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PART III.
GEOLOGY OF THE BLACK AND GRAND PRAIRIES.
The formations which relate to the Black Prairie and Grand Prairie subprovinces belong to three categories, as follows:
- (1) The Azoic and Paleozoic rocks, making the basement or foundation;
- (2) the Cretaceous rocks, which are the chief formations of the Black and Grand prairies proper;
- (3) the surficial post-Cretaceous formations which in places veneer the surface.
BASEMENT ROCKS OF THE BLACK AND GRAND PRAIRIES.
The subhorizontal Cretaceous rocks which make the chief formations of the Black Prairie and Grand Prairie regions overlie an older floor or substructure of Azoic and Paleozoic rocks. The composition and structure of the latter rocks have an important bearing upon the conditions of underground water, for they are frequently penetrated by the artesian drills.
The interior edges of the subhorizontal sheets of Cretaceous rocks of the prairies under discussion everywhere rest unconformably upon and against the Paleozoic rocks. (See P1. XIV.) These older rocks of the Paleozoic floor constitute a plexus of formations ranging in age from Algonkian to Permian, inclusive; they are entirely different in stratigraphic arrangement from the overlying Cretaceous and dip at different angles and in different directions.
From Arkansas west to the Missouri, Kansas and Texas Railway near Boggy Depot, a distance of 200 miles, the rocks are Carboniferous, or older Paleozoic. From Atoka westward toward Duncan, in the Chickasaw Nation, they are granites, Silurian limestones, and Carboniferous strata. From Duncan, following the western border southward, the Cretaceous rocks overlie the Permian Red Beds as far south as Chico, Wise County, Texas. From the latter point they overlie Carboniferous rocks to Burnet County, where Cambrian and Silurian rocks again form the floor for a short distance. Near the Colorado they rest upon the Carboniferous, while west of that stream they overlap Cambrian formations and granite. The northern border region, in Indian Territory, composed of the Ouachita Mountains, has been a land area since early Mesozoic time, and probably was never completely overlapped by the Cretaceous rocks. "
See pocket maps, Pls. LXIII, LXIV.