dune strata. Instead, these packages preserve a scour-fill architecture created by large migrating dunes within a dry, mature dune field. Lower and middle Page strata associated with early phases of dune-field construction were cannibalized by the deepest scours of later, large dunes. Interpretations are supported by the relatively large coefficient of variation (CV) of lower and middle Page set thicknesses, consistent with set production by successive deepest trough scours, and the relatively low CV of the depression-filling basal Page sets. Numerical modeling demonstrates how this cannibalization of early-phase stratigraphy is autogenic, and that early-phase strata can be preserved within antecedent depressions. Relative rise of the inland water table from basin subsidence and Carmel sea level forced preservation of multiple stacked packages composed of scour-fill architecture. Without these allogenic forcings, the Page would be little more than an erosional surface. Reservoir models informed by these results should consider the permeability of mature dune strata representing the majority of the record and regional bounding surfaces.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019