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Atlantic Continental Shelf and Slope of the United States - Gravels of the northeastern part
| Content Provider | Semantic Scholar |
|---|---|
| Author | Schlee, John Stevens Pratt, Richard M. |
| Copyright Year | 1970 |
| Abstract | Gravel is concentrated mainly on the glaciated part of the continental margin-the Gulf of Maine, Scotian Shelf, and northern part of Georges Bank. Most coarse detritus in the Gulf of Maine is exposed on ledges and shallow banks as well as on the hummocky topography between basins. It is a very poorly sorted mixture of gravel, sand, silt, and clay. The fragments are subrounded to .angular, and some gravels have multimodal grain-size distribuition. Rock types are varied, and the detritus seems t.o be derived fl'lom local bedl'lock. On Georges Bank and Nantucket Shoals, the gravel is better 'Sorted, more quartzose, and better rounded. It is associated with sand waves afld tidal ridges in both areas. Gravel is both coarse and abundant in the exit channels that lead s?award from the Gulf ·of ::\Iaine across the continental shelf; the abundance of gravel in these channels indicates that they were occupied by lobes of glacial ice during the Pleistocene. Gravel on the Scotian Shelf resembles that of both the Gulf of Maine and Georges Bank in that well-sorted sanely gravel is closely associated with till-like mixtures of gravel and finer sediment. As in the Gulf of Maine, basin sediment is silty clay or clayey silt. Echosounding records suggest that these fine-grained sediments mask the gravel and were deposited during the Holocene rise in sea level. Scattered occurrences of gravel are found on the continent slope as far ~outh as Hudson Canyon. The gravel fraction on the slope is a minor part of the sediment (most is silt and clay) and shows a wide range in size and roundness. On the nonglaciated shelf south of Xew England and Long Island, gravel is distributed sporadic-ally; largest concentrations are associated with the drowne!d Hudson ChannE'l east of New .Jersey. The gravel is moderately sorted quartzose, and commonly in a bimodal grain-size distribution with sand. Interpretation of the areal distribution and properties of gravel allows us to (1) infer the bedrock geology for most of the Gulf of :Maine and Scotian Shelf, (2) fix the approxima.te limits of glaciation on the continental shelf, and (3) list the agents that dispersed the gravel. Sedimentary rock of probable Triassic age contributed detritus to much of the northeastern Gulf of Maine and Bay of Fundy and probably underlies these areas. Sedimentary rock ,of Cretaceous and younger :age was a ~ource for rock fragments in the southern Gulf and some of the "vein" quartz pebbles so abundant on Georges Bank and NantuckE't Shoals. Granite and felsi,te clasts are abundant off 1 Contribution 2107 of the Woods Hole Oceanographic Institution, based on work done under a program conducted jointly by the U.S. Geological Survey and the Woods Hole Oceanographic Institution and financed by the U.S. Geological Survey. the central 1\Iaine coast, southeasitern New England, and the Scotian Shelf. Spotted schist and mica gneiss arE' concentrated southwest of Nova Scotia; along will Basin) are a mixture of all rock types. Resistant rock types ("vein" quartz, quartzite, and chert) are concentrated mainly south of the Gulf of Maine and probably represent a mixed provenance; some fragments were probably brought in from crystalline bedrock to the north by glaciers. Some also may have come from quartzose conglomerates in strata of Cretaceous age, which are thought to underline Georges Bank and the southern Gulf of 1\Iaine. Utilizing the concentration of coarse detritus to mark the sea ward extension of ice, we find that the boundary extends enstward as a lobate line from glacial moraines on Nantucket and :Martha's Yineyard, across Great South Channel and the northern part of Georges Bank. It continues across the seaward terminus of No,rtheast Channel and along the seaward edge of the Scotian Shelf. 1\Iost of Georges Bank was subaerially exposed during low ,stands of st>a level, so that melt-water ~.treams drained south to the shelf edge, where they clumped detritus into the many submarine canyons 'that incise d1e southern part of Georges Bank. The boundary indicates that ice c>xtended at least to the edge of the Scotian Shelf, where it formed a floating, calving margin in the sea. Glaciers moving southward from New England and Canada sculptured the northern continental margin and contributed the poorly sorted till-like mixtures of gravel, sand, silt, and clay. They dumped debris along northern Georges Bank, probably, as moraines and outwash plains. Hence, the moderate sorting, better roundness, and increase in resistant rock types noted in gravels on Georges Bank, Nantucket Shoals, and parts of the Scotian Shelf reflect some current transport by melt-water streams 'and by marine bottom currents. Coarse dt>bris was rafted by floating ice to the continental slope and rise; this is reflected in the wide variation in rock types and roundness (pointing to multiple sources) and in the "tacked on" nature of the gra Yel fraction to the main part of the grain-size distribution. l\Iost of the shelf off New England, Long Island, and New Jersey is mantled by sand and lesser amounts of gravel in amounts probably sufficient to constitute an economic asset. A drowned river terrace on the shelf soutlwast of New York City and isolated glacial gravelly sands offshore from Boston are promising deposits meriting further detailed study. Other deposits are off Rhode J,sland, Cape Cod, and Long Island. A few |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://pubs.usgs.gov/pp/0529h/report.pdf |
| Alternate Webpage(s) | https://doi.org/10.3133/pp529h |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
