A long-term record of climate-controlled hillslope sedimentation

Rare exposures of buried hillslopes and colluvium are found in the upper Miocene and Pliocene basin-fill records of two extensional basins in southeastern Nevada. Study of the stratigraphy, sedimentology, and petrology of these hillslope deposits, along with the Quaternary record and modern slopes of the study area, produces a long-term record of variable hillslope-sediment production and delivery to piedmonts as a function of climate change. Though rare, ancient colluvium exists in the stratigraphic record, and its study is the most direct method of linking hillslope processes to basin depositional systems. Results enhance our ability to interpret the sedimentologic record, particularly in terms of climate controls, and clarifies the applications of arid-environment geomorphic research to sedimentology and basin analysis.;The upper Miocene and Pliocene Muddy Creek Formation and Pliocene Panaca Formation of southeastern Nevada are analogous in their role as late-stage (post-tectonic) basin fill deposited prior to basin integration and incision. The Muddy Creek Formation exposed in Table Mesa basin consists of three members defined by sharp changes in depositional style. The lower member, dominated by internally-drained spring travertine and saline lacustrine deposits, underlies a relatively sharp, undulating contact with mostly extrabasinally-derived red, siliciclastic alluvium of the middle member. The middle member of the Muddy Creek Formation coarsens upward at its top in a gradational contact with the locally-derived coarse fluvial upper member, which, in turn, is truncated at its top by QTg gravel deposited after external drainage was established. The Panaca Formation consists of distal deposits of pond and littoral/marsh environments and medial and proximal alluvial-slope, fluvial, eolian and colluvial deposits. Alluvial-slope deposits account for much of the exposed fill in the Panaca basin, and its fine-grained, roughly parallel-bedded and commonly massive character probably led to previous interpretations of these deposits as lacustrine.;A petrologic weathering index for sand, clay-mineral data, and stratigraphic relations all indicate that, in the Neogene, episodes of greater production and delivery of colluvium from hillslopes in the study area were marked by more intense chemical weathering and thus greater effective moisture. Strata that record only limited colluviation, less intense weathering, and piedmonts dominated by eolian processes very likely correspond to drier conditions. The climate-driven hillslope and piedmont landscape changes evident in these older stratigraphic records corroborate the conceptual model developed by Quaternary researchers for and environments. Quaternary models of and landscape change can therefore be used to interpret the stratigraphic record at least back into the Miocene, but though the mode of response is consistent through the record, the magnitude of sedimentologic response has varied greatly.;A combination of field and laboratory geomorphic, sedimentologic, and sedimentary petrographic methods are employed to document the sources of basin fill sediment, the character and relative amount of sediment produced on local hillslopes through time, the hillslope weathering and transport processes occurring through time, and the role that rock type has played in regional landscape evolution. Results of these studies indicate rock type is the dominant control on sediment yield and landscape development in tectonically inactive, dry landscapes. Hillslope processes and products in the study area have been consistent through time, but process rates have varied greatly, indicating that orbital-scale climate cyclicity can be, but is not always, well-expressed in the stratigraphy of continental basins.