Monday, 6 October 2008: 4:00 PM
George R. Brown Convention Center, 320F
Long cores from Bristol Dry Lake (BDL) show that the basin has hosted playas, but no deep perennial lakes, for more than 3 Ma, and that the basin-center sedimentation has been continuous. However, examination of the Holocene and late Pleistocene alluvial fan deposits surrounding the playa suggests that periods of non-deposition and down-cutting in the fans have made highly variable amounts of sediment available to the playa. Mid-fan deposits (~50 m above the playa) consist of braided-stream sediments composed of poorly-sorted gravel and sand. An extensive pedogenic calcic horizon (Bkm) in this sediment is widespread, although it is best developed on the north side of the basin. The calcic zone is approximately 1-3 m below the surface and is overlain by an associated red Bt horizon in many places. In many locations, this old fan deposit is disconformably overlain by a thin, braided stream deposit with little or no pedogenic characteristics. The observation that this stratigraphy is widespread indicates that there were one or more major periods of reduced sediment delivery to the playa during the development of the old soil and before the deposition of the overlying fan deposit. Similar stage III calcic soils elsewhere in the Mojave Desert took at least 30,000 years to develop. During Pleistocene pluvials, other basins in southern California, such as Manix, Searles, and Panamint Lakes, were experiencing high lake levels. The BDL fan stratigraphy, coupled with the lack of evidence of paleoshorelines cut into Pleistocene fan deposits surrounding the BDL basin, indicates that BDL did not host a deep perennial lake at this time. Complexly varying patterns of sediment delivery from fans must be reflected in the playa sediments. Non-deposition in the fans may correspond to periods of significant diagenesis in the basin center halite units, where primary halite textures have been obliterated.