--> Quantitative and Geologic Constraints for Regional Scale, Shallow-Burial Seepage Reflux Dolomitization, Late Devonian, Western Canada Sedimentary Basin, by M. J. Shields, P. A. Scholle, and P. V. Brady; #90986 (1994).

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Abstract: Quantitative and Geologic Constraints for Regional Scale, Shallow-Burial Seepage Reflux Dolomitization, Late Devonian, Western Canada Sedimentary Basin

Michael J. Shields, Peter A. Scholle, Patrick V. Brady

It is generally agreed that most Late Devonian replacement dolomites in the Western Canada Sedimentary basin formed in a burial environment after significant lithification of host strata. However, the origin of the Mg, and the hydrologic mechanism of water circulation, remain controversial. Basin-scale mass balance analysis strongly suggests that residual seawater brines, associated with Late Devonian evaporite deposits, provided the Mg; hydrologic modeling indicates that seepage reflux of brines could have provided the transport mechanism.

Basinwide distribution of comparable dolomites indicates that Mg-rich parent waters flowed laterally for tens to hundreds of kilometers. The assumption that brines were derived only from Devonian evaporites artificially constrains the duration of the modeled reflux system to 16 Ma or less. Late Devonian evaporites are composed mainly of anhydrite, indicating that residual brines had densities of between 1120 and 1200 kg/cu m and, therefore. differential heads between 9 and 17% greater than seawater. Basin-fill relationships indicate that platform-to-basin relief ranged from several tens to a few hundred meters, thus constraining the base level for reflux. Assuming that Mg supply was the limiting factor for dolomitization, a conservative volumetric flow model, based on the above constr ints, indicates that regional-scale reflux could have supplied the requisite Mg during the Late Devonian. High permeability zones, such as platform margins, fractures and karst systems, could have acted as conduits for significant volumes of brine moving hundreds of kilometers from recharge areas during this time period. Such long distance lateral flow can explain dolomitization at sites where local evidence for evaporite deposition is lacking. Seepage reflux, thus, is a viable mechanism for regional dolomitization in the Western Canada Sedimentary basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994