It is widely accepted that carbonates, especially where these give evidence of origin as banks or reefs, and the primary evaporites with which they are associated closely, represent two distinct 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 and chloride accumulation under conditions of restricted circulation. Examples of the application of this concept are found in the literature and in current discussions of

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the Silurian of Michigan, the Middle Devonian of Alberta and Saskatchewan, and the Permian of Germany, among several evaporite-carbonate complexes. Support for the two-stage concept derives from interpretations of physical relations, geochemistry, the ecologic requirements of contributing organisms, the relative rapidity of evaporite deposition, and the assumed time-stratigraphic significance of extrapolated biostratigraphic data.

When the carbonate-first--evaporite-later model is applied to specific cases, at both local and regional scales, there emerge patterns of thickness distribution, water depth, and paleogeography that appear bizarre in view of the epeiric setting of the deposits. Further, demonstrable evaporite-carbonate intertonguing and lateral intergradation require an explanation other than accumulation in two successive, temporally distinct phases.

Consideration of diffusion rates compared with discharge rates required for evaporite deposition leads to the development of a model in which the density stratification of the waters is such as to permit the simultaneous deposition of chlorides and biogenic carbonates, including isolated "pinnacle" reefs, in the same basin, perhaps in close juxtaposition. The concept involved is implicit in several older papers on classic evaporite occurrences. Newer data permit a restatement in more quantitative terms.

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