Clay Mineralogy and Illite Crystallinity of the Atoka Formation, Arkoma Basin, and Frontal Ouachita Mountains

Christoph Spötl1, David W. Houseknecht2 and Robert Jaques
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211
1 Present address: Münzergasse 2, 6060 Hall in Tirol, Austria.
2 Present address: U.S. Geological Survey, 915 National Center, Reston, Virginia 22092.

Abstract: Clay mineralogy (including illite crystallinity) was studied in Pennsylvanian synorogenic sediments (Atoka Formation) in the subsurface of the Arkoma Basin and the adjacent Ouachita thrust belt. Vitrinite reflectance values range from ≥0.8% at the surface up to as high as 4.7% Ro at the base of the Atoka Formation. The mineralogy of the <2 µm fraction of the mudrocks is fairly monotonous and composed of illite (<10% interstratified smectite), Fe-chlorite, kaolinite, quartz, and traces of feldspars. Kaolinite is common at shallow levels and “disappears” in most wells at a thermal maturity of 1.9–2.1% Ro, suggesting its possible use as an independent paleothermal indicator in this basin. Illite crystallinity (IC) values are fairly high (0.3–0.5° 2θ) and show little variation throughout the entire maturity range. In addition, no relation was observed between vitrinite reflectance and illite crystallinity, indicating that IC is not a useful paleothermal indicator in these rocks. Illite is almost exclusively of the 2M1 polytype, suggesting a predominantly detrital origin. Incipient metamorphic and low-grade metamorphic mudrocks in the Ouachita thrust belt to the east of the Arkoma Basin are regarded as the source rocks for the clays of the Atoka Formation. Rapid transportation and deposition by turbidity currents probably played a key role in protecting these unweathered micas from pervasive alteration in the terrestrial environment.

Key Words: Anchimetamorphism • Diagenesis • Illite • Illite crystallinity • Polytype • Provenance • Vitrinite reflectance • X-ray powder diffraction

Clays and Clay Minerals; December 1993 v. 41; no. 6; p. 745-754; DOI: 10.1346/CCMN.1993.0410614
© 1993, The Clay Minerals Society
Clay Minerals Society (www.clays.org)