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Brent E. Owens

Publications and source records attributed to Brent E. Owens.

5 recordsLinked to original sources
USGS2025

Pocoshock Creek Gneiss and redefinition of the Petersburg Granite, central-eastern piedmont of Virginia

The Petersburg Granite (sensu lato) was formerly interpreted to be a single late Paleozoic composite batholith in the central-eastern part of the Piedmont Physiographic Province of Virginia. Detailed mapping beginning in the mid 2000s resulted in the subdivision of these voluminous granitic rocks into five units—layered granitic gneiss, foliated granite, porphyritic granite, subidiomorphic (equigranular) granite, and pegmatitic granite. U-Pb zircon geochronology demonstrate that foliated granite and layered granitic gneiss currently assigned to the batholith are 100 million years (m.y.) older, and yield Silurian to Devonian ages (ca. 425 mega-annum (Ma) to ca. 403 Ma). We propose the name Pocoshock Creek Gneiss for these strongly foliated and layered gneisses. Geochronologic data also require redefinition of the Petersburg Granite (sensu stricto). Massive equigranular and porphyritic rock types of the Petersburg Granite (sensu stricto) are geochemically distinct from a subset of Pocoshock Creek Gneiss samples. Massive equigranular and porphyritic rock types are similar to another, more granitic subset of the Pocoshock Creek Gneiss, which indicates different magma sources for the Petersburg Granite (sensu stricto) and Pocoshock Creek Gneiss but also likely indicates assimilation of Pocoshock Creek Gneiss during late Paleozoic intrusion of these Petersburg Granite (sensu stricto) rock types. Equigranular granite southwest of Petersburg, Virginia is 15–20 m.y. older than porphyritic granite, massive granite, and monzodiorite (ca 300 Ma) near Richmond, Virginia. The Petersburg Granite (sensu stricto) consists of two regional plutons—the northern Richmond pluton and the southern DeWitt-Sutherland pluton. The DeWitt-Sutherland pluton of Early Pennsylvanian age is separated from Late Pennsylvanian to early Permian granite of the Richmond pluton by a map scale septum of Pocoshock Creek Gneiss. Geochemical analyses of 15 samples of Pocoshock Creek Gneiss are compared to other igneous suites throughout the Appalachian orogen. Whole rock geochemistry shows elevated average thorium and uranium values in porphyritic granite compared to massive equigranular granite and Pocoshock Creek Gneiss and suggest that homes and businesses built on that rock type of the Richmond pluton may be most at risk for uranium and (or) dissolved radon in groundwater or radon gas in confined spaces.

Professional Paper↗
USGS2023

Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA

Field relations as well as geochemical and petrologic studies of metaigneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.

Virginia↗
USGS2021

Stop 3 – The Petersburg “Granite” redefined: Recognition and implications of Silurian to Devonian rocks in central-eastern Virginia

Introduction Although the Petersburg Granite had long been in practical use as a building stone since the 1830s (Watson, 1906; 1907; 1910; Darton, 1911; Steidtmann, 1945), it was first formally defined as a geologic unit by Anna Jonas on the 1928 geologic map of Virginia. Anna Jonas defined this unit as a Precambrian coarse-grained porphyritic biotite granite that was intruded by finer grained granite and cut by pegmatite (Nelson, 1928). This belt of mostly granitic rocks extends from near Ashland, Virginia north of Richmond, to near Stony Creek, south of Petersburg, Virginia (e.g., Virginia Division of Mineral Resources, 1993) and is bounded by the Hylas fault zone to the northwest, the Mesozoic Richmond basin to the west, and the newly recognized Nottoway River fault zone to the southwest (e.g., Carter and others, 2020; 2021). The eastern boundary of this belt is covered by Coastal Plain sediments, but geophysical and deep borehole data suggest an orogen-scale suture separates it from the Neoproterozoic Chesapeake block to the east (Figure 1; Carter and others, 2021).

Virginia↗
USGS2017

Geology of the Petersburg batholith, eastern Piedmont, Virginia

The 295-300 Ma Petersburg batholith in east-central Virginia forms one of the largest and northernmost of the Alleghanian plutonic complexes in the southern Appalachian Piedmont. The batholith is primarily composed of granite including massive and foliated (both magmatic and solid-state fabrics) varieties. The plutonic complex intruded medium-grade metamorphosed volcanic/plutonic rocks of the Roanoke Rapids terrane. The western edge of the batholith experienced right lateral transpressional deformation associated with movement on the Hylas fault zone during the Alleghanian orogeny; this was followed by normal faulting and exhumation during the development of the Triassic Richmond basin. Much of the batholith was buried by a thin veneer of primarily Cenozoic siliciclastic sediments at the western edge of the Atlantic Coastal Plain. Granite rocks of the Petersburg batholith have long been quarried for both dimension and crushed stone. The purpose of this trip is to discuss the age, origin, and tectonic significance of the Petersburg batholith.

Virginia↗