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Along-Strike Architectural and Sedimentologic Variation of a Distributive Fluvial System, Cretaceous Straight Cliffs Formation, Utah


Models of distributive fluvial systems (DFSs) highlight architectural and sedimentologic trends that occur down-depositional dip from apex to toe. Along-strike trends (e.g., across the main trend of the feeder system) are less well-understood. In the western Kaiparowits Plateau of southern Utah, continuous exposures of fluvial sections in the John Henry Member (Cretaceous Straight Cliffs Formation) provide opportunities to rectify this knowledge gap and inform predictive models of fluvial architecture in part of a preserved DFS. In addition to traditional field work, new data also highlight use of UAV-based stereo-photogrammetry, and a workflow for photorealistic point cloud interpretations brought into Petrel. Previous work in these strata documented 7 depositional units (DUs) based on internal channel belt architecture, median bedload grain size, net-to-gross, degree of tidal influence, lateral versus vertical amalgamation, and width of channel belts. The DUs were found to be regionally consistent for ~30 km in the dip-direction by previous workers, and this study has found them to be consistent over at least ~15 km in the strike direction as well. Regional trends in net-to-gross and grain size are also consistent in the strike direction, expressed first as an upward decrease followed by an increase in both metrics. Over larger strike-distance scales (at least 50 km to the north), the DUs are not as clearly defined, and the section is 78% thicker. The northward thickening has also been observed in the shoreface deposits to the east, suggesting a regional pattern of increased accommodation across the basin, generally parallel to the thrust front that was presumably creating flexural accommodation. Differences in fluvial architecture, indicated by discontinuity of the DUs observed along strike, suggests that fluvial deposits in the north may have been deposited by a different distributive fluvial system than in the south. However, similar trends in net-to-gross and grain size indicate that these different DFSs could have been influenced by similar allogenic forces. An alternative hypothesis suggests that the discontinuity in DUs is only caused by a disparity in accommodation, resulting in different depositional style between the north and the south, but as part of the same DFS. Ongoing provenance studies will help to clarify these controls on fluvial architecture which are critical to understanding analogous hydrocarbon reservoirs.