Bristol Lake's mineralogy is described as having a bullseye pattern of minerals with lithofacies consisting of halite at the center surrounded by mud, gypsum, and finally a sand flat playa margin. These minerals also have vertical lithofacies which resemble the horizontal facies stratification with gypsum occurring deeper in the playa followed by mud-halite and halite on top. The mud lithofacies consists of thick detrital mud, and the halite lithofacies is defined by giant hopper shaped crystals. Gypsum occurs in large lenticular crystals throughout the playa but is mostly concentrated around the mud lithofacies. Gypsum crystal sizes increase toward the center of the playa.
Interpretation
Gypsum
Past studies have determined that the gypsum occurring in Bristol Lake precipitated displacively within the sediment where groundwater saturated with gypsum recharges around the mud lithofacies. This is supported by the geometry of the deposit and by chemical ldata, which suggests that water precipitating gypsum in the playa is more associated with groundwater than the brine at the basin center. large size of the gypsum crystals may be due to several reasons; inflow waters containing low Ca/SO4 ratios may result in large lenticular crystals, microorganisms have the potential to rework large lenticular crystals, high concentrations of NaCl in inflow waters can decrease nucleation density of minerals resulting in larger crystal sizes. It is likely that a combination of these processes was needed in order to form the gypsum crystals mentioned because gypsum crystals formed from low Ca/SO4 ratios or from microbial activity alone would not result in gypsum crystals large enough, and because gypsum size increases toward the center of the playa where NaCl concentrations are greatest.
Halite
Thin crusts and hopper-shaped halite crystals that occur in the sediment are caused by evaporative growth from capillary brines discharging at the surface.
Mud
Sediment mineral composition found in the desert saline sediments of southern California are predominantly influenced by the composition of the source rock, this is true for Bristol Lake as well.
The brine chemistries at Bristol Lake are different from those predicted to form by the evaporative concentrations of the 2 inflow waters currently accounted for.
Na-HCO3_SO4 will precipitate CaCO3 which will deplete the water in Ca. These waters evolve into Na-HCO3-CO3-Cl-SO4 brines with minor Mg and K. They will precipitate halite, Na-Sulfate, and Na-carbonate mineral upon further evaporation.
Cl-SO4 predicted to precipitate calcite and then gypsum and form neutral Na-SO4-Cl brines with subordinate K and Mg. These brines are predicted to precipitate halite and Na-Sulfate salts during further evaporative concentration.
The basin center brines of BDL are Na-Ca-Cl rich with lower concentrations of K and Mg and little SO4 and HCO3. Differing from their predicted chemical composition mostly by lacking SO4, CO3 and HCO3 and having high levels of Cl. Rosen 1991 attributed increased concentration of Cl to be from atmospheric precipitation, however the Ca-Cl concentrations present at Bristol Lake are not compatible with normal low temperature surface weathering and evaporative concentration processes. It is speculated that a magma chamber drives the formation of Ca-Cl brines at elevated temperatures and drives the transportation of these brines to the surface. Other evidence of a magma chamber in the area is the Amboy crater and its associated lava flows, which occur directly North of Bristol Lake.
