ISOTOPIC EVIDENCE FOR CARBONATE CEMENTATION AND RECRYSTALLIZATION, AND FOR TECTONIC EXPULSION OF FLUIDS INTO THE WESTERN CANADA SEDIMENTARYBASIN

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 signi ficant deep-burial cementation and recrystallization, (2) calcites exp erienced more extensive geochemical alteration than did dolomites unde r deep-burial conditions, and (3) the fluids that facilitated deep-bur ial carbonate diagenesis probably were partially derived from the Rock y Mountain fold-and-thrust belt, The more extensive degree of recrysta llization of calcite is shown especially by its higher Sr-87/Sr-86 rat ios, A lesser degree of C-13 depletion in dolomites indicates that dol omite recrystallization partially coincided with hydrocarbon oxidation , Evidence supporting interpretation 3 (above) includes fractures and vugs bearing late-diagenetic calcite cements that have extremely high Sr-87/Sr-86 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 ela stic rocks in the Rocky Mountains and in the Obed platform about 100 k m into the foreland basin, The late diagenetic calcite cements also ha ve highly depleted delta(13)C values (minimum -27.1 parts per thousand relative to PDB [Peedee belemnite]), indicating incorporation of oxid ized carbon from thermochemical sulfate reduction, The process of carb onate cementation and recrystallization in strata of the Obed platform probably occurred during deep burial (maximum 5-7 km) and was effecte d by a hot (>100 degrees 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 th at (1) the common practice of using limestones to establish marine or original geochemical baselines for stable and radiogenic isotope inter pretations must be conducted with caution, and (2) replacement burial dolostones are quite resistant to burial recrystallization. Finally, t he geochemical trace of tectonically expelled fluids may be limited to about 100 km into the foreland basin, implying that the volumes and/o r fluxes of fluids produced by tectonic expulsion are rather low.