The Balance between Deposition and Subsidence (Tectonics) in a Rift Basin Playa and Its Effect on the Climatic Record of an Area: Evidence from Bristol Dry Lake, California
ROSEN, MICHAEL R., Division of Water Resources, CSIRO, Floreat Park, Western Australia
Two continuous core intervals drilled in Bristol Dry Lake, a large (150 km) playa in the central Mojave Desert of California, penetrated over 500 m of sediment and did not reach basement. One core, drilled in the center of the playa, consists of alternating beds of halite (up to 10 m thick) and calcareous, siliciclastic muds, and sand, containing minor amounts of anhydrite. The other core, drilled on the playa margin, consists of muds and sands in the upper 150 m but then abruptly changes to approximately 100 m of chaotic muddy halite. Below this chaotic salt, the rest of the core returns to siliciclastic muds and sands with minor anhydrite intervals.
The repetitious nature of the alternating shallow brine pond halite and siliciclastic and the consistency of the carbonate isotopic data from the surface and core indicate a relatively stable brine composition for most of the history of Bristol Dry Lake. All sedimentary structures and primary halite fabrics in the core indicate shallow-water, brine-pond halite alternated with halite-saturated siliciclastic muds in the basin center. A delicate balance of subsidence and mechanical and chemical deposition of evaporite and siliciclastic minerals was necessary to maintain the largely ephemeral lake environment of deposition through over 550 m of basin fill.
The alternating brine pond/saline lake setting in Bristol Dry Lake is not directly related to climatic influences, and the sediments do not record major climatic events demonstrated in other closed-basin lakes. The reason for this insensitivity to climatic events is explained by the interior location of the basin, the low relief of the mountains surrounding the catchment, the large surface area of the catchment, and the low average sedimentation rates. All of the above criteria are at least partially controlled by the tectonics of the area, which, in turn, affect the sedimentation rate and supply of water to the basin. Therefore, it is important to consider the influence of the above factors in determining global versus local, or regional, climate curves for a particular basin.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)