ORIGIN OF SOAPSTONE WITHIN THE WISSAHICKON FORMATION: ANYALSES OF NATIVE AMERICAN QUARRIES ALONG THE LOWER PATUXENT RIVER, MARYLAND

In the Mid-Atlantic region, soapstone, or steatite, was quarried for bowl manufacture by early Native Americans during the Late Archaic Period, between 3000 and 5000 years ago. Along the lower Patuxent River, Howard County, Maryland, their quarrying efforts focused on predominantly talc-rich soapstone deposits, some containing over 90% talc. Petrographic and geochemical investigations at several quarry sites along the Patuxent River sought to determine the nature and origin of talc-rich soapstone, which occurs as thin layers and lenses of ultramafic rock within the Maryland Piedmont. Archeological excavations confirm that soapstone blocks here are isolated blocks surrounded by Wissahickon Formation, a Cambro-Ordovician metapelite. Soapstone petrofabrics are similar in style and sequence to those in the surrounding schist, suggesting emplacement of the soapstone blocks occurred prior to or during the regional Taconic metamorphic event.

In the soapstones, actinolite is replaced to varying degrees by talc and chlorite. In some samples, foliated talc is crenulated and magnetite crystals occur along the crenulation limbs. Chlorite is typically in either random orientations or, less often, interleaved with talc, signifying formation during retrograde metamorphism. Whole-rock major and trace element compositions of the soapstones vary considerably from outcrop to outcrop. The soapstone samples from this area are high in Mg and Fe (~ 45-80% Mg and 20-55% Fe) but very low (~3%) in alkalis (Na + K), reflecting extensive alteration.

Ultramafic bodies within the Wissahickon may derive from blocks eroded from an advancing thrust sheet of Baltimore Mafic Complex. During ensuing progressive regional metamorphism, mafic pheonocrysts likely changed to amphiboles like tremolite-actinolite, and later to talc at higher metamorphic grade. The variable degree of talc formation in the ultramafic blocks may reflect patterns of fluid flux in the rock during metamorphism, particularly along shear zones surrounding the blocks, with talc favored under relatively aqueous hydrothermal conditions. Although a specific geochemical “fingerprint” for soapstone bowls from the Mid-Atlantic has not yet been found, future work in the area may lead to more precise methods for determining artifact provenance.