NEW INSIGHTS INTO THE EARLY CAMBRIAN IGNEOUS AND SEDIMENTARY HISTORY AND PRECAMBRIAN FRAMEWORK OF THE SOUTHERN OKLAHOMA AULACOGEN FROM BASEMENT WELL PENETRATIONS

The Southern Oklahoma Aulacogen (SOA) contains a bimodal large igneous province in excess of 250,000 km³ emplaced in the Neoproterozoic/Early Cambrian. Using cuttings from 21 wells that span a 35-km-long band in the Arbuckle Mountains area and drilled igneous sections ranging from 300 m to 4.8 km thick, we have described a total of 26.6 km of uncorrelated subsurface igneous section comprising volcanic rocks intruded by diabase sills and sheets of granophyric granite. Previously unrecognized is the widespread intercalation of rhyolite lavas with basaltic to andesitic lava flows. Discernible subsurface volcanic features include rhyolite weathering surfaces, formerly glassy flow margins, and basaltic phreatomagmatic sections. Single rhyolite flow units hundreds of meters thick contain tridymite needles (now inverted to quartz) and show similar textures to those observed in rhyolite surface exposures; very little felsic pyroclastic material is present. The rhyolites have highly fractionated, within-plate, A-type trace element signatures. Basaltic to andesitic lavas are tholeiitic to slightly alkaline, and mafic flow packages can be geochemically correlated between wells.

We have re-interpreted the rhyolitic section of the Frankfort Sparks well from Ham et al. (1964) to consist of a total of 350 m of rhyolite flows intercalated with 1020 m of texturally immature rhyolitic volcaniclastic rocks in a basin or apron adjacent to volcanic vents. The Pan Am #1 Jarman (Sec.19-T1N-R2W) penetrated a 496-m-thick clastic fan-delta that originated outside the SOA and was deposited onto and later covered by volcanic flows. Post-volcanic deposits derived from clastic fan deposits and resedimented closer to the aulacogen axis document a persistent negative topographic profile. We are extending these techniques to SW Oklahoma to update the basement subcrop map of Ham et al. (1964). One new finding is the presence of intercalated rhyolite and basalt flows in the Sohio #1-3 Traub well 6.8 km NNE of Bally Mtn. at depths of 3960-4880 m.

Subsurface studies have also been extended to older Precambrian metamorphic rocks flanking the SOA, including a previously unknown sequence of amphibolite and quartzo-feldspathic gneiss encountered in a core at 5465 m in Sec.23-T9N-R26W in the Frontal Fault Zone on the north side of the SOA.