BASEMENT-TECTONIC SIGNATURE IN THE ORIENTATION FREQUENCIES OF STREAMS IN THE WESTERN TWO-THIRDS OF LOUISIANA

Stream segments in ten hydrologic units that cover portions of the western two-thirds of Louisiana show a consistent pattern of orientation-frequency trends. The pattern, as revealed in high-resolution (1:24,000-scale) files of the National Hydrography Dataset (NHD), consists of four trends comprising two orthogonal pairs, with the dominant pair aligned with the cardinal directions and the other pair diagonal to them. This coupling of orthogonal and diagonal pairs of frequency trends is reminiscent of a pattern of lineament trends and of fracture strikes interpreted as indicative of basement-tectonic influence in other regions. A preliminary analysis of four high-resolution NHD datasets available for hydrologic units in Mississippi has shown the same essential pattern as that seen in Louisiana, except that for the smallest of these files the diagonal trends show as somewhat more northerly, which may reflect primarily its small sample size (N=313). The distribution of this pattern, extending over a continuous area from extreme southern Louisiana northward into Arkansas and spanning both growth-faulted and nongrowth-faulted areas, shows the essential autonomy of these trends from the suite of conditions conducive to growth faulting. It thus also reflects autonomy from the stress regime in the growth-faulted portion of south Louisiana through at least the latter half of the Cenozoic, and supports the case for basement influence on the formation of the stream-orientation trends.

The high-resolution NHD data for Louisiana contain, in addition to the cardinal and diagonal trends, indications of four yet-weaker sets of trends: two sets oriented near N 25° W and N 65° W, and two sets oriented in most places near N 25° E and N 65° E. These trends collectively would also comprise two orthogonal pairs of sets (N 25° W and N 65° E, and N 65° W and N 25° E), with each pair nearly but not precisely diagonal to the other (intersecting at 40° and 50° angles rather than at 45° angles). Continuing work in Louisiana will include applying multiple techniques toward the evaluation of the significance and potential validity of these apparent weak subsidiary trends.