MODELING CHEMICAL SEDIMENTATION IN CLOSED BASINS

Lacustrine evaporites present an opportunity to reconstruct the geochemical processes governing chemical sedimentation in saline lakes. The conceptual framework of saline lake sedimentation and geochemistry were pioneered by Lawrence Hardie and Hans Eugster. A number of computer programs exist that can model the evaporative concentration of closed-basin brines. However, these programs are limited as the chemistry of one parent water is modeled to a highly concentrated end point with no account for changing hydrological conditions. Different parent water chemistries can be linked to changes in sediment mineralogy but abrupt mineralogical changes in the sediment record may indicate entirely different depositional environments with no evidence of how the transitioning may have occurred.

The model developed for this study merges EQUIL with a three-dimensional, physical mass balance model that determines basin water chemistry and mineral precipitation in an environment that is in constant flux from a hydrological standpoint. The addition of inflow water, its chemical and volumetric variability and its influence on both delivering solutes to, and diluting lake water is accounted for. The model stores data for inflow, air temperature, evaporation, lake level, lake salinity, lake volume, and precipitated minerals.

Pleistocene-to-Holocene deposits of Searles Lake were chosen to test the model. Searles Lake deposits comprise a well-preserved record of sedimentation in a closed basin (Smith, 1979; 2009). The results of the model, to varying degrees, match the outcrop and subsurface deposits and shed light on brine evolution during transitional lake stages.