GOOCHLAND TERRANE UNVEILED: USING AIRBORNE GEOPHYSICS TO REFINE GEOLOGIC MAPPING IN CENTRAL VIRGINIA

The Goochland terrane in the central Virginia Piedmont is bounded on the west by the Spotsylvania and unnamed (Nutbush Creek?) high-strain zones (HSZ), and on the east by the Hylas HSZ. Since 2013, Virginia’s geological survey (Geology and Mineral Resources Program, Virginia Energy) has been conducting 1:24K geologic mapping, funded in part by the USGS STATEMAP program, focused on a twelve-quadrangle area covering a large part of the Goochland terrane. The USGS recently acquired airborne geophysical survey data in eastern Virginia and North Carolina that covers much of the Goochland terrane (Shah and Connell, 2022). This high-resolution dataset has enabled critical refinements to recent bedrock mapping, in an area where a thick mantle of saprolite conceals bedrock in many places:

--Aeromagnetic (Mag) and Thorium (eTh) imagery each highlight internal complexity within the State Farm Gneiss dome including the presence of two secondary domes; positive Potassium (K) anomalies highlight several Neoproterozoic plutons within the dome.

--On the east flank of the dome, the ca. 550 Ma Sabot gneiss, which includes magnetite-rich quartzofeldspathic gneiss, has a strong Mag signature and is in fault contact along the Dover Creek HSZ with the low-Mag ca. 1046–1030 Ma State Farm gneiss.

--South of the James River, also on the east flank of the dome, Mag imagery indicates the Hylas HSZ extending SSW across the area, juxtaposing Sabot gneiss, on the west, against the Mosely felsic gneiss (uncertain age and affinity) on the east.

--West of the State Farm dome the ca. 403–385 Ma Maidens biotite gneiss has strong eTh and Uranium (eU) signatures relative to the Sabot gneiss to the east; field mapping coupled with geophysical signatures indicates Maidens and Sabot are in fault contact across the Jones Creek HSZ.

--Preliminary mapping indicates strongly negative eTh and eU anomalies SW of the Maidens outcrop belt coincide with interlayered biotite gneiss and granitic rocks of the Trenholm terrane, age and affinity uncertain, which are in fault contact with Maidens gneiss of the Goochland Terrane along the Nutbush Creek(?) HSZ.

--Mag imagery shows numerous NNW-trending Mesozoic diabase dikes in the western portion of the map area.

--K imagery highlights extensive terrace deposits in several areas on top of bluffs north and south of the James River.