Application of a Quantified System Scale Analyses of DFS to Predicting Basin Scale Facies Distributions

Abstract

Analysis of modern continental sedimentary basins indicates that distributive fluvial systems (DFS) account for a large proportion of modern continental sedimentary basins and will thus account for a significant proportion of the continental geologic record. A system-wide study on the remarkably well exposed Salt Wash DFS of the Late Jurassic Morrison Formation, SW USA, was conducted in order to test the presence of trends cited in published generic DFS models. A downstream decrease in the sand:mud ratio and presence of channel belt deposits, and a downstream increase in floodplain proportion were quantitatively determined. Analyses of the occurrence and thickness of ribbon fluvial channel fills indicate a relatively uniform presence across the DFS. A consistent change in fluvial architecture was qualitatively identified, with proximal regions dominated by stacked channels belt deposits with a high degree of amalgamation and distal regions dominated by floodplain muds and sheet sandstones and sparse ribbon channels, with little to no amalgamation of channel deposits. This study on the Salt Wash DFS quantitatively and qualitatively demonstrates the robustness and predictability of published DFS models as well as providing additional statistics on trends cited. Results and trends found on the Salt Wash DFS have been utilised to aid mapping efforts at a basin scale of the Paleogene Fort Union and Willwood Formations in the northern portion of the Bighorn Basin, NW USA. Preliminary analyses of multiple successions across an E-W trending cross-section within a NW-SE trending basin reveals the presence of smaller transverse, easterly flowing DFS and the presence of a much larger axial DFS in the basin center flowing northwards. The presence of large scale (up to 30 m thick) amalgamated channel belt deposits separated by equally thick floodplain packages, with no visible connectivity between the channel belt deposits, in the center of the basin indicates that the deposits in this portion of the basin represent the medial to distal portion of a system. These combined studies demonstrate the predictability of the DFS concept at both the system and basin scale, with clear implications in resource exploration efforts.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015