Petrology of Potomac Group sands in Fairfax County, Virginia

The Potomac Group sands sampled in Fairfax County are dominantly microcline-rich lithic arkoses. Quartz averages 59%, microcline 25%, plagioclase less than 1% and lithic grains 16%. Most sands are texturally submature and medium grained. Microcline is somewhat more abundant in the deeper beds in the subsurface than in the shallow subsurface and outcrop sections, with quartz correspondingly more abundant at the surface. Zircon is the only abundant non-opaque heavy mineral in the outcrop section as well as in the shallow subsurface, but tourmaline, rutile and staurolite are also common. In contrast, the lower beds in the subsurface have a heavy mineral assemblage with zircon, garnet and apatite as the dominant species.

Several alternative explanations for the vertical variation in mineral assemblages from less to more stable types in the Potomac Group sands are possible: 1) removal of primary Piedmont apatite-bearing igneous and garnet-bearing metamorphic source rock by erosion or burial by overlapping Potomac Group sediments, 2) deep weathering of primary Piedmont source areas and destruction of less stable minerals (apatite and garnet) in Cretaceous time, 3) destruction of less stable minerals (apatite and garnet) near the surface and in the shallow subsurface by post-Cretaceous deep weathering and/or intrastratal solution.