Origins of Relief Along Contacts Between Eolian Sandstones and Overlying Marine Strata

Abstract

Origins of large-scale relief along eolian-marine unit contacts, which form significant stratigraphic traps for hydrocarbons, can be recognized as inherited, reworked, and/or erosional. The Permian Rotliegende-Weissliegende Sandstone and Yellow Sands of Europe may best exemplify inherited relief in that dunes are preserved largely intact. Reworked relief, which shows significant destruction of original dune topography but with remnants of the bedforms preserved, is shown by relict Holocene dunes of coastal Australia, the Jurassic Entrada Sandstone of the San Juan basin, and the Pennsylvanian-Permian Minnelusa Formation of Wyoming. Erosional relief results from post-eolian processes and is exemplified by the the Jurassic Entrada Sandstone of northeastern Utah. Whether inherited to reworked relief is preserved depends on factors that relate to the budget of the dunes and the erg and to the nature of the transgressive environment. Sand budget refers to the maintenance, gain, or loss of dune prior to and during initial transgression. Sand can be conserved and the dune form stabilized by vegetation, early cementation, and a wind regime conducive to sand trapping by the dune. Dune building can continue during initial transgression by maintenance of sand-drift paths, best accomplished by offshore winds. In contrast, dunes are degraded by deflation, a cutoff of the supply, and a variety of marine processes. Factors related to the nature of the transgressive environment include (1) orientation of the dunes to destructive marine pro esses, (2) types of processes, (3) energy level, (4) type and rate of sedimentation, and (5) rate of marine encroachment. Preservation of inherited to reworked relief requires that the dunes become stabilized or remain active into the early phases of the transgression. Effects of destructive marine processes must be minimized by resistant, early-cemented dunes, by an orientation of the dunes such that marine energy is dissipated along a minimum area of the dunes, by a low-energy, non-wave-dominated marine environment, by a rapid transgression that reduces the length of time for reworking, or by high rates of marine sedimentation burying dune forms. Erosional relief requires selective downcutting through erg deposits and can (1) reflect original erg topography (e.g., by preferential channeling through interdune corridors), (2) be totally independent of erg topography and deposits, or (3) be controlled by erg facies with preferential downcutting through less resistant deposits. The last case could result in relief that mimics original topography where interdune deposits are less resistant, or show an inverse of original topography where dune sands are less resistant.