Comments on Geologic Parameters
16 Rock Mineralogy:
Several authors presented descriptions of the mineral composition of the lower Potomac interval (Anderson, 1948; Kasabach and Scudder, 1961; Gill and others, 1963; Maher and Applin, 1971; Trapp and others, 1984; Benson and others, 1985). Review of these reports indicates that the mineralogy is quite variable: Anderson (1948), Hansen (1984), and Trapp and others (1984) reported that feldspar comprises between 20 and 50 percent of the light minerals, but Gill and others (1963) and Benson and others (1985) reported that the Potomac Group sands are quartzose with very little feldspar. The lithologic descriptions of Gill and others (1963) indicate high percentages of calcite and shell fragments, but those of Anderson (1948), Trapp and others (1984), and Benson and others (1985) indicate low to moderate amounts of calcite and shell. Most reported lignite and/or glauconite. The heavy minerals typically comprise less that 1 percent (by weight) of the sand fraction and are predominantly composed of epidote, garnet, staurolite, zircon, and tourmaline. We attribute the variability in mineralogy to deposition of the sands in a variety of fluvial and deltaic environments. To typify the mineralogy, we present data from Trapp and others (1984, their table 13) in the GIS. However, the other references cited earlier provide additional information.
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Gill, H. E., Seabar, P. R., Vecchioli, J., and Anderson, H. R., 1963, Evaluation of geologic and hydrologic data from the test-drilling program at Island Beach State Park, N.J.: New Jersey Department of Conservation and Economic Development, Water Resources Circular 12, 25 p.
Hansen, H. J., 1984, Hydrogeologic characteristics of the Waste Gate Formation, a new subsurface unit of the Potomac Group underlying the eastern Delmarva Peninsula: Maryland Geological Survey Information Circular 39, 22 p.
Trapp, H., Jr., Knobel, L. L., Meisler, H., and Leahy, P. P., 1984, Test well DO-CE 88 at Cambridge, Dorchester County, Maryland: U.S. Geological Survey, Water-Supply Paper 2229, 48 p.