Abstract

The Cambro-Ordovician Arbuckle Group in the Arbuckle Mountains, south-central Oklahoma, had a complex history of dolomitization that resulted in two different geometries of dolomite bodies: stratal dolomite of stratigraphically consistent, widespread distribution, and non-stratal dolomite of stratigraphically inconsistent, local occurrence. Stratal dolomite includes the Royer and Butterly units in the lower Arbuckle Group. Most stratal dolomite samples are coarsely crystalline and have 87 Sr/ 86 Sr ratios (0.70890-0.70919) similar to Late Cambrian limestone and coeval seawater (0.7089-0.7092). All stratal dolomite and Arbuckle limestone samples have low delta 18 O values (- 5.3 per thousand to - 11.9 per thousand ). The cross-platform distribution of stratal dolomite, coupled with dominant 87 Sr/ 86 Sr ratios similar to coeval seawater, suggests that stratal dolomite formed during early diagenesis. However, the low delta 18 O values indicate that all stratal dolomite and Arbuckle limestone samples suffered postdepositional modification, probably as the result of infiltration of meteoric water during development of the Ordovician unconformities. Nonstratal dolomite is present in two areas: the Tishomingo Anticline and the Arbuckle Anticline. In the Tishomingo Anticline area, massive bodies (> 10 km 2 ) of nonstratal dolomite are present in a paleokarst system of pre-Middle Ordovician age. Some of the nonstratal dolomite bodies were displaced by Pennsylvanian faults, whereas many dolomite bodies are related to the faults. Petrographically, nonstratal dolomite is finely to medium crystalline and has replaced recrystallized limestone. The dolomite has lower 87 Sr/ 86 Sr ratios (0.70772-0.70900; average 0.70834) and higher delta 18 O values (+0.6 per thousand to -5.8 per thousand ; average -2.9 per thousand ), relative to associated (Lower Ordovician) limestone ( 87 Sr/ 86 Sr, 0.70878-0.70898, average 0.70887; delta 18 O, -6.1 per thousand to -8.9 per thousand , average -7.4 per thousand ). The petrographic and isotopic characteristics suggest that the nonstratal dolomite probably resulted from dolomitization of recrystallized limestone by post-Early Ordovician seawater. The relation of the nonstratal dolomite bodies to Pennsylvanian faults may indicate that the dolomite originated from multiple times of seawater circulation through a paleokarst system. In the Arbuckle Anticline area, nonstratal dolomite is present as small (< 1 km 2 ) irregular bodies that are related to Pennsylvanian faults and are associated with the margins of stratal Butterly dolomite. The nonstratal dolomite, medium to coarsely crystalline and brightly luminescent, is characterized by high (radiogenic) 87 Sr/ 86 Sr ratios (0.70951-0.70974), delta 18 O) values (average -1.1 per thousand ), and Fe and Mn concentrations (average 1.32 mol % and 0.65 mol %, respectively), relative to all Arbuckle carbonates. Such compositions suggest that this type of dolomite probably originated from fluids that were derived from the adjacent basin(s) during late Paleozoic time.

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