A reinterpretation of the Jelm and Popo Agie formations (Triassic Wyoming) and distributive fluvial system (DFS) and the role of the accommodation/sedimentation ration in DFS Deposition

Distributive fluvial systems are common river environments where the stream is distributary, rather than tributary. These fluvial systems are common in modern basins, where preservation potential is highest, but relatively few have been interpreted from the rock record. This indicates that some previously described fluvial sedimentary units are likely misinterpreted as tributary systems, when in actuality, they were deposited in a distributive system. The Jelm and Popo Agie formations are two poorly understood sedimentary units from west-central Wyoming, which were previously described as part of a tributary fluvial system. In contrast to prior interpretations, we demonstrate that features such as sedimentary structures (i.e., antidunes and chutes-and-pools), facies architecture (i.e., extensive crevasse splays and few channels), and depositional trends, (i.e., fining of sediment and decreased shear stress down-dip) are more consistent with the distributive fluvial system model. Similar to previously described distributive fluvial systems, the Jelm Formation exhibits progradational stacking of the fluvial facies associations. This is contrasted by the overlying Popo Agie Formation, which shows abrupt retrogradational stacking, a pattern that is previously undescribed from any distributive system. The stacking patterns in both the Jelm and Popo Agie formations are interpreted to be the result of changes in the accommodation/sedimentation ratio along depositional dip. Accounting for accommodation and sedimentation in distributive fluvial systems helps better understand how these units were deposited through time and allows for better prediction of stacking patterns found in other distributive fluvial deposits.