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Abstract:
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Samples from more than two hundred and twenty wells penetrating basement rock have been examined and described from a 61 ,000 square mile area in adjoining parts of Oklahoma , Kansas , Missouri , and Arkansas . Twenty -five isotopic age determinations were made on twenty samples of basement rock from wells and outcrop areas . Nine basement rock units can be defined within the study area on the basis of petrography and isotopic age . 1 . "Older granite and gneiss" is a loose association of granitic rocks considered to be older than 1400 million years . The unit cannot be precisely defined within the study area . 2 . The Lyon County Quartzite is a micaceous quartzite restricted to relatively small areas in the buried basement of Kansas . The age of deposition is unknown but preceded the 1400 million year age of metamorphism . 3 . The Chase County Granite Group is composed largely of granite and gneissic granite . Rocks of this group underlie most of the Nemaha Uplift in Kansas and also occur considerably east and west of the Uplift . The rocks were intruded at about 1400 million years , probably as a composite batholith . At about 1200 million years , four petrographically related rock units were intruded or extruded . The age difference between these units cannot be distinguished by isotopic ages . The four units are assembled into the Northeast Oklahoma Province , a petrographic grouping of volcanic rocks and chemically equivalent hypersolvus type epizone granites . 4 . The Washington County Volcanic Group is composed mostly of rhyolite but also contains andesite and metarhyolite . The majority of the rhyolites were probably extruded as welded tuffs . 5 . The Spavinaw Granite Group is composed of generally micrographic granite porphyries . The intrusions are considered to be largely sills on textural evidence and by analogy with outcrop areas elsewhere . 6 . The Woodson County Granite is texturally variable . Samples from drill holes in Kansas are petrographically identical to boulders found at Rose Dome in Woodson County , Kansas . 7 . The Osage County Microgranite is the most uniform unit in the area . The microgranite occurs in a roughly circular area in Osage County , Oklahoma . The unit was probably intruded as a sill within rhyolite flows . Two other petrographic units are interpreted as being younger than parts of the Northeast Oklahoma Province , although in the same 1200 million years isotopic age range . 8 . The Vernon County Metamorphic Group is composed mostly of low rank metamorphic rocks derived from clastic sediments . The grade of metamorphism is from incipient to middle greenschist facies . Rhyolite detritus occurs in some samples . 9 . The Central Oklahoma Granite Group is composed of two feldspar mesozone type granites . The unit is not distinguishable from the Chase County Granite Group on the basis of petrography . Uplift and erosion followed the intrusion of the Central Oklahoma Granite Group . The main uplifts were along the present Nemaha Uplift and along a northeast -southwest axis from southwest Missouri to central Oklahoma . There is no evidence for any igneous or metamorphic activity between 1200 million years and the deposition of lower Paleozoic sediments . |