BASEMENT ROCKS AND STRUCTURAL EVOLUTION OF SOUTHERN OKLAHOMA Basement rocks in the 17,000-square-mile region of southern Oklahoma crop out in the Wichita

Basement rocks in the 17,000-square-mile region of southern Oklahoma crop out in the Wichita and Arbuckle Mountains and have been studied in subsurface samples from 175 wells. The rocks are divided into two contrasting provinces, each completely different from the other in age and petrologic character. Each province has played a different role in guiding the stratigraphic and structural evolution of overlying Paleozoic strata. The older or Eastern Arbuckle Province consists dominantly of Precambrian massive granites isotopically dated at 1,050-1,350 million years. They are part of an extensive continental craton, on which the Paleozoic sediments are thin and have been only slightly disturbed, chiefly by gentle folding and block-faulting. Basement rocks of the younger or Wichita Province are sediments, flows, and intrusive igneous rocks of probable Early and Middle Cambrian age. The sediments and flows consist of graywacke, bedded chert, spihtic basalt, and rhyolite. As shown by drilling and supporting seismic data, these framework rocks accumulated to a thickness of about 20,000 feet within an elongate downwarp upon the craton. Overlying strata of Paleozoic age are 40,000 feet thick. The 60,000-foot sequence in this trough, here called the southern Oklahoma geosyncline, was strongly folded and faulted in Pennsylvanian time, the structural grain and some of the major faults having been established during the time of formation of the basement rocks. wan time and have exerted a significant effect upon the area up to the present.