FACTORS CONTROLLING AUTHIGENIC MINERALOGY IN THE PLIO-PLEISTOCENE LAKE TECOPA BEDS, SOUTHEASTERN CALIFORNIA, USA

The stratigraphy and mineralogy of the Plio-Pleistocene Lake Tecopa Beds in southeastern California were investigated in nine correlated sections to elucidate relationships between sedimentary facies, authigenic mineralogy, and lake level in a saline, alkaline-lake deposit. The alluvial facies have mineral compositions dominated by quartz, albite, potassium feldspar, trioctahedral and dioctahedral(?) smectite, and illite, with variable quantities of calcite, chlorite, clinoptilolite, phillipsite and soluble salts. Lacustrine margin to shallow lacustrine facies along the basin margin contain a variety of compositions ranging from calcite- and trioctahedral smectite-rich lacustrine-margin mudstones to phillipsite- and/or clinoptilolite-dominated shallow-lacustrine mudstones and sandstones. Perennial lake deposits along the basin margin are composed chiefly of trioctahedral smectite, calcite, and dolomite with lesser and variable quantities of clinoptilolite, phillipsite, sepiolite, feldspars, quartz, and soluble salts. Playa and shallow lacustrine facies near the basin center are composed primarily of authigenic potassium feldspar, albite, illite, and searlesite, with lesser quantities of calcite, opal C-T, trioctahedral smectite, zeolites and soluble salts. Perennial lake deposits near the basin center partially preserve the trioctahedral smectite, calcite, and dolomite assemblage observed at basin margin locations, but also have variable quantities of authigenic potassium feldspar, illite, searlesite, zeolites, and soluble salts. Most of the minerals in the alluvial facies, aside from calcite, zeolites, and some clay minerals are interpreted to be detrital in origin. Aside from quartz, some feldspars, and potentially part of the clay mineral fraction, most of the minerals in the lacustrine facies are interpreted to be authigenic in origin. The distribution and composition of authigenic minerals in the lacustrine and lacustrine margin facies in basin margin locations are interpreted to be controlled by initial sediment composition and coupled lake level and composition. The authigenic mineral distribution and composition of lacustrine facies in the basin center and axis locations are interpreted to be chiefly controlled by reactions with basin-center brines, although a depositional signature may be retained in some facies.