FLUVIAL SYSTEMS IN MODERN SEDIMENTARY BASINS AS ANALOGS TO CENOZOIC FLUVIAL ROCKS

Many advances are being made in understanding Cenozoic fluvial successions; however, these concepts must be placed into the context of their sedimentary basins. Distributive fluvial systems (DFS) cover the greatest area of fluvial deposition in most modern continental sedimentary basins, thus we expect DFS deposits to comprise much of the fluvial rock record, including that of the Cenozoic of the Western US. Commonly on DFS, proximal deposits are dominated by relatively coarse-grained, amalgamated channel belts with soils that are well drained. Down-DFS, the degree of channel belt amalgamation and the width of the active channel commonly decreases with a greater proportion of the deposited sediments representing floodplain environments. Distally, many DFS display isolated channels that are surrounded by floodplain deposits. Commonly, the soils that develop in the distal reaches are hydromorphic due to discharge of groundwater. These geomorphic element distributions are not only important for understanding ancient record facies distributions, but they also partially control the distribution of taxa in the basin ecosystem and the conditions influencing fossil preservation, and hence the expected distributions and abundances of fossils. These proximal to distal geomorphic element changes lead to a distinctive increasing soil drainage and coarsening upward vertical profile as the DFS prograde into their sedimentary basins.

Many Cenozoic successions in the Western US display this progradational profile. The Oligocene White River Group to Miocene Arikaree Formation and equivalent units in WY, NE, and SD display an increasing paleosol drainage upward succession that has often been attributed to climate change; however, this is also coincident with greater amalgamation of channel belts and a remnant form of a megafan is observed on satellite imagery. The Nacimiento Formation of the San Juan Basin in NM also displays a coarsening and increased paleosol drainage upward, consistent with prograding DFS. However, not all successions may fit this simplified model, so some caution is needed in its application. Further work is needed to better integrate observations of geomorphic and ecologic distributions in modern sedimentary basins into our understanding of Cenozoic fluvial successions.