Jeffery Valenza and Dr. Tom Morris, Geological Sciences
Tempestites are sedimentary deposits traditionally thought to be formed by high-energy storm events. Typically, water depth for tempestites has 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), yet appear to be imbedded within very shallow to emergent settings such as coastal sabkhas and alluvial plains.
This project was predominantly a field survey, which included locating, measuring, and sampling from exposed tempestites in cliffs of the San Rafael Swell of Central Utah. In order to locate tempestite beds, we used all-terrain vehicles to drive along the base of cliffs, and when we spotted a possible bed we dismounted and hiked the rest of the way. There, we measured the thickness and lateral extent, took photographs, GPS coordinates, and obtained a sample of the bed. Data was obtained from a previous study of the same area, providing porosimeter and permeameter statistics.
Approximately five miles were explored along the San Rafael Swell cliffs near Moore road. We were able to obtain five samples from five tempestite beds, as well as the corresponding coordinate locations and descriptions. Descriptions of these beds yielded information about the thicknesses and lateral extents. Along the five mile stretch of cliffs that we examined, we found that the beds containing AHCS were thinner than we anticipated, with the thickest bed being 1 meter thick, while the thinnest was 10 cm.
Typically, the beds occurred in sets of three to five, with clay-rich interbeds. While the most recent samples have not been analyzed for porosity and permeability to date, data obtained for past measurements yield an average porosity of 4.61% and an average permeability of 1.04 mD at 500 psi confining pressure.
Initial interpretations of the study area yield some significant insight into AHCS. The limited thickness of these tempestite beds indicates either a shorter period of deposition, or a limited amount of sediment available for deposition. The substantial lateral extents of the beds indicate that whatever event produced these beds was not merely a localized event, but perhaps a regional one. Bed sets of 3-5 hummocky beds suggest repeated energy events. These occur only in the upper third of the Entrada section, suggesting they were not cyclic deposits occurring during the 4 million years of deposition of the Entrada.
Past and recent research conducted on the mudstone-dominated portion of the Entrada Sandstone has categorized its depositional environments to consist predominantly of terrestrial depositional systems. These include wadi-type alluvial systems and inland to coastal sabkha facies. 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 for non-cyclic but repeated energy events could include storm surge, tsumanis, or very large tidal bores. This work will be the starting point for my Master’s thesis, which will focus on further evaluation of these bedforms, and the possible mechanisms that produced them.