Isotopic composition and diagenetic history of carbonate cements in Devonian Golden Spike reef, Alberta, Canada
Extensive cementation in the margin of the Golden Spike (Upper Devonian, Alberta) reef complex is due largely to formation of carbonate cements in a marine environment. Subaerial cements are absent in the reef margin, but they occur in shallow-water reef interior facies. Middle and late burial calcite cements are less abundant but occur vertically and laterally throughout Golden Spike. The δ 13 C and δ 18 O values of selected carbonate cements and sediments, mainly from the reef margin, show: (1) a narrow range (+4.0‰ to +1.6‰) of δ 13 C values for all samples; (2) the most positive (X = +3.8‰) δ 13 C values occur in lime mudstones; (3) radiaxial calcite cements (submarine) and associated (often inter-layered) marine internal sediments have similar δ 13 C (+3.5‰ to + 1.6‰) and δ 18 O (−5.8‰ to −7.7‰) values; (4) submarine radiaxial and radial fibrous calcites (X = −6.7‰), middle burial non-ferroan calcites (X = −8.8‰), and late burial ferroan calcites (X = −13.0‰) contain progressively more negative δ 18 O values. Textural evidence (that is, cement fabrics, their distribution, superposition, and relationships with associated sediments) and isotopic compositions considered in light of the Golden Spike burial history indicate the following. Submarine cements of the reef margin apparently have not been significantly modified by meteoric waters ( 18 O, 13 C depleted) throughout their diagenetic history. Submarine cements and associated sediments have δ 13 C values similar to modern marine cements and sediments, and any diagenetic re-equilibrations occurred in a “closed system” with little or no addition of 12 C. δ 13 C values in middle and late burial stage cement are the result of calcite precipitation, probably in equilibrium with subsurface fluids. The 18 O/ 16 O ratios for marine sediments, fossils, and submarine cements are probably due to isotopic re-equilibration of originally enriched (marine) 18 O/ 16 O at elevated burial temperatures before the end of the Mississippian. On the basis of burial history, middle and late burial calcite cements are post-Mississippian to pre-Late Cretaceous; the Late Cretaceous is the most likely time of major petroleum entrapment.