Abstract

Certain sedimentary rocks cropping out in the Ouachita Mountains of southeastern Oklahoma and western Arkansas are unusual in that they contain large amounts of silica disseminated through a relatively thick series of beds which are laterally continuous over large areas. They range in age from Ordovician to Pennsylvanian and may be divided into (1) sedimentary rocks of the main anticlinorium of the Ouachita Mountains in southeastern Oklahoma and southwestern Arkansas, and (2) sedimentary rocks exposed in the fault blocks lying between the central Ouachita Mountains and the Arbuckle Mountains and McAlester Basin. The former group consists largely of chert and siliceous shale of typical geosynclinal facies; the latter group consists largely of chert and siliceous limestone intermediate in lithology and fabric between rocks of geosynclinal facies and those of foreland facies exposed near by in the Arbuckle Mountains.

The siliceous sedimentary rocks of typical geosynclinal facies comprise the Bigfork chert (Ordovician); the Arkansas novaculite (Devonian-Mississippian); the siliceous shales of the Stanley (Mississippian), Jackfork (Pennsylvanian), and Atoka (Pennsylvanian) formations; and the Hatton tuff lentil (Mississippian). Siliceous sedimentary rocks of atypical geosynclinal facies exposed in the fault blocks are the Pine-top chert (Devonian), the Woodford chert (Devonian ?), and the Chickachoc chert (Pennsylvanian).

Although there are conspicuous lithologic differences between these siliceous sedimentary rocks of Ouachita facies, it is considered that most of the silica was supplied by extrusive vulcanism and submarine weathering of volcanic ash. Furthermore, all the siliceous sediments apparently were deposited when relatively little clastic material was being supplied to the “Ouachita geosyncline”. Subsequent to their deposition, the siliceous sediments have been modified by epigenetic redistribution of part of their silica, resulting partly from metamorphism accompanying the Ouachita orogeny and partly from surface exposure.

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