Heavy Mineral Distribution in the Lower Chesapeake Bay, Virginia

Heavy minerals from 195 bottom sediment samples of the lower Chesapeake Bay, Virginia have been studied. In comparison with other areas such as the Gulf of Mexico, qualitative and qualitative mineralogical variations were subdued, because of the absence of widely divergent source areas. The effect of present-day drainage system on the mineralogical distribution patterns is not clear, largely because of the complex Pleistocene history of the study area. Seven geographic provinces were defined and treated statistically. An analysis of variance was conducted on the seven provinces. The more significant mineral species from the study area are zircon, hornblende, pyroxene, and garnet. In addition, the more significant ratios are zircon/hornblende, garnet/hornblende, and zircon + rutile + tourmaline/hornblende + pyroxene. Percentage ispleth maps were made of the more common heavy minerals found in the lower bay, and mineral ratios contour maps also were prepared. In absolute abundance, hornblende and garnet show the most variation. Hornblende ranges from 7 percent to .56 percent, whereas garnet ranges from 1 percent to 34 percent of the heavy-mineral fraction. Correlation coefficient matrices, in conjunction with the percentage isopleth maps, established certain unique mineral suites associated with each defined province. The Baymouth province has a garnet-hornblende-pyroxene suite, which is derived from influx of shelf sediments. The Northern province has a zircon-tourmaline-staurolite assemblage, which is derived from a Piedmont-Blue Ridge and Appalachian source-land. The combined river provinces have a zircon-tourmaline assemblage, which is derived from shoreline erosion of Pleistocene material and Piedmont-Blue Ridge rocks. The Eastern Shore province has a garnet-hornblende-pyroxene suite, which is derived from influx of present-day beach sediments. The Western Shore province is associated with a zircon-epidote-staurolite assemblage, which is derived from tributary effluent and shoreline erosion. Factor analysis identified two factors that account for 6J percent of the variation in the relative amounts of the , seven most common heavy minerals that were used as variables. Factor I has a grouping of zircon, hornblende, staurolite, and pyroxene; whereas, Factor II has a cluatering of tourmaline, epidote, staurolite, and garnet. The associations of Factor I-II sediments indicates that mineral stability and source have largely determined the variability within the data.