NEW INSIGHTS INTO THE EARLY CAMBRIAN IGNEOUS AND SEDIMENTARY HISTORY OF THE SOUTHERN OKLAHOMA AULACOGEN FROM BASEMENT WELL PENETRATIONS

The Southern Oklahoma Aulacogen records intraplate magmatism related to opening of the Iapetus Ocean during breakup of Rodinia in Neoproterozoic-Early Cambrian time. Striking inward from the Laurentian margin, the rift contains over 250,000 km³ of bimodal igneous rock in southern Oklahoma and adjacent parts of Texas. Later tectonic inversion of the rift caused thrusting of thick sections of Lower Cambrian igneous rocks over younger oil-bearing strata. In the Arbuckle Mountains area of south-central Oklahoma we are using 29 drilling penetrations along a 42 km band parallel to the rift axis to gain a more complete understanding of the igneous history of the rift than available from outcrop studies alone. Thicknesses of volcanic rocks penetrated in the wells range up to 4.3 km, and the base of the volcanic section has not been reached. Epizonal granite sheets and diabase sills or dikes intrude the volcanic succession. Rhyolites in the rift are A-type lava flows in some cases exceeding 400 m in thickness. Some cuttings show relict perlitic texture, defining originally glassy flow margins. Many cuttings are felsitic and contain tridymite needles (replaced by quartz) of varying sizes, recording cooling gradients in thick flows. Cuttings containing unwelded pumice and tricuspate shards represent thin pyroclastic interbeds between flows. Numerous occurrences of lithic-rich volcaniclastic detritus derived from erosion of rhyolite lavas include one interval ~800 m thick. Previously unrecognized tholeiitic to transitional basalt and intermediate lava flow accumulations up to 2.8 km total thickness are intercalated with rhyolite flows and have an OIB geochemical signature. Mafic flow packages can be geochemically correlated between wells. Explosive basaltic phreatomagmatism is indicated by altered, lapilli- and ash-sized, variably vesicular sideromelane pyroclasts in some intervals. Intercalated within the volcanic section is a ~630-m-thick, arkosic sequence inferred to represent a clastic wedge derived from Mesoproterozoic granite on the north side of the rift. Significant granite intrusions have been drilled. In contrast to the thin (~0.5 km) sheet granites exposed in the Wichita Mountains in SW Oklahoma, one well penetrated 1.3 km into a granite body, the base of which was not reached.