Abstract

Exposure of the igneous floor of the Southern Oklahoma Aulacogen in the Arbuckle Mountains area is limited to Cambrian-aged rhyolite outcrops in the core of the Arbuckle Anticline. A deep drilling test in 1980-81 on the flank of the anticline penetrated the Washita Valley Overthrust Fault. In addition to the normal felsic suite of rhyolite, microgranite, and granite, this well encountered 9,230 ft of basalt and altered basalt as well as a section of rhyolite interbedded with early rift-fill sediments. Beneath these rocks the Washita Valley Fault was crossed and the well encountered Ordovician sediments, the original target of the test. Prior to this well, the only basalt penetrated in deep drilling in this area was interpreted as dikes and sills. Using drill cuttings and well logs the geologic section in this borehole is described. Major-oxide chemical analyses are compared with similar analyses of the Navajoe Mountain Basalt-Spillite Group in southwestern Oklahoma. Although basalt extrusion is thought to have been widespread, most of these rocks have been removed by erosion or buried by later sediments. Geophysical evidence suggests the possibility of a similar thickness of basalt to the northwest along strike with regional faulting. Basaltic volcanism appears to have played a more significant role in rift evolution than previously recognized.