Abstract

The Pliocene–Pleistocene Lake Tecopa beds present a well-documented example of authigenic silicate diagenesis in an ancient saline, alkaline lake environment. Controls on authigenic mineral formation and distributions were investigated in nine stratigraphic sections aligned along a north-south transect in the Tecopa basin. Specifically, potential depositional and hydrologic controls on mineral assemblages and distributions were addressed by correlating detailed sedimentological data and basin hydrology with authigenic mineral facies distributions.

Deposition occurred within the Lake Tecopa basin in environments ranging from alluvial and eolian around the basin margin to lake margin, mudflat, and shallow and perennial lacustrine in the basin center. The authigenic silicate minerals include trioctahedral smectite, phillipsite, clinoptilolite, opal C-T (cristobalite-tridymite), potassium feldspar, illite, albite, and searlesite, as well as many other minor or less commonly observed phases. Authigenic mineral distributions along the margin of the basin are strongly controlled by sediment composition (primarily tuffaceous component) and lake-level variations. Authigenic mineral compositions in the center of the basin are dominated by feldspar, illite, and searlesite, and are less influenced by sediment composition or short-term changes in lake level. The authigenic silicate mineral composition in the central part of the basin is interpreted to be a result of chemical interaction with a saline, alkaline brine that moved in accord with lake-level changes and induced density-driven circulation. The results suggest that distributions of authigenic silicate minerals in saline, alkaline lake deposits are complexly related to depositional and hydrologic processes and may be of limited utility in resolving lake-level changes in ancient lacustrine systems.

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