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Abstract

A thick sequence of Paleozoic sedimentary and very low-grade metamorphic rocks of geosynclinal facies crops out in the Ouachita Mountains of eastern Oklahoma and western Arkansas. Similar rocks crop out also in the Marathon and Solitario Uplifts of Trans-Pecos Texas. Data obtained from numerous well borings indicate that these outcrops are merely exposed minor segments of a long sinuous geosynclincal structural belt that stretches from eastern Arkansas into Mexico and is now buried beneath younger sediments. The term ‘Ouachita facies’ is applied to both the rocks of the outcrop areas and the buried rocks of the structural belt.

In periods of active downwarping of the geosyncline, when large amounts of elastic detritus were available, the typical deposits in the Ouachita geosyncline were poorly sorted subgraywacke sandstone and impure clay shale. During periods of relative quiescence, when little clastic material was being supplied to the geosyncline, the sedimentary deposits were largely argillaceous limestone, siliceous shale, siliceous limestone, bedded chert, and novaculite.

In the Marathon Uplift the major siliceous units are the Fort Peña Chert (Ordovician), Maravillas Chert (Ordovician), and Caballos Novaculite (Devonian-Mississippian). In the Ouachita Mountains the major siliceous units are the Bigfork Chert (Ordovician) and the Arkansas Novaculite (Devonian-Mississippi.).

It has been suggested by some authors that the high silica content of some of the cherts and novaculites is an epigenetic feature resulting from a combination of surface exposure and from alteration of originally non-cherty sediments during the orogeny in which the Ouachita geosyncline was compressed, deformed, and uplifted. However, the cherts and novaculities maintain similar lithologic and faunal characteristics in wells throughout the entire Ouachita structural belt, although most of these rocks never have been exposed at the surface.

The majority of the silica in the cherts and novaculites of Ouachita facies probably was supplied by extrusive vulcanism and submarine vveathering of volcanic ash. There has been some epigenetic redistribution of silica in the cherts and novaculites which crop out in the Ouachita Mountains and Marathon Uplift, but this redistribution is considered to be relatively unimportant.

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