SUBAQUEOUS CLINOFORMS FED BY SUBAERIAL DUNES, ENTRADA SANDSTONE OF EASTERN UTAH

In 1973, E.A. Shinn published a paper entitled “Sedimentary Accretion along the Leeward, SE Coast of Qatar Peninsula, Persian Gulf”. He showed how small desert dunes along the Abu Dhabi coast are migrating into the Persian Gulf, generating a 20-m-thick, progradational wedge composed of a single set of steeply dipping, angle-of-repose cross-strata. Northeast of Moab, Utah, the Dewey Bridge Member of the Entrada Sandstone contains subaqueous, heterolithic cross-strata up to 10 m thick that accumulated via processes similar to those described by Shinn. The major difference between the Holocene and Jurassic settings is that we envision a Jurassic water body that 1) occupied a closed basin; 2) was surrounded by eolian dunes; and 3) alternated between a lake and a sabkha. In several ways, it thus may have resembled modern Lake Chad.

Our conclusion that the rocks are non-marine is based on the very large, reversing, base-level changes recorded by the strata and on the presence of chert nodules with radial structure within redbeds that were downlapped by clinoforms. Large sandstone dikes and pipes are abundant within the clinoform-bearing sequences. Some dikes appear to have been injected downward from well-sorted sand bodies into underlying silty sandstones. Some deformation may have been caused by large sand dunes migrating over uncompacted, prograding strata. Progradational wedges containing subaqueous clinoforms were repeatedly downlapped by eolian strata that were, in turn, onlapped by red, silty sandstones of sabkha/lacustrine origin. The uppermost wedge-capping eolian strata constitute the lowermost portion of the Slickrock Member of the Entrada in the study area. The clinoforms lie within a 30 km-wide area that apparently occupied a small structural depression. Base level likely fluctuated with regional climatic conditions. Lacustrine facies record pluvial episodes when water-table rise led to flooding of the basin floor. Delivery of dune sand into the lake eventually outpaced subsidence, leading to long-lived erg conditions.