Abstract

After six years of studying the provenance of sands on eastern and southern U.S. shelves, we conclude that sand provenance patterns observed on these shelves reflect the late Pleistocene paleogeography of the shelves, and have for the most part been undisturbed by modern shelf currents. Three examples are offered to illustrate this idea. The first deals with the provenance of sand on the southern New England shelf. It will be shown that the distribution of glacial sands on this shelf coincides with the location of the late Pleistocene glacial deposits, while the distribution of fluvial sands follows the traces of relict stream systems. The second example comes from the South Texas shelf, where two sand types, mature multicyclic sands and immature first cycle sands, are found in modern nearshore and ancient shelf deposits. The sands of the modern Guadalupe River are dominated by the mature sand type while those of the modern Rio Grande and Colorado are dominated by the immature sand type. During the Pleistocene, these rivers deposited sand in what are now relict deltas and river valleys on the outer shelf. The sands overlying the relict Colorado and Rio Grande deltas are, like their modern counterparts, enriched in first-cycle sands, whereas those overlying the ancestral Guadalupe River valley are enriched in multicyclic sands. The last example is taken from the Georgia Bight, where two sand types, highly reworked coastal plain sands and less reworked fluvial sands derived from the Piedmont, are found in the shelf sediments. The Piedmont sands are found in abundance in a series of coast-perpendicular strips, which run from the shore to the shelf edge and which coincide with the trends of Pleistocene stream systems that crossed this shelf during lowstands. These three studies illustrate the point that the patterns of sand provenance types on the eastern U.S. and northern Gulf of Mexico shelves are mostly controlled by the late Pleistocene paleogeography of these shelves and have undergone little net movement, which would have obscured these patterns. The preservation of such well defined provenance patterns seems to contradict physical measurements of bottom currents, suggesting major transport of sand on these shelves.