Meteoric Modification Of Early Dolomite And Late Dolomitization By Basinal Fluids, Upper Arbuckle Group, Slick Hills, Southwestern Oklahoma (1)

Abstract

Thick, massive dolomite units from the upper Arbuckle (lower Ordovician) peritidal carbonate sequence, Slick Hills, southwestern Oklahoma, provide insight into two important problems: (1) modification of early dolomite, and (2) late dolomitization. Early dolomite, volumetrically dominant, is nonluminescent to dully luminescent. {87}Sr/{86}Sr ratios of most early dolomite samples are similar to both Early Ordovician seawater and associated limestones, consistent with an early origin from coeval seawater. All early dolomite samples are characterized by low delta{18}O values (-5.1 to -10.0 o/oo PDB), indicating postdepositional modification. Meteoric water, buffered by younger carbonates with lower {87}Sr/{86}Sr ratios, was probably responsible for modification because lower Sr concentrations, delta{18}O values, and {87}Sr/{86}Sr ratios characterize the more intensively modified early dolomite samples. Meteoric modification probably occurred during emergence of the carbonate platform in the Ordovician. Late dolomite, volumetrically minor, is brightly luminescent and occurs mainly in transition zones between early dolomite and associated limestone. Late dolomite has high {87}Sr/{86}Sr ratios, delta{18}O values (+5.9 to -2.5 o/oo, PDB), and Fe and Mn concentrations, relative to early dolomite and associated limestone. {87}Sr-, {18}O-, Fe-, and Mn-enriched fluids, derived from the Anadarko basin during the late Paleozoic, are proposed to have formed the late dolomite. The fluids migrated through preexisting, early dolomite aquifers, and dolomitized limestones near the early dolomite-limestone contacts which existed at that time. Although early dolomite remained more porous than limestone during early burial, burial alteration by solution-compaction and cementation by late dolomite have reduced porosity considerably. Better reservoir quality is expected to be associated with late dolomite in the Arbuckle Group.