APPARENT HUMMOCKY CROSS-STRATIFICATION OF THE UPPER ENTRADA SANDSTONE: EVIDENCE FOR HCS-DOMINATED TEMPESTITES INTERBEDDED WITH VERY SHALLOW TO EMERGENT SILICLASTIC ENVIRONMENTS

Tempestites are sedimentary deposits traditionally thought to be formed by high-energy storm events. Typically, water depths for tempestities have been interpreted to be between fair weather and storm wave base. Tempestites are primarily identified in the rock record by hummocky cross-stratification (HCS). HCS is a typical indicator of oscillatory flow in subaqueous settings, where strong wave action can scour and redeposit sediment. Tempestites may also contain sole marks and various forms of ripples. Some work has been done on tempestites in lower shoreface and carbonate ramp settings, but little research has been done on tempestites in very shallow water to emergent siliciclastic settings. Utah’s Jurassic (Callovian) Entrada Sandstone hosts fine-grained sand units that demonstrate apparent hummocky cross-stratification (AHCS). Past and recent research conducted on the mudstone-dominated portion of the Entrada Sandstone has categorized its depositional environments to consist predominantly of wadi-type alluvial systems and inland to coastal sabkha facies similar to the present day Rann of Kutch in eastern India. Assuming the facies interpretations are correct, AHCS observed in the Entrada Sandstone suggests that high energy marine events inundated (e.g. affinities to inundites) subaerially-exposed coastal environments such as wadis and sabkhas. Because time equivalent known marine deposits are more than one hundred kilometers away, this area of Utah must have been low in elevation and relatively flat. Potential driving mechanisms include storm surge, tsumanis, or very large tidal bores. Additionally, we consider alluvial flood deposits as the energy source for these puzzling Entrada tempestites.