Abstract

Numerous discoveries of Holocene penecon-temporaneous dolomite forming mainly in the supratidal zone have prompted environmental analogies with ancient dolomites. Although most of these may be justified, overgeneralization has resulted. Many dolomites lack definitive evidence of either supratidal accumulation or supratidal dolomitization.

Detailed field and laboratory studies of three dolomitic units in New York (Little Falls, Upper Cambrian; Herkimer, Middle Silurian; and Lockport, Middle Silurian) and one in California (Lost Burro, Middle and Upper Devonian) show that the dolomite, apparently metasomatic, represents original calcium carbonate sediments of variable environmental deposition. Portions of the Little Falls and Upper Lockport Formations contain evidence (algal stromatolites, desiccation cracks, intraclasts) of tidal flat (peritidal) deposition, whereas the other occurrences (Herkimer, Lost Burro, and lower Lockport) contain evidence (bioherms, abundant open-marine fossils) of subtidal accumulation. Dolomitization is primarily diagenetic. Even in those cases which possibly represent supratidal dolomitization, the dolomite is ordered and of much coarser grain size than that in the modern analogues.

There is considerable evidence that much Holocene dolomite, as well as a substantial part of the dolomite in the geologic record, formed under evaporitic conditions, especially in contact with brines of high Mg++/Ca++ ratios. However, findings in Holocene sediments as well as the lack of indication of the initial presence of evaporites in many ancient dolomites suggest that dolomite need not be an evaporitic mineral.

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