The stratigraphy and stratigraphic nomenclature of the Goochland Terrane in the piedmont province of east-central Virginia

Abstract

The Goochland terrane is a structurally isolated crustal block in the eastern Piedmont of Virginia. It is composed of the previously named State Farm Gneiss, Montpelier Anorthosite, Sabot Amphibolite, and Maidens Gneiss, but also includes the Scotchtown Gneiss, Teman Gneiss, and Old Bandana Gneiss which are formally named and defined herein. The eastern part of the Goochland terrane is antiformal and cored by Mesoproterozoic rocks (the State Farm Gneiss and the Montpelier Anorthosite). These basement units are overlain by a late Neoproterozoic to early Paleozoic (Ediacaran to Early Cambrian) saprolitic, metavolcanic, and metasedimentary sequence that sequentially includes the Scotchtown Gneiss, Sabot Amphibolite and Maidens Gneiss. The western part of the terrane is synformal and includes in its core two additional units that overlie the Maidens Gneiss: the Teman Gneiss and the Old Bandana Gneiss. Based on mineralogy and zircon grain morphology, the protoliths of the Maidens, Teman, and Old Bandana gneisses were predominantly sedimentary rocks. The protoliths of the Teman Gneiss and Old Bandana Gneiss were deposited unconformably upon the protolith of the Maidens Gneiss. The eastern and western parts of the Goochland terrane are separated by the Dabneys fault, which has considerable east-side-up vertical offset and possibly also significant transverse displacement. Correlation of the upper part of the Goochland terrane (Teman and Old Bandana gneisses) with the Setters and Cockeysville gneisses in the Baltimore region suggests that the Goochland terrane was left about 135 miles (ca. 220 km) southwest of its original North American location, which was to the east of Baltimore, Maryland. This displacement was caused by the oblique collision of the eastern North American continent with the western edge of the Gondwanan craton during the later Carboniferous (Pennsylvanian) Period.