Abstract

Mississippian rocks in north-central Oklahoma were deposited on a ramp-shelf system that trended along an approximate northeast–southwest strike and that deepened to the southeast and southwest into the Arkoma and Anadarko basins. The system is bounded on the east by the Ozark uplift. Structure in this area is dominated by extensional and transverse faulting associated with the Transcontinental arch (Nemaha uplift). Shallower water (shelf) depositional settings dominate in the northern part of the study area and deepen toward the south into the Anadarko and Arkoma basins.

Sedimentary rocks on the carbonate ramp are dominated by cyclic, partially dolomitized, argillaceous mudstones interbedded with fine-grained wackestones to grainstones. Intergrain pore space is filled by bladed, isopachous, and syntaxial marine calcite cements followed by blocky calcite cements. Limestone is commonly replaced by chert with intergrain open space filled by fine crystalline quartz (chert) cement. Late diagenetic fracture, breccia, and vug (FBV) porosity are filled by calcite and less commonly, by quartz cement that displays a coarse, blocky habit.

Carbon and oxygen isotope values for limestones and replacive dolomite are consistent with precipitation from Mississippian seawater and mixed seawater–meteoric water; values for FBV-filling calcite cements indicate precipitation from evolved basinal waters. The ⁸⁷Sr/⁸⁶Sr values of calcite micrite, replacement dolomite, and fracture-filling calcite range from 0.7077 to 0.7112. The lower values are consistent with equilibration with Mississippian seawater through most of the study area. More radiogenic ⁸⁷Sr/⁸⁶Sr values for fracture-filling calcite cements in the northeast part of the study area indicate interaction with continental basement rocks or siliciclastic rocks derived from continental basement.

Two-phase (liquid plus vapor) aqueous and petroleum inclusions were observed in FBV-filling calcite and quartz cements. The aqueous inclusions have homogenization temperatures of 48°C to 156°C and salinities ranging from 0 to 25 equivalent weight % NaCl equivalent, and reflect the presence of distinct dilute and saline fluid end-members. Calculated equilibrium δ¹⁸O_water values (VSMOW) for fluids that precipitated fracture-filling calcite cements are variable, ranging from –0.3 to +14.5‰ and do not reflect a single end-member water.

Early diagenesis was dominated by seawater-involved cementation, with modification by meteoric water during sea-level low-stands. FBV-filling calcite and quartz represent a later stage of diagenesis associated with petroleum generation and migration. Formation of fractures in the Mississippian section in north-central Oklahoma likely is related to fault movement along the Nemaha ridge instigated by Ouachita tectonism during the Pennsylvanian and extending into the Permian. This timing corresponds with regional flow of saline basinal fluids associated with the orogenic activity. These fluids ascended along faults and contributed to precipitation of FBV-filling cements. Calculated δ¹⁸O_water values for calcite cement in some areas of north-central Oklahoma suggest that cement-depositing fluids approached isotopic equilibrium with the host carbonate rocks. In other areas, however, cement-depositing fluids have oxygen isotope signatures that reflect nonresident fluids whose flow was restricted to fault and fracture pathways, which did not permit isotopic equilibration with the host limestone. In particular, fracture-filling calcite veins from Osage County, with high ⁸⁷Sr/⁸⁶Sr (>0.710) and low δ¹³C values (–2.3‰ to –4.1‰), reflect fluids that retained isotopic characteristics that were derived through interaction with subjacent shale source rocks.