Quantification of a Distributive Fluvial System: The Salt Wash DFS of the Morrison Formation, SW U.S.A.

Abstract

Recent analysis of modern aggradational continental sedimentary basins reveals that sedimentation patterns are dominated by distributive fluvial systems (DFSs). The Salt Wash Member of the Late Jurassic Morrison Formation has previously been described as a fan-shaped fluvial system. This study characterizes facies variations across the Salt Wash DFS to quantitatively test predicted trends in conceptual DFS models. Notable proximal-to-distal trends include a change in total thickness of the fluvial succession from 174 m to 40 m, and an average grain size from coarse sand to silt, while the percentage of sand decreased from 70% in the proximal region to 8% in the distal region. The proportion of amalgamated channel-belt deposits decreased from 67% to 0%, while floodplain facies and lacustrine deposits increase (38% to 94% and 0.1% to 7% respectively). A downstream decrease in average channel-belt thickness (15 m to 3.8 m, from thickest to thinnest) and average story thickness (7.7 m to 2.3 m, from thickest to thinnest) is also recorded. Significant downstream changes in deposit architecture were also noted, with proximal regions dominated by stacked channel-belt deposits with a high degree of amalgamation. Distal deposits are dominated by floodplain muds and sheet sandstones and sparse ribbon channels, with little to no amalgamation of channel deposits. This study provides quantified information for an ancient DFSs with the aim of providing a dataset that can be used for objective comparison between different DFSs, as well as providing numerical data to aid resource exploration and modelling efforts.

A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

“…Raines et al (2013) attributed this to the propagation of transverse drainage systems across the foredeep. Additionally, the Morrison Formation in Utah has been shown to include the record of transverse sediment dispersal by distributive fluvial systems Owen et al, 2015). Previously, Upper Jurassic strata in Montana have been assigned to a backbulge setting with passage of the forebulge interpreted to have created the sub-Cretaceous un con formity (DeCelles, 2004;Fuentes et al, 2009;.…”

Fluvial geomorphic elements in modern sedimentary basins and their potential preservation in the rock record: A review

“…An exception to this meanderbelt 825 form is observed on the Beni River DFS, where the channel belts remained in a location for long enough 826 to produce an amalgamated channel belt form (Fig.21C). These different forms of meander belts will 827 produce very different sand body geometries in the rock record, where lack of amalgamation will result 828 in discrete ribbon sandstone geometries (e.g., Owen et al, 2015b) and an amalgamated meander belt 829 will produce a complex sheet sandstone with multiple pointbar accretion directions (e.g., Hartley et 830 al.,2015). Thus, channel belt sandstone geometries offer clues as to the position or process in the 831 sedimentary basin.…”

Recognition and importance of amalgamated sandy meander belts in the continental rock record

“…The succession is interpreted to represent a large DFS that flowed in a north to northeast direction ( Fig. 1; Craig et al, 1956;Mullens and Freeman, 1957;Owen et al, 2015aOwen et al, , 2015b. The system comprises large-scale amalgamated channel belt deposits that can extend tens of kilometers laterally in the proximal region.…”

“…The system comprises large-scale amalgamated channel belt deposits that can extend tens of kilometers laterally in the proximal region. Downstream, channel belts pass progressively into floodplain facies composed of poorly developed paleosols, ribbon channels, and minor lacustrine units (Owen et al, 2015b).…”