USING BASEMENT WELLS TO INVESTIGATE THE SUBSURFACE CAMBRIAN BIMODAL VOLCANIC RECORD IN THE SOUTHERN OKLAHOMA AULACOGEN

The Southern Oklahoma Aulacogen 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, and previous information on basalts in the rift has come from eleven scattered thin 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 aulacogen. Extending for ~40km northwest of the rhyolite outcrops in the Arbuckles, 35 wells have penetrated the overthrust igneous rocks in the subsurface, with drilled thicknesses of igneous section ranging from 457m to 4.3km. We are using drill cuttings from these wells to characterize the igneous stratigraphy and geochemistry of the buried volcanic section. Rhyolite lava flows penetrated by these wells include single cooling units up to 100’s of m thick, which are comparable to those documented by surface mapping in the Wichita and Arbuckle Mountains. An important finding of the subsurface work is that voluminous basalts are intercalated with the rhyolites and include thick sequences of lava as well as significant amounts of basaltic pyroclastic rock. The deepest well penetrated a cumulative section of 2.2km of basalt before reaching the thrust contact with structurally underlying Paleozoic sediments. This ongoing investigation suggests a complex bimodal petrogenesis for the southeastern part of the aulacogen and a much more important role for basaltic volcanism than previously documented.