--> Abstract: Origin of Replacement Dolomite from the Northwest Margin of the Southesk-Cairn Carbonate Platform, Late Devonian (Frasnian), West-Central Alberta, Canada, by M. J. Shields; #91012 (1992).

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ABSTRACT: Origin of Replacement Dolomite from the Northwest Margin of the Southesk-Cairn Carbonate Platform, Late Devonian (Frasnian), West-Central Alberta, Canada

SHIELDS, MICHAEL J., Southern Methodist University, Dallas, TX

Platform-margin carbonates of the Blue Ridge, Nisku, and Upper and Lower Leduc formations from the northwest margin of the Southesk-Cairn Carbonate Platform (Frasnian) are variably dolomitized. Replacement dolomite decreases upward and bankward in the section--a common pattern in the Devonian of western Canada. Field and petrographic evidence strongly suggest dolomitization occurred in the subsurface prior to deep burial. Within partially dolomitized units early (meteoric?) phreatic and vadose cements precede dolomitization. Furthermore, solution collapse breccias probably formed before dolomitization. Finally, replacement dolomites were succeeded by two generations of dolomite cement, which are themselves separated by stylolites.

Although replacement dolomites have different crystal forms and preserve depositional fabrics to varying degrees (obliterating through near-mimetic), there are no apparent geochemical distinctions between them. All have nearly stoichiometric compositions (>48 mole % MgCO3), nonzoned dull-red luminescence patterns, very low trace element concentrations, moderately depleted Oxygen 18 (average -4.8o/oo PDB) and Carbon 13 values similar to the limestones they replace. These data suggest either dolomitization occurred at slightly elevated temperatures or burial recrystallization has homogenized their chemistries. Significant small-scale Oxygen 18 variations within different dolomite types argue against the latter.

Dolomitization probably occurred in the shallow subsurface prior to the Late Mississippian. The Antler orogeny, which began in the Late Devonian, provided a thermal anomaly and a westward structural dip, while brines from primary evaporites (Late Devonian to Mississippian) were the most likely dolomitizing waters. These relatively dense brines moved westward through the relatively permeable carbonates. In addition to their inherent density drive, evaporitic brines have up to five times more Mg2+, higher Mg2+/Ca2+ ratios, and lower SO4= concentrations (a postulated dolomitization inhibitor) than sea water. Unlike other dolomitization models this hypothesis is consistent with the petrographic and geochemical observations cited above.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)