Offset Shotholes in West Texas: Evidence for Neotectonics, Stress Relief, or Blasting Phenomenon?

Abstract

The Downie Roadcut on U. S. Highway 285, 14.6 km (9 mi) north of Sanderson in Terrell County, Texas, exposes the gently dipping Cretaceous Santa Elena Formation. The north-south trending roadcut was excavated in 1965 and was one of the first applications of the presplitting blasting technique in west Texas. The walls of the roadcut provide excellent exposures of the formation, the structural features developed within the limestone, and the scars of shotholes drilled for the presplitting blast. On the northern third of the east wall of the roadcut, some shothole traces are offset along three prominent bedding planes in an apparent updip direction. The magnitude of the offsets ranges from 2.5 cm (1 in.) to 49.3 cm (19.4 in.). Even though the area in which the roadcut is located has been seismically inactive for approximately the last 13 million years, one explanation advanced for these displacements is that they are evidence of neotectonic movement due to wrench faulting. Other mechanisms potentially responsible for the shothole offsets include post excavation stress relief and construction blasting effects. In order to determine the most plausible mechanism for shothole offset, an extensive field investigation consisting of detailed structural mapping and fracture orientation measurements was conducted, and a program to monitor possible block movements along the bedding planes is ongoing. The portion of the roadcut wall exhibiting offset shotholes was divided into separate blocks whose movements were determined by measuring the magnitude and direction of shothole displacements. An analysis of almost 6,000 fracture orientations indicated that the structural pattern exposed in the roadcut does not support the postulated wrench-faulting model. Instead, the fracture pattern typifies the regional structural grain, characterized by Precambrian basement structures reactivated during Paleozoic and Laramide deformation. The monitoring system indicates no recent block movements. The offset shotholes, which are localized on a portion of the wall that forms a thin ridge of rock between the roadcut and a steep draw immediately to the east, are believed to be due to block movements created by blasting operations conducted during construction of the roadcut.