Sediments of Ouachita facies extend subsurface from the Ouachita Mountains of Oklahoma and Arkansas to the Marathon and Solitario uplifts of extreme southwest Texas. Although these sediments have been subjected to strong dynamic metamorphism, the metamorphic grade attained is uniformly low. Sediments of Ouachita facies are essentially unaltered in some areas; elsewhere they range in degree of metamorphism as high as the "biotite zone."

Mineralogical change and cataclastic effects are key criteria for discerning the stages of progressive metamorphism in sediments of Ouachita facies. Metamorphic changes are most easily observed in shales and argillaceous sandstones--limestones and cherts react less conspicuously to metamorphic stress. Mineralogical change resulting from metamorphism reduces the porosity and non-fracture permeability of sandstones to such an extent that they are not good petroleum reservoirs.

The petrography of the so-called "schists" of Luling field, Caldwell County, Texas, and of some other deep wells along the Luling-Mexia-Talco fault system is discussed briefly. It is shown that they are generally comparable in mineralogy and degree of metamorphism with sediments exposed in the Ouachita Mountains of McCurtain County, Oklahoma.

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