A Test Comparison of Orientations of Interpreted Surface Drainage Lineaments and Deep-Subsurface Dipmeter Records in North-Central Louisiana

An ongoing program in the Water and Environmental Section of the Louisiana Geological Survey seeks to gather and sort deep-subsurface dipmeter records into two databases, one for depositional interpretation and the other for structural interpretation. For this purpose, dip angles of 20° or greater magnitude are treated as reflecting structural influence, and those less than 20° are treated as depositional in origin. In connection with this effort, we selected a test area in north-central Louisiana for comparison of the resultant strikes of the “structural” dipmeter entries (predominantly from 6,000+-ft depths) with the orientations of interpreted straight hydrographic segments or surface drainage lineaments. The Monroe South 30 x 60 minute quadrangle includes the greater part of the largest known array of surface drainage lineaments in the state; its western half was the area chosen for focus. The bedrock geologic units exposed there comprise Eocene formations of the Claiborne Group (Cane River, Sparta, Cook Mountain, and Cockfield).

We examined the sixteen 7.5-minute topographic quadrangle sheets covering this area for straight drainage-course trends, interpreted them at 1:24,000 scale using a length cutoff of 300 m, and summed their measured orientations as a simple (nonlength-weighted) count for each 7.5-minute quadrangle and for all the quadrangles combined. This exercise resulted in an average of ~89 interpreted surface lineaments (range 63–119) per individual 7.5-minute quadrangle, and a collective total of 1,422 lineaments for all 16 quadrangles. Rose plots of unaveraged counts prepared in 10° frequency classes for each 7.5-minute quadrangle and for the combined total data set show a very strong trend oriented NW-SE, a strong though weaker trend oriented NE-SW, and much weaker though discernible trends oriented ~N-S and ~E-W. These drainage-course trends show no statistically significant difference from the examined dipmeter subpopulation, except for the NW quadrant when the quadrants are considered separately.