Sedimentology of playa lakes of the northern Great Plains

Abstract

The semi-arid plains of western Canada and northern USA contain a large number of saline and hypersaline lakes, with many of the shallow lakes exhibiting playa characteristics. For over 60 yr a number of these playas have provided a source of valuable industrial minerals. The modern sediments and sedimentary processes operating in selected playa basins located throughout the northern Great Plains region are studied. The playas generally occupy small, closed basins commonly having an elongate, riverine shape. Most of the brines are dominated by Na, Mg and SO42- ions; but examples of lakes rich in Cl- and HCO3- ions are also present. Significant variation in the brine chemistry can exist on a seasonal as well as a diurnal basis. The modern sediments of the playa consist of: very soluble salts of mainly Na and Mg sulfates and carbonates; sparingly soluble precipitates, including calcite, protodolomite, gypsum and mixed layer clays; detrital minerals consisting dominantly of quartz, carbonates, feldspars and clays; and organic matter. Much of the physical and mineralogical character of the playas is due to processes that are largely of a chemical nature and are associated with either evaporative concentration of the brines or groundwater discharge. The most significant processes include: cyclic flooding and desiccation of the playa surface, formation of salt crusts, efflorescent crusts, spring deposits, and intrasedimentary salts, formation of solution pits and chimneys, wind displacement of brines, and periodic detrital sedimentation by sheet flow and wind. These processes combine to create a very dynamic modern depositional environment. The establishment and prolonged maintenance of delicate physical and chemical equilibria have allowed the deposition of over 40 m of salt in some basins.