CLAY MINERALOGY AND ILLITE CRYSTALLINITY OF THE ATOKA FORMATION, ARKOMA BASIN, AND FRONTAL OUACHITA MOUNTAINS

Clay mineralogy (including illite crystallinity) was studied in Pennsy lvanian synorogenic sediments (Atoka Formation) in the subsurface of t he Arkoma Basin and the adjacent Ouachita thrust belt. Vitrinite refle ctance values range from greater-than-or-equal-to 0.8% at the surface up to as high as 4.7% R(o) at the base of the Atoka Formation. The min eralogy of the < 2 mum fraction of the mudrocks is fairly monotonous a nd composed of illite (< 10% interstratified smectite). Fe-chlorite, k aolinite, quartz, and traces of feldspars. Kaolinite is common at shal low levels and ''disappears'' in most wells at a thermal maturity of 1 .9-2.1% R(o), suggesting its possible use as an independent paleotherm al indicator in this basin. Illite crystallinity (IC) values are fairl y high (0.3-0.5-degrees 2theta) and show little variation throughout t he entire maturity range. In addition, no relation was observed betwee n vitrinite reflectance and illite crystallinity, indicating that IC i s 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 th e Ouachita thrust belt to the east of the Arkoma Basin are regarded as the source rocks for the clays of the Atoka Formation. Rapid transpor tation and deposition by turbidity currents probably played a key role in protecting these unweathered micas from pervasive alteration in th e terrestrial environment.