Paper No. 5
Presentation Time: 2:35 PM
INSIGHTS INTO AVULSION OFFSET DISTANCE USING STRATIGRAPHIC INDICATORS: EXAMPLE FROM THE MORRISON FORMATION
HELLER, Paul L., Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, HAJEK, Elizabeth, Department of Geosciences, The Pennsylvania State University, University Park, PA 16802 and LAREAU, Heather Jones, Encana Natural Gas Inc, 370 17th St. Suite 1700, Denver, CO 80202, heller@uwyo.edu
While modern avulsions are rare events, ancient deposits preserve many iterations of the process. Two types of avulsion stratigraphy are observed in ancient deposits:
transitional stratigraphy, where crevasse-splay and other proximal-overbank deposits immediately underlie channel-belt deposits, and
abrupt stratigraphy, where channel deposits directly overlie fine-grained, distal overbank material. Although controls on the size and distribution of splay deposits are relatively unconstrained, within any given system, coarse overbank material accumulates adjacent to active channels. Consequently, the proportion of transitional and abrupt deposits in ancient successions reflects characteristic length scales of 1) proximal-overbank deposits, and 2) cross-stream avulsion-offset distances. Assuming no significant change in splay/proximal-overbank production within a given succession, relative avulsion-offset distances can be compared.
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.
