Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, U.S.A

Abstract

Two continuous cores over 400 m in length drilled in Bristol Dry Lake, a large playa in the Mojave Desert, California, U.S.A., provide sedimentologic and isotopic information which can be integrated with surface trenches and groundwater analyses. The combination of these data provide three-dimensional data on the facies distribution and geochemical evolution of the basin for the last 4 Ma. At the surface, the basin exhibits a generalized bulls-eye facies distribution. Low-gradient alluvial fans ring the playa. Extensive calcrete and pedogenic calcite associated with halophyte plants cement the mid-to-distal fan gravels and sands. Basinward of the distal fans, in the playa margin facies, an extensive gypsum zone is over 300 m wide. Within the gypsum zone, celestite forms large decimetre-size nodules which may coalesce into metre-size patches. Basinward of the playa margin facies, halite hopper crystals (up to 0.5 m in diameter) form in water-saturated muds of the saline mud-flat facies. Finally, in the basin-centre a 0.2-m thick chevron halite crust (salt pan) forms from evaporation of ponded ephermeral water. The lateral facies distribution seen at the surface is found throughout the entire length of the cores. The core drilled in the basin-centre exhibits beds of alternating salt pan and saline mud-flat facies, whereas the core drilled in the playa margin exhibits alternating beds of playa margin sediments and saline mud-flat for most of the length of the core. Towards the base of the playa-margin core, distal alluvial fan sediments are also present. Stable oxygen isotopes of basin-centre calcite concretions from the surface and core indicate that the isotopic composition of the water precipitating the concretions has not changed substantially throughout the evolution of the basin. The sulfur isotopic composition of the surface gypsum, anhydrite and celestite show little variation. However, δ 34S values of anhydrite from core are isotopically lighter relative to surface gypsum and anhydrite. This isotopic depletion indicates that bacterial sulfate-reduction is involved in the dehydration of gypsum to anhydrite, and is not caused by evolving isotopic composition of sulfate in the brine. At the surface, gypsum δ 34S values show no statistically significant trends with lateral distribution. Chemical analyses from shallow groundwater wells and basin-centre brines from up to 150 m depth indicate that: (1) the brine is dominated by Na, Cl, and Ca. (2) Na, Ca, Mg, K, and Cl contents increase toward the basin-centre, SO 4 increases from the alluvial fan facies to the playa margin and then sharply decreases in the basin-centre, and Si, and HCO 3 decrease slightly toward the basin-centre. This pattern is consistent with the observed evaporite mineral distribution in the basin. (3) molar Na:Cl and Ca:Cl ratios demonstrate that the simple dissolution of previously deposited halite deposits, as has been suggested for other playa basins, cannot account for the proportions of these ions present in the brine. (4) Chemical budget calculations suggest that Cl can be accounted for by atmospheric input, although hydrothermal sources cannot be discounted. The repititious nature of the alternating shallow brine-pond halite and siliciclastics, the consistency of the carbonate isotopic data from the surface and core, and the agreement of water chemistry data with observed evaporite sequences, 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-centre. A delicate balance between subsidence, and mechanical and chemical deposition of minerals was necessary to maintain the largely ephemeral environment of deposition during the deposition of over 500 m of basin fill.