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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 56 (1972)

Issue: 10. (October)

First Page: 2000

Last Page: 2012

Title: Bristol Bay: Model Contemporary Graded Shelf

Author(s): G. D. Sharma (2), A. S. Naidu (2), D. W. Hood (2)

Abstract:

A veneer of contemporary sediments covers the continental shelf in the Bristol Bay region of the southeast Bering Sea. The continental shelf is broad, covering an area of 1,500,000 sq/km, and has a relatively smooth bottom and a gentle slope of about 2.4 percent. Nearshore sediments are extremely poorly sorted gravelly sands which grade to well-sorted fine sands in the central bay. The far offshore sediments are extremely poorly sorted muddy sands. The mean size of the sediments generally decreases with increasing depth and distance from the coast. An overall decrease in the weight percent of total heavy mineral contents also is observed from the coast to the outer bay. The mineralogy of the sand reflects a complex provenance--sedimentary, high-grade metamorphic and igneo s sources in the north and east, and a volcanic source in the south. The heavy mineral dispersal pattern presumably is influenced by fluvial and marine currents. The general paucity of clay minerals and the dominance of feldspars in the less than 2-micron fractions of Bristol Bay sediments indicate the absence of intense chemical weathering in the source areas. Organic carbon content in the sediments is uniformly low (0.5 percent), and is closely related to the amount of clays in the sediments.

On the basis of scatterplots of grain-size parameters, the Bristol Bay shelf can be delineated into two broad depositional environments, the inner and the outer shelves. Textural differences in the two environments are believed to be caused by storm-generated waves. These waves may be as great as 200 m long and are believed to elutriate the silts and clays from sediments up to a depth of 80 m. Grading of the shelf also is achieved principally by these waves that bring about a net seaward movement of nearshore sediments.

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