Texture of Arkansas Novaculite Indicates Thermally Induced Metamorphism

Abstract

ABSTRACT Arkansas in the Hot Springs-Little Rock area exhibits the texture, in scan electron micrographs, of a thermally metamorphosed rock formed ostensibly from cryptocrystalline chert. The stratigraphic formation, named Arkansas Novaculite, grades from a metamorphic rock in central Arkansas to typical cryptocrystalline chert at Atoka, Oklahoma, 250 air-km west. The Bigfork chert behaves similarly. It appears desirable to restrict the term novaculite to a petrologic name for a thermally metamorphosed chert which shows polygonal, triple-point texture; Arkansas is the type occurrence. Quartz crystals 60 µm in diameter yielding relatively very coarse texture are developed in within 100 m of Mesozoic intrusives, e.g., at Magnet Cove. Coarse texture, crystals 10-20 µm in diameter, is widespread at Hot Springs and Little Rock quarries. Fine texture (metamorphic) which ranges down to 1 to 2 µm, occurs in the western part of Trap Mountain, 12 air-km southwest of Hot Springs. Similar texture occurs in the Broken Bow, McCurtain County, Oklahoma, region. Incipient-metamorphic texture appears in the Potato Hills region, Oklahoma, but at Atoka, Arkansas novaculite is cryptocrystalline chert. Thermal metamorphism of chert from vulcanism yields novaculite. In Ouachita locales of intense structural deformation coarse-textured is developed. Whether deformation alone generates adequate heat to produce is uncertain. Novaculite may be a geologic clue to a hidden intrusion, a thermal ore deposit (e.g., the vanadium in Arkansas), or a source of geothermal energy.