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Quantitative Analysis of the Dimensions and Distribution of Channelized Fluvial Sandbodies within a Large-Scale Outcrop Dataset: Upper Cretaceous Blackhawk Formation, Wasatch Plateau, Central Utah, USA


Conceptual and quantitative models of fluvial stratigraphic architecture typically focus on the basin-scale spatial and temporal distribution of channel-belt sandbodies. Such models argue that alluvial architecture may be driven by processes that are allogenic (e.g. tectonic subsidence, sea level, sediment supply), or instead via autogenic self-organization (e.g. avulsion). Avulsion models typically assume that channelized sandbodies are distributed either randomly or show spatial regularity, however recent observations have indicated the potential for localized clustering. This research involves the analysis of a 200 m thick by 300 km wide alluvial-to-coastal plain succession, the Upper Cretaceous Blackhawk Formation (Wasatch Plateau, Utah, U.S.A.) that corresponds to shoreline progradation in response to decreasing tectonic subsidence rate. Oblique aerial photographs and architectural panels of six near-linear, near-vertical cliff face sections aligned oblique to depositional strike were used to characterize the dimensions (apparent widths, thicknesses) and spatial distribution of sandbodies. Two statistical parameters, lacunarity and Ripley's K-function, were applied to determine whether channelized sandbodies show significant spatial regularity, randomness, or clustering within the cliff-face sections. Over 500 channelized sandbodies are identified in the studied cliff-face sections; and consist of single storey, multilateral, and multistory categories. The size and abundance of sandbodies broadly increases from base to top of the Blackhawk Formation, but there is much local variation. Weak clustering of sandbodies is noted locally in lower coastal plain strata (<50 km from the coeval shoreline) over length scales of 1-5 times the mean sandbody dimensions. Weakly defined, regular spacing over comparable length scales occurs locally in upper coastal plain strata (>50 km from the coeval shoreline). A weak negative correlation between lacunarity and stratigraphic position is also observed, such that greater spacing occurs between sandbodies in the lower Blackhawk Formation than between sandbodies in the upper Blackhawk Formation. These results support an avulsion-related control on sandbody distribution laterally and vertically, which operated under a range of distances from the coeval shoreline (c. 0-100 km) and tectonic subsidence rates (c. 80-700 m/Ma). The comparison of these results to modern day analogues will be the focus of further study.