TAPHONOMY AND PALEOECOLOGY IN A DISTRIBUTIVE FLUVIAL SYSTEM FRAMEWORK: MOST OF THE PRESENT ISN’T THE KEY TO MUCH OF THE PAST

There is increasing evidence that most modern fluvial systems, deposited in degradational settings, provide poor analogues for fluvial systems in the rock record that will have been deposited in aggradational settings. A more appropriate model to understand the facies distributions and depositional processes of such deposits may be that of Distributive Fluvial Systems (DFS), a term used to describe features such as alluvial fans and fluvial megafans, which cover large parts of modern sedimentary basins. Recognition of these differences may change our understanding of the taphonomic and paleoecologial patterns exhibited by most fossil assemblages from terrestrial fluvial environment. Here we explore and quantify these changes for the first time.

DFS sediment packets are predicted to show a consistent drying-upwards pattern as coarser, proximal deposits prograde over finer distal deposits. Any lengthy DFS fossil successions should reflect this trend with a shift from more water-dependent taxa to more arid-tolerant taxa up section, which will be diachronous across the DFS. Preliminary data support such a trend for the White River Group DFS in WY, NE and SD.

Unlike tributary systems, where stream order increases downstream, channels in a DFS commonly become smaller and more dispersed distally from the apex. Additionally, as deposition in a DFS is not constrained to a valley, reworking may be concentrated in proximal deposits, with distal deposition building alluvial ridges and filling accommodation through avulsion, rather than incising. Two-dimensional modeling of extra-sedimentary basin tributary systems vs. DFS shows that these properties lead to different patterns of fossil concentration and residency within the taphonomically active zone (TAZ), with important taphonomic and paleoecological implications.

Finally, distal-most DFS environments can be extremely extensive (100s of kilometers in length and 10s to 100s of kilometers in width) and uniform in channel and interfluve morphology. Fossil assemblages sampled from these distal-most deposits should show a greater degree of temporal and spatial uniformity than many modern ecosystems. Examples of such deposits include the Hell Creek/Fort Union Formations of MT, WY, SD and ND, and eastern exposures of the White River Group.