ABSTRACT

Previous investigations dealing with the geology of the inner Choctaw Anticlinorium have been contradictory and generally unsatisfactory, and have usually interpreted the area as very gently deformed. This study attempts to provide an accurate map of lithologic distributions and a more realistic structural interpretation.

Map units were defined on the basis of lithologic contrasts. Shale is the most common rock type in the area, with less common occurrences of sandstone and limestone. Samples of the three map units were examined, in hand specimen and thin section, but no significant differences between samples of similar lithology (that is, belonging to the same map unit) could be observed. Structural data such as orientations of fold hinges and cleavage and bedding surfaces were collected where possible. The major structural elements found in the map area are numerous small, disharmonic, isoclinal recumbent folds, and prominent slaty cleavage at consistently low angles that tends to obscure bedding in shales.

Field observations fail to demonstrate large scale folding or high angle faulting, but do indicate that the area has been subjected to severe deformation with a pronounced low angle component.

The distribution of rock types, the evidence of intense deformation, and the lack of evidence for large scale folding and/or high angle faulting suggest that a mechanism involving low-angle thrust faulting is responsible for the features observed in the map area. Such a mechanism of thrust faulting, combined with the lithologic types common in the map area, suggest that the pre-Bigfork Chert rocks of the inner Choctaw Anticlinorium acted as a major decollement zone, deforming in response to loading and subsequent gravity sliding of the entire sequence. Intense deformation accompanied by large accumulations of strain occurred in the pre-Bigfork rocks, while the younger strata were transported with relatively little deformation.