ABSTRACT

Fisk's 1944 monograph on the lower Mississippi valley postulates a rectilinear grid of NE- and NW-striking faults traversing the Mississippi embayment. The fault interpretation was made directly from stream lineaments discerned on aerial photographs. New 7.5-minute topographic quadrangles in Louisiana confirm that the straightness, orientation, and spacing of stream courses in many parts of the state constitute highly suggestive patterns of NE- and NW-trending drainage lineaments. None can yet be linked to mapped surface faults; however, the suggestive indicators are collectively so numerous and widespread that joint control of stream courses cannot be dismissed.

One likely example of joints influencing orientations of stream courses occurs at Longleaf Vista, Red Dirt Wildlife Management Area, Kisatchie National Forest, in southern Natchitoches Parish. Multidirectional extension joints in sandstone of the Catahoula Formation include one predominant set having measured strikes in the range of N 70° E to N 90° E. Two tributaries of nearby Bayou Cypre show relatively straight courses with overall orientations of N 75° E and N 83° E, near the middle of the range of strikes measured for the joints. The N 75° E course includes eight straight segments with orientations ranging from N 50° E to N 88° E; five of these range between N 69° E and N 88° E, virtually identical to the strikes of joints.

The joints in this example are formed in the most lithified surface-stratigraphic unit in the state. If joints influence other stream courses in Louisiana, as is here considered probable, they are predominantly in less lithified stratigraphic units and may be largely concealed by alluvial, colluvial, pedologic, and vegetative cover.