STREAKING ACROSS THE GABBRO: THE WICHITA MOUNTAINS’ QUANAH GRANITE DIKES EXPOSED

The Wichita Mountains are largely composed of igneous rocks, products of the Eocambrian Southern Oklahoma Aulacogen. The Quanah Granite is a well-exposed pluton in the southeastern part of the mountains. This pluton produced numerous dikes found north of its margin in the Central Lowlands. Our study documents the distribution and orientation of these dikes that were previously unmapped in detail.

We plotted dike outcrops using a handheld GPS-WAAS unit and walked trends. Where possible, we recorded orientations using a Brunton pocket transit on outcrops that appeared to be robustly intact. We uploaded our waypoints into Topo North America 10.0 and annotated waypoint and track sets with dike trends. We then resolved dike widths, waypoint frequency, and response to topography. Annotated dikes were plotted on top of the U.S.G.S. Quanah Mountain 7.5’ quadrangle topographic sheet. Orientations of dikes that cut three or more contours were resolved through three-point analysis. For dikes that did not cross sufficient contours, we used the strike from a linear trend and assumed a 90° dip. We then ranked our data with one of 4 grades; in descending order these are: A. Brunton measured dikes, B. orientation resolved by 3-point solutions, C. an estimated orientation from dikes that crossed two contours or followed topography, and D. lines that failed to cross more than one contour. We uploaded the dike orientations into Stereonet (Almendinger, 2013) to graphically evaluate orientation. We constructed rose diagrams to determine prominent strike directions using two data sets: 1. our A- and B-grade data, and 2. our A-, B-, and C-grade data.

These dikes range from a few centimeters to over a meter thick. The textures vary within and among dikes, but all appear to be similar in composition. We noted that some dikes contain quartz veins or vugs. The dike-orientation data were somewhat scattered, but three principal orientations emerged from set 1: 090° 60°S, 120° 60°S, and 045° 33°. The latter was defined by three dikes of common strike, one dipping northwest, one southeast, and one vertical. These trends were noted with a 10° bin rose diagram, but similar orientations are determined by a variety of bin sizes (5°-35°). Set 2 data was more scattered but also produced our three principal orientations.