THE PULASKI FAULT IN THE PULASKI QUADRANGLE: 100 YEARS OF COLLABORATION BETWEEN USGS, VIRGINIA GEOLOGICAL SURVEY AND VIRGINIA TECH GEOLOGY DEPT

The Virginia Division of Geology and Mineral Resources (VDGMR) has completed the geologic map of the Pulaski, Virginia 7.5 min quad. under a U.S Geological Survey (USGS) StateMap agreement. During the last 100 years, successful collaboration of VDGMR, USGS, and Virginia Tech Geology Dept. (VT) has produced a variety of interesting interpretations of the geologic structures of this area. In the 1920s, Marius Campbell (USGS), and Roy Holden (Dept. Head, VT) worked in the greater Pulaski area. Their report, published by VDGMR, included a map and cross section that outlined a folded thrust sheet, which Campbell named the Pulaski overthrust (Campbell and others, 1925). In addition, they described an anomalous belt of rocks on Draper Mtn., adjacent to the town of Pulaski, as “the most nearly incomprehensible fault of this region”. They were referring to a thin belt of Cambrian Elbrook Dolomite bounded top and bottom by faulted Devonian age rocks. Earlier, Campbell (1893) had described these Elbrook outliers as islands emergent in a Devonian sea. In 1939, the VDGMR published Byron Cooper’s Ph.D. dissertation on the geology of Draper Mountain. Cooper posited that the Elbrook is a sliver of the Pulaski thrust sheet trapped during emplacement of the Clayton Hollow fault (Cooper’s name), a southeast dipping, high-angle reverse fault, that post-dated Pulaski thrusting. Over the next 20 years, Cooper, Dept. Head for VT Geol., published various interpretations of the Clayton Hollow fault. In the early 1980s, Art Schultz, funded by VDGMR, with guidance from Wallace Lowry (VT), and Jerry Barthomew (then with VDGMR) was part of a team that mapped the Pulaski fault in the greater Blacksburg area. Schultz (1988) pushed the naming envelope by describing “horses in schurflingsfensters” for complexly deformed tectonic slices exposed in windows of the Pulaski thrust sheet. Not to be outdone, Bartholomew (2010) devised an entirely new structural nomenclature for these rocks. Today, the Clayton Hollow fault on Draper Mtn., with its adjacent sliver of Elbrook Dolomite, has become an eastward dipping floor thrust of the Draper Mtn. allochthonous “orphan”, part of the “Radford orphanage” group. Perhaps a future EdMap student at VT working with VDGMR staff might once again reinterpret this “incomprehensible fault”.