Evaporite Deposition from Layered Solutions

It is widely accepted that carbonates, especially those which originate as banks or reefs, and the evaporites with which they are closely associated represent mutually exclusive, successive phases of deposition: (1) a stage of carbonate bank or reef deposition under conditions of normal marine circulation, followed by (2) a time of sulfate deposition under conditions of restricted circulation, (3) an episode of further restriction leading to chloride precipitation, and, in some instances, ending in (4) an ultimate stage of bittern-salt accumulation. A multistage history of carbonate-evaporite deposition requires paleogeographic interpretations that appear bizarre in view of the epeiric setting of most deposits. An alternative is sought in deposition from density-layered waters with an upper layer of nearly seawater salinity and a lower layer or layers saturated with respect to one or more evaporite salts. Brine is supplied to such a system by evaporation on shelf areas surrounding a basin, and saturation is maintained by contact with salts on the basin floor. In this manner layered solutions persist for geologically significant spans of time, in spite of diffusion and mixing, while carbonate deposition continues in the upper dilute waters. Episodic exposure of the lower brine to evaporation is required for evaporite deposition. Such exposure may be the result of displacement of the upper layer by wind, or may accompany long-period, high-amplitude, internal seiches. Layered systems and the effects of winds and internal seiches are demonstrable in present-day saline lakes; the processes involved are amenable to laboratory investigation.