A THICK SECTION OF RIFT RELATED BASALTS AND RIFT FILL SEDIMENTS IN THE ARBUCKLE MOUNTAINS, OKLAHOMA, AS REVEALED BY DEEP DRILLING

Bimodal volcanism and interbedded volcanic and rift fill sedimentary rocks are characteristics of continental rifts. Although rhyolites are present, mafic volcanic rocks and rift fill sedimentary rocks are not exposed in the outcrops of the Southern Oklahoma Aulacogen. Drilling penetration of basalt has been limited to a maximum of 300m in southwestern Oklahoma. Although basalt extrusion is thought to have been widespread, most of these rocks have apparently been removed by erosion or deeply buried by Paleozoic sedimentation and later tectonic events. This paper describes the geologic section encountered in a deep exploratory well in the Arbuckle area in south central Oklahoma that penetrated 4800m of over-thrust Cambrian igneous rocks including 2,813m of basalt and altered basalt. This section occurs in the hanging wall of the Washita Valley Fault, a Cambrian normal fault bounding the aulacogen which was re-activated as a regional thrust fault in Pennsylvanian time. Also present is a lower section of 500m of apparently interbedded rhyolite and rift fill sedimentary rocks. Although alternative structural interpretations for the lower section must be considered, the lithologic evidence strongly favors a rift fill origin. The sedimentary rocks contain a total of 155m of dolomite and lithic dolomitic conglomerate. The lithic clasts in the conglomerate were possibly derived from 1.4Ga granites north of the fault. Magnetic survey evidence suggests the possibility of a significant basalt accumulation to the northwest along strike with regional faulting.