Potomac Terrane Focus Area
The Maryland Geological Survey (MGS) is working in cooperation with the U.S. Geological Survey (USGS), Towson University and Johns Hopkins University on an Earth Mapping Resources Initiative (EMRI) study on mineralization in the Potomac terrane, which is part of Maryland’s central Piedmont physiographic province. The Potomac terrane is a 100-km long, 20-km wide zone of metasedimentary rocks with included mafic-ultramafic blocks that trends NE across central Maryland. Ultramafic bodies in this region were historically mined for chromite, talc, and magnesite.
Geologic mapping, geochemical analysis, and study of newly acquired aeromagnetic data across the Potomac terrane will be used to study the relations of mafic-ultramafic bodies and the rocks that surround them.
New and revised geologic mapping of four 7.5-minute Potomac terrane quadrangles is ongoing (Rockville, Finksburg, Reisterstown, and Delta) and aims to resolve confusion arising from naming differences in similar metasedimentary rock units along its length. Detailed mapping, geochemical and petrographic analysis, and drill core at depth of the mafic-ultramafic bodies will provide a basis for understanding the distribution, composition, and tectonic origin of these bodies. Geochemical data for the Potomac terrane focus area will be collected and analyzed by the U.S. Geological Survey for chromium, cobalt, zinc, nickel, magnesium, and titanium, and copper and iron resources.
The two largest ultramafic bodies in the central/southern portion of the Potomac terrane, the Soldiers Delight Ultramafite and Rockville quarry, have been sampled and analyzed for whole-rock and trace element geochemistry in cooperation with Towson University. Both are serpentinized peridotite with disseminated chromite and magnetite (Knopf and Jonas, 1929; Larrabee, 1961), but differ in three macroscopic ways: Soldiers Delight contains podiform chromite, the Rockville Quarry is 10-20% rodingized gabbroic dikes, and they lie within different metasedimentary units. Their rare earth element (REE) and trace element compositions also differ. The Soldiers Delight samples are strongly depleted, a geochemical signature uniquely characteristic of ocean crust remnant mantle rocks. Rockville Quarry REE and trace element content is an order of magnitude higher, possibly reflecting more complete serpentinization, and leaves broader possibilities for its origin as ocean crust or continental intrusion.
Guice, G.L., Ackerson, M.R., Holder, R.M., George, F.R., Browning-Hanson, J.F., Burgess, J.L., Foustoukos, D.I., Becker, N.A., Nelson, W.R., and Viete, D.R., 2021, Suprasubduction zone ophiolite fragments in the central Appalachian orogen: Evidence for mantle and Moho in the Baltimore Mafic Complex (Maryland, USA): Geosphere, v. 17, no. X, p. 1–21.
Knopf, E.B., and Jonas, A.I., 1929, Geology of the crystalline rocks of Baltimore County: Maryland Geological Survey, Baltimore County Report, p. 97-199.
Larrabee, D., 1969, Serpentinite and rodingite in the Hunting Hill quarry, Montgomery County, Maryland, U.S. Geological Survey Bulltein 1283, 34 p.
Contact Information
Christopher P. Williams, Program Chief, Coastal and Environmental
(410) 554-5559 | christopherp.williams@maryland.gov
Rebecca Kavage Adams, Geologist
(410) 554-5553 | rebecca.adams@maryland.gov