INSIGHTS INTO AVULSION OFFSET DISTANCE USING STRATIGRAPHIC INDICATORS: EXAMPLE FROM THE MORRISON FORMATION
We evaluate avulsion stratigraphy in the fluvial Morrison Fm (Upper Jurassic, eastern Utah). The Sand Wash Member of the Morrison Formation, a sand-rich unit, underlies the Brushy Basin Member, an overbank-dominated unit. Channel belts in both units have similar paleoflow depths, sand body thicknesses, number of stories, maximum grain size, and show similar crevasse-splay deposits. As such there are no significant changes in channel properties that would suggest a potential change in crevasse-splay production between the two units. The Sand Wash and Brushy Basin members contain 20% (n = 46) and 73% (n = 51) abrupt avulsion deposits, respectively. Therefore, it is possible that short avulsion-offset distances (indicating small avulsions) were common for Salt Wash rivers whereas Brushy Basin rivers were more prone to avulsing longer distances.
Further analysis of proximal-overbank deposits and channel spacing in these and other units may help constrain paleo-avulsion offset distances in different types of fluvial systems. This type of information is useful for predicting the behavior of modern avulsive systems as well as subsurface distributions of reservoir units.