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X-ray diffraction analysis of Upper Mississippian-Lower Pennsylvanian shales from the Ardmore and Anadarko basins indicates that the Meramecian Caney shales are characterized by the abundance of illite and the scarcity of chlorite and kaolinite, the Chester shales by the presence of illite and montmorillonite, the Springer shales by the presence of montmorillonite and the absence of illite, the Lower Pennsylvanian Morrow shales by an abundant illite content, and the Atoka shales by the relative abundance of mixed-layer clay.
The Chester and Springer formations, which can be divided into three geographic provinces, are overlain by a 200±-foot "mixed" interval which consists of a mechanical mixture of the Morrow and underlying formation clays. The type of reworked Springer clay in the "mixed" interval can be used to identify the geographic province of the underlying Springer section. Both the Morrow and Atoka shales contain distinctive interfingering beds of reworked Springer shales which appear to be restricted to wells situated near the downthrown sides of faults.
In the Ouachita geosyncline the Upper Mississippian Stanley shale is characterized by the presence of illite, chlorite, and feldspar, the Lower Pennsylvanian Jackfork and Johns Valley shales by the presence of mixed-layer clay and the relative scarcity of kaolinite and chlorite. Near the northern margin of the Ouachita area the lower Stanley shale is similar in composition to the Caney and Springer shales, indicating a transition between the two basins.
The Stanley illite-chlorite shales and high-rank graywacke sandstones were deposited in a rapidly sinking eugeosyncline. The sediments were derived from a metamorphic-acid igneous complex on the south. The closely related Chester and Springer montmorillonitic shales and orthoquartzite-quartzose graywacke sandstones were deposited in a rapidly sinking intracratonic basin. They were probably derived from a broad, low-lying sedimentary-volcanic source north and south of the basin. The southern source area may have extended toward the east and partly separated the two depositional areas.
The Pennsylvanian (Morrow) was ushered in by a major uplift and metamorphism in the southern part of the Ouachita eugeosyncline, and minor faulting along the southern flank of the Anadarko and Ardmore basins. The micaceous Jackfork shales (Ouachita basin) and orthoquartzite sandstones were derived from this Ouachita uplift. The micaceous Morrow shales (Ardmore and Anadarko basins) were derived in part from the Ouachita area and in part from the periodically uplifted montmorillonitic sediments along the south flank of the basins. At the beginning of Atokan time there was increased uplift along the south flank of the basins (the Wichita Mountains and Criner Hills) and as a result the Atokan shales are more montmorillonitic than the Morrow shales, and the sandstones are typical low-rank raywackes.
Regionally the clay mineral suites in the Arkansas and Illinois basins and in the north Texas area can be related to those in Oklahoma.
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