Abstract

Ferroan dolomite and calcite cement fossiliferous sandstone unite in the Choptank and Calvert Formations (Miocene) of western Maryland and northern Virginia. The average composition of the dolomite is Ca (sub .55) Mg (sub .43) Fe (sub .02) CO (sub 3') , though nonferroan dolomite is also present. The dolomite is generally euhedral and zoned, and it may have fluid-inclusion-rich cores. Calcite cements are dominantly composed of equant spar, though a fine, acicular, isopachous rim cement may be present. Calvert Formation dolomite is chemically similar to Choptank Formation dolomite, though the former contains both calcite inclusions and hollow centers. A relatively simple burial history for ferroan dolomite and calcite cement, coupled with petrographic and isotopic evidence, indicates that some of the dolomite has a marine origin, possibly under the influence of sulfate-reducing organisms. Later recrystallization may have occurred under the influence of alternating marine and meteoric-phreatic conditions. Zoned dolomite rhombs, dolomite with calcite cores, and clear dolomite rims indicate episodic crystallization, possibly due to multiple periods of phreatic conditions. Most of the calcite cement is demonstrably meteoric in origin. Dolomite cement is generally confined to permeable sandy intervals bounded on the top and bottom by clays of low permeability. When buried, this sequence would be similar to dolomite sequences inferred to be of deep burial in origin. The timing of dolomite formation in Chesapeake Bay sediments is significant in that most of the dolomite has derived its magnesium from sources other than the clays, as has been suggested in deep-burial dolomitization models.

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