GEOLOGIC MAPPING AND GEOCHRONOLOGY OF THE DABNEYS 7.5-MINUTE QUADRANGLE, GOOCHLAND TERRANE, VIRGINIA

The Dabneys 7.5-minute quadrangle was mapped as a part of the Virginia Department of Energy, Geology and Mineral Resources Program’s long-range plan to map the Richmond Metropolitan Statistical Area, funded by the USGS STATEMAP Program. This quadrangle, which includes the enigmatic Goochland lithotectonic terrane, had never been fully mapped at the 1:24,000 scale, despite its relative proximity the city of Richmond, Virginia’s capital.

The Goochland terrane is a complex area in the eastern Piedmont province of Virginia. It is bounded on the east by the Hylas zone and on the west by the Spotsylvania high-strain zone, which separates the Goochland terrane from the Chopawamsic terrane. There are three main map units (from youngest and structurally highest to oldest and structurally lowest): the Devonian Maidens Gneiss (Dmg), the late Neoproterozoic Sabot gneiss (Zsg), and the Mesoproterozoic State Farm Gneiss (Ysf).

Detailed mapping found that most exposures in the quadrangle are Maidens Gneiss (Dmg), which is primarily a biotite gneiss with accessory garnet, kyanite, or sillimanite. Variations in mafic versus felsic mineral assemblages and whole rock geochemical compositions make this unit dependably heterogeneous. The Maidens Gneiss is also chaotically folded, which introduces additional complexity when understanding internal lithologic relationships. The informal Sabot gneiss (Zsg) is used in conjunction with the Sabot Amphibolite because Zsg is composed of less than 50% of amphibolite and includes a range of gneissic compositions. There is clear delineation between the Maidens, Sabot, and State Farm gneisses on the EarthMRI airborne geophysical dataset of the Virginia and North Carolina Fall Zone. Geologic mapping newly identified the Jones Creek high-strain zone (JCHSZ), which is a steeply west-dipping fault boundary between the Maidens Gneiss and Sabot gneiss. Exposures in the JCHSZ record dextral, west-side-north shear sense. Additional mapping of the area beyond is required to understand the regional significance.

Preliminary zircon analysis of the Maidens and Sabot units provides a range of ages from ca. 420 to ca. 310 Ma. One basal metasedimentary Sabot sample is multimodal from ca. 665 Ma to ca. 300 Ma, likely from core rim-interaction and Alleghanian overprinting. Further analysis is ongoing.