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The AAPG/Datapages Combined Publications Database
Even though, in any given area, shelf sands deposited during the Holocene-Pleistocene may have a common provenance, sands deposited during the latest transgression can be distinguished from sands delivered earlier, by using Fourier Grain Shape Analysis. Sands with longer residence times on the Long Island shelf have smooth abraded grain profiles whereas the youngest sand tends to be much more irregularly shaped. In the nearshore zone, the percentage of irregular grains grades rapidly from approximately 100% at the beachface to 70% at the 10 m depth. Seaward of the 10-m isobath, the proportion of irregular sand decreases at a much slower rate. This change in gradient defines a boundary between nearshore sands and more abraded sands of the middle and outer shelves where lit le onshore-offshore mixing of sediments occurs. This boundary, the littoral energy fence, also has been seen on the South Carolina shelf.
Sands on the Long Island middle and outer shelf are characterized by relatively high percentages of highly abraded sand. Samples from this zone show areas with slightly higher (80 to 100%) or lower (50 to 80%) proportions of abraded quartz. This pattern appears to be related to morphologic elements on the shelf. In contrast, the South Carolina middle and outer shelf contains broad, coast-perpendicular stripes of abraded sand alternating with stripes from 10 to 30 km wide that are strongly dominated by first-generation irregular sand. Stripes containing high proportions (over 75%) of irregular sands are interpreted to be understories of alluvial valleys of the ancestral Cooper, Santee, and Waccamaw Rivers.
These results indicate that shelf sediments preserve a
rich record of Pleistocene-Holocene events of both subaerial and subaqueous nature. In addition to clarifying the history of modern shelves, aerial patterns of quartz grain shape variation can be carried into the stratigraphic record, thereby allowing a more detailed paleogeographic-paleoenvironmental reconstruction.
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