--> Evidence for a Paleoaquifer from Early Diagenetic Carbonates of the Cardium Alloformation, Alberta, Canada, by G. D. Thyne and C. J. Gwinn; #90986 (1994).

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Abstract: Evidence for a Paleoaquifer from Early Diagenetic Carbonates of the Cardium Alloformation, Alberta, Canada

Geoffrey D. Thyne, Cynthia J. Gwinn

Early diagenetic cements can significantly reduce sandstone porosity. In marine sandstones initiation of carbonate cementation is often attributed to dissolution/reprecipitation of shell fragments in zones of microbial activity. However, sandstones in coastal depositional regimes may serve as paleoaquifers depending on basin geometry and hydrologic conditions. As the freshwater-seawater interface moves through these sediments, chemical conditions in the mixing zone may initiate precipitation of authigenic mineral phases, and thus, control spatial distribution of cements. An example from the Cretaceous Cardium sandstone of Canada is presented to illustrate the concept.

Characteristics of the paleowater of early diagenesis may be inferred from the isotopic evidence in the early diagenetic cements precipitated in the aquifer matrix. For this study, cores from wells in the Raven River Allomember and Carrot Creek Member of the Cardium Alloformation were sampled at closely-spaced vertical intervals. If the temperature of carbonate precipitation can be constrained by petrographic means, then the degree of dilution of the connate marine water can be estimated from the oxygen isotopic content of the siderite. Sedimentological and stratigraphic features have established an offshore, shallow marine shelf environment of deposition for the Cardium Alloformation; however, geochemical and isotopic data suggest early diagenesis in a freshwater environment. This ap arent discrepancy could be resolved by postulating early meteoric water infiltration of these nearshore sediments during stillstands or relative drops in sea level. However, there is little or no evidence of subaerial exposure; therefore, formation of an aquifer with a mobile mixing zone is proposed.

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