Abstract

Carbon, oxygen, and strontium isotope data for calcites and dolomites from the Devonian Obed platform in Alberta, Canada, demonstrate that (1) both limestones and dolostones of the Obed platform underwent significant deep-burial cementation and recrystallization, (2) calcites experienced more extensive geochemical alteration than did dolomites under deep-burial conditions, and (3) the fluids that facilitated deep-burial carbonate diagenesis probably were partially derived from the Rocky Mountain fold-and-thrust belt. The more extensive degree of recrystallization of calcite is shown especially by its higher 87Sr/86Sr ratios. A lesser degree of 13C depletion in dolomites indicates that dolomite recrystallization partially coincided with hydrocarbon oxidation. Evidence supporting interpretation 3 (above) includes fractures and vugs bearing late-diagenetic calcite cements that have extremely high 87Sr/86Sr ratios, including the highest ratios reported thus far for any diagenetic carbonates from western Canada (0.7252). In carbonates, values this high are found only in tectonic veins in Proterozoic clastic rocks in the Rocky Mountains and in the Obed platform about 100 km into the foreland basin. The late-diagenetic calcite cements also have highly depleted δ13C values (minimum −27.1‰ relative to PDB [Peedee belemnite]), indicating incorporation of oxidized carbon from thermochemical sulfate reduction.

The process of carbonate cementation and recrystallization in strata of the Obed platform probably occurred during deep burial (maximum 5–7 km) and was effected by a hot (>100 °C) mixture of connate brines and hydrothermal or metamorphic fluids that were expelled from the Rocky Mountain fold-and-thrust belt during the Laramide orogeny. The data also suggest that (1) the common practice of using limestones to establish marine or original geochemical baselines for stable and radiogenic isotope interpretations must be conducted with caution, and (2) replacement burial dolostones are quite resistant to burial recrystallization. Finally, the geochemical trace of tectonically expelled fluids may be limited to about 100 km into the foreland basin, implying that the volumes and/or fluxes of fluids produced by tectonic expulsion are rather low.

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