RECONSTRUCTING PROCESSES ASSOCIATED WITH TURBIDITY CURRENTS BUILDING SANDY CLINOFORMS IN THE CRETACEOUS FERRON SANDSTONE, UTAH

Field data from an exceptionally well-exposed set of clinoforms in the Ferron sandstone at Ivie Creek, Utah, has been analyzed to reconstruct depositional conditions associated with turbidity currents that built this delta. Approximate height, length and maximum thickness for the individual sand-rich clinoforms are 15m, 500m and 0.4m, respectively. A bottom set accounts for roughly 70% of each length and the foreset makes up about 60% of the height. The representative slope to clinoform foresets is six degrees. The foreset/bottom set transition is characterized by upper plane-bed stratification composed of fine sand and distal bottom sets consist of climbing-ripple stratification and very fine sand. This spatial assemblage of structures and textures, and particularly the ripple stratification, has been used to reconstruct the near-bed suspended-sediment concentrations and durations associated with turbidity currents that deposited 11 beds. Quantitative estimates for concentration of suspended sand within the depositing currents range from 0.3% to 3.0% by volume. Durations for these same depositional events range from 0.5 to 7 hours. Suspended sand concentrations for associated distributary channels have also been estimated using the suite of structures and textures preserved in these fills. These concentrations are an order of magnitude smaller than those recorded in the clinoforms. This difference in suspended sand and the relatively short durations for the depositional events are not consistent with direct river discharge being the only sand source. Mining of sediment from pre-existing mouth bars is proposed as the additional source for sand within the depositing turbidity currents.