PALEO-WETLAND FACIES OF STONEWALL FLAT, NEVADA, DURING THE PLEISTOCENE/HOLOCENE CLIMATE SHIFT

Stonewall Flat, SW Nevada, is one of several playas that contain remnants of Pleistocene pluvial lakes or wetlands, but it is unique in the completeness of its preservation of fossilized wetland facies indicators and changing sedimentation patterns for the end of the Pleistocene. Stonewall Flat playa contains four distinct horizons. The lowest surface (H-1) is characterized by a mud-cracked clay pan that lacks fossils. It is at the center of Stonewall Flat and represents the open water facies. The H-2 surface lies 1 m above H-1, manifests 0.3 - 0.6 m of low-amplitude relief, consists of light grey paludal clay with fossil root casts, burrows, and some algal mats, and represents the deep marsh facies enveloping the central H-1 open water pond. The H-3 surface lies 1.0-1.5 m above H-2 and consists of the same light grey clay capped by fossil algal mats in the north, by thin fluvial sandstone in the east, and by thin marlstone containing fossil insect burrows to the south and west. H-3 represents the shallow marsh and wet meadow facies, and preserves locations of groundwater seeps (marlstone) and shallow creeks (fluvial sandstone) discharging surface flow to the wetland. A planar color change occurs 0.5 m above the H-3 surface where the light grey paludal clay is overlain by 0.5 m of light yellowish-brown silty clay resulting from increased eolian deposition. This represents the initial sedimentation change resulting from the Holocene climate shift. The H-4 surface at the top of the paludal stratigraphic sequence is capped by a thin veneer of alluvium representing alluvial fan aggradation following the change to a more arid climate. C-14 ages from the H-3 marlstone date groundwater seepage into the final Stonewall wetland highstand at 10,600 radiocarbon years BP. This correlates to Holocene Wet Period 11, which is well represented in Varnish Microlamination data from the surrounding alluvial fans. The subsequent shift to a warmer and drier climate terminated the wetland and promoted fan aggradation onto former wetland scrub and wet meadows.