Abstract

Anhydrite nodules in the Late Devonian Dismal Creek and Blue Ridge Members in the subsurface of Alberta, Canada, recovered from depths in excess of 3000 m, resemble sabkha nodules. However, several features are inconsistent with a sabkha origin or are consistent with both a shallow and a deep burial origin. These nodules occur in facies that represent deposition on a marine carbonate platform or ramp close to or below fair weather wave base. Almost all nodules are associated with dark, bituminous, argillaceous seams and pockets, carbonaceous halos, anastomosing bituminous veinlets, and stylolitic contacts. Petrographic, UV-fluorescence, x-ray diffraction, and pyrolysis data indicate that the dark material consists of diagenetic residues from pressure solution and concretionary cementation, and that the organic matter is mature kerogen, solidified crude oil, or a mixture of the two. Sulfur and oxygen isotope ratios of the anhydrite suggest bacterial sulfate reduction before or during nodule growth. All features taken together indicate a burial-diagenetic origin for the Dismal Creek and Blue Ridge nodules. It is suggested that gypsum or anhydrite nodules resembling those commonly formed in sabkhas can be formed during burial at depths of at least several tens to several hundreds of meters. The main criteria for recognizing a burial origin for sulfate nodules are: 1) nodules occur within, or juxtaposed to, residues of pressure solution seams, stylolites, and diagenetic argillaceous and carbonaceous pockets between diagenetic carbonate nodules; 2) nodules replace host rock, and they may crosscut the boundaries between host rock and pressure solution accumulates; 3) nodules contain inclusions that had formed during burial, e.g., authigenic pyrite, dolomite, etc.; 4) there is an absence of features indicative of breakage during mechanical compaction, e.g., broken pile-of-brick texture within the nodules; 5) anhydrite crystals in nodule margins have circum-nodular, tangential alignment; 6) compared to sabkha nodules, burial-diagenetic nodules may occur in facies that are not lagoonal to peritidal and supratidal, and that are unlikely to have been subaerially exposed in evaporitic climates.

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