LONG-AXIS ORIENTATION OF BRECCIA CLASTS AT THE KILMICHAEL STRUCTURE, MISSISSIPPI

The Kilmichael structure, first identified around 1930, is a roughly circular feature eight to ten kilometers in diameter located in north-central Mississippi. The structure deforms Paleocene and Eocene sediments and has been of great interest and debate among structural geologists, geophysicists and residents since its initial discovery. Theories regarding the origin of the Kilmichael structure include meteor impact, restraining bend uplift, pluton or diapir emplacement and clastic intrusion from over-pressured sediment. In our previous work, we observed small faults (normal, reverse and strike-slip), ptygmatic folds, small-scale domes (possible diapirs), clastic dikes and matrix-supported breccias.

Breccias within the Kilmichael structure are derived from soft sediments, mainly sands and clays, which are supported in a fine to medium grained clayey sand matrix. The breccia zone studied is greater than 100 meters in width with both large (up to two meters) and small (less than one centimeter) clasts. If due to shearing, this large breccia zone suggests fault displacement of hundreds of meters. Alternatively, sediments may have been brecciated and fluidized by over-pressuring. Interpreting long-axis orientation of the smaller clasts within this breccia zone should allow for distinction between these mechanisms. A shearing hypothesis predicts clasts aligned with shear zone margins, whereas a sediment-flow hypothesis predicts turbulence within the breccia zone.

Clast orientations within the breccia zone were measured by excavating adjacent vertical and horizontal faces at several locations along the outcrop. Clasts completely enclosed within the faces and larger than one and a half centimeters were measured. In general, orientations of clasts are not parallel to breccia margins. Abrupt alignment domain boundaries have been found within the breccia; for example, small clasts near large clasts change alignment becoming parallel to the large clasts. These abrupt boundaries suggest turbulence and support a sediment-flow hypothesis; however, a component of associated shearing is not conclusively ruled out.