Textures of Paleozoic chert and novaculite in the Ouachita Mountains of Arkansas and Oklahoma and their geological significance

Abstract

Scanning electron micrographs of cherts and novaculites from the Ouachita Mountain fold belt of Arkansas and Oklahoma show a sequential range in textures from cryptocrystalline, anhedral quartz < 1 µm in diameter in the nonmetamorphosed chert and novaculite, to coarse, polygonal triple-point, euhedral quartz 100 µm or more in diameter where the cherts have been thermally metamorphosed, the coarsest being in xenoliths. The textures have been correlated with similar texture and crystal sizes in the chert from a contact metamorphic aureole on the Isle of Skye, Scotland, where classic metamorphic mineral assemblages from talc through tremolite, diopside, and forsterite grades have been identified. Texture of chert thus can be used as an indicator of elevated temperature. Mean apparent crystal diameters measured of the quartz in the Arkansas Novaculite and associated cherts were plotted on a map of the Ouachita Mountain fold belt extending from Little Rock, Arkansas, to near Broken Bow, Oklahoma. The regional trend in texture parallels the structural core trend and, in addition, is strongly overprinted by localized areas of coarser crystallinity near Little Rock, Magnet Cove, and Potash Sulphur Springs, Arkansas, where igneous intrusions have occurred, and near Broken Bow, Oklahoma. Temperature estimates from studies of fluid inclusions, stable isotope ratios, mineral-chemical phase relationships in associated rocks, and novaculite as xenoliths suggest that maximum temperatures as high as 760 °C may have been reached by portions of the novaculite. Other examples of triple-point texture in chert collected from distant localities, which range in age from Precambrian to Tertiary, show that polygonal triple-point texture in chert is world-wide in occurrence. It follows that small samples of chert-novaculite can yield evidence of a history of elevated temperature. Such evidence may be used to estimate maturation or degradation of hydrocarbons in the rocks and to furnish clues during exploration for thermally related metallic and nonmetallic minerals.