PROFILING THE BURIED CAMBRIAN SEDIMENTARY AND BIMODAL IGNEOUS STRATIGRAPHY OF THE SOUTHERN OKLAHOMA RIFT ZONE USING BASEMENT WELL PENETRATIONS

The Southern Oklahoma rift zone forms a major, late Neoproterozoic (?) to Early Cambrian failed rift that strikes inward from the ancient continental margin across older crust of the southern mid-continent. Rifting was accompanied by bimodal basalt-rhyolite magmatism and included extensive volcanic activity. Scattered rhyolite outcrops occur in the Wichita and Arbuckle Mountains of southwestern and south-central Oklahoma. Relatively thin sheet granites injected at the contact between the volcanic pile and underlying mafic plutonic rocks are exposed in the Wichita Mountains. Previous information on basalts in the rift has been restricted to eleven scattered thin, incomplete penetrations of these rocks in the subsurface. There are no known surface basalt exposures other than dikes. Total eruptive rhyolite volumes across southern Oklahoma and extending into the Texas Panhandle in the subsurface have been estimated at 40,000km³, but basalt volume and stratigraphic position have remained poorly constrained. Most of the volcanic section along the rift has been buried by Paleozoic sediments and Pennsylvanian thrust sheets.

In the western Arbuckle Mountains, the volcanic rocks were thrust over younger, oil-bearing sediments during Pennsylvanian inversion of the rift along a reactivated Cambrian normal fault, which originally formed the north boundary of the rift. Extending for ~40km northwest of the rhyolite outcrops in the Arbuckles, 42 wells have penetrated the overthrust igneous rocks in the subsurface, with drilled thicknesses of igneous section ranging from 457m to 4.3km. Although well cuttings are inherently low resolution compared with surface mapping, the use of deep drilling data to characterize the igneous stratigraphy of the buried volcanic section has revealed igneous and sedimentary sections which are not present in the surface exposures. These include voluminous basalts intercalated with rhyolites, significant pyroclastic deposits of both basaltic and rhyolitic compositions, a thick clastic section representing a fan-delta deposit, and a 1.38km penetration of a yet to be investigated epizonal granite section. These features are discussed as a work-in-progress analysis of basement well data in conjunction with surface mapping of nearby outcrops.