A REVIEW OF AND GEOCHEMICAL EVOLUTION OF THE BRINES IN THE BRISTOL TROUGH, CALIFORNIA

The geochemical evolution of the brines in the SE to NW trending basins of the Bristol Trough (Danby (DL), Cadiz (CL), and Bristol (BL) Dry Lakes) has been the subject of considerable debate since they were included as end members of closed basin geochemical evolution on the “Eugster and Hardie” diagram. The composition of the brines vary from Na-Cl brine at DL, to Na-Ca-Cl brines at CL and BL basins; an unusual composition for continental brines. The total dissolved solids concentrations increases from roughly 200 g/L at DL to 350 g/L at BL. Minor and trace element compositions indicative of geothermal fluids (Li, B, Sr, Si) are low (<24 mg/L) at DL moderate (< 100 mg/L for Li, B, and Si) at CL and BL, but with Sr concentrations of <2 mg/L at DL, but around 500 mg/L in CL and BL. While an argument for the need of geothermal fluids to evolve the brine to the composition of CL and BL is possible, it is unlikely that geothermal fluids have influenced the DL basin. However CL and BL have little evidence of geothermal fluids in the basin presently. Silica concentrations are low (<40 mg/L), current brine temperatures are approximately 25 degrees centigrade, and no geothermal springs or fumaroles have been found. However, geothermometry (using Mg-Li) of the brine fluids indicate temperatures may have been as high as 160 degrees centigrade in BL, 150 in CL and about 100 in DL in the past, assuming the Li comes from heating Given the age of the Trough (> 5 mya), and the possibility that all three basins were connected into the same lake or open estuary during Bouse Formation time (about 5 mya ago), the brine composition of DL likely has evolved separately from CL and BL since Bouse time. In addition, because BL is more concentrated and has higher geothermometry temperature estimates than CL, the likely source of the geothermal brine (if present) was to the northwest of BL. Current information on brine chemistries in all 3 basins is limited. Until more information from deep wells can be obtained in all three basins, the definitive origin of Bristol Trough brines can only be postulated.