The origins of most secondary dolomites are difficult to determine. Currently, many secondary dolomites are being interpreted as resulting from the mixing of fresh and marine waters in the phreatic zone (Dorag model), although no situations are known from the Holocene and Pleistocene, where widespread and complete dolomitization has occurred.

In some sequences, coarse, well-crystallized dolomites are the last significant diagenetic event to have occurred, postdating the main stages of cementation and lithification.

In areas where there is no evidence of evaporites of supratidal dolomites, and the geologic and diagenetic histories have been worked out in detail (as for some isolated Devonian reef complexes in Alberta), the following evidence supports an origin from compacting subsurface brines: (1) late-stage formation of dolomites and their transection of earlier burial cements and stylolites; (2) insufficient subaerial exposure during deposition and early burial for extensive Dorag-type dolomitization; (3) geochemical and isotopic data; and (4) burial by relatively impervious calcareous clays, preventing

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downward percolation of fresh waters during periods of exposure and erosion in younger, overlying strata.

The transection of stylolites indicates that dolomitization took place at moderate depths of burial, from 500 to 1,000 m or greater.

The only fluids available for dolomitization during intermediate burial were subsurface brines released from adjacent and underlying compacting strata, a model first proposed by L. V. Illing. Detailed information on the diagenetic and geologic histories is needed before the origin of secondary dolomites can be interpreted widely.

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