Hornblende content of Georgia-South Carolina, U.S.A., nearshore sands: support for shoreward sediment transport in the nearshore zone

Abstract

Core top samples from 51 box cores taken about 1 km apart in several transects off Hilton Head Island, South Carolina, and Tybee, Wassaw and Ossabaw Islands, Georgia, have been analyzed for heavy-mineral composition of acid-insoluble 3-2 φ fractions to determine hornblende content, and for acid-soluble (principally calcium carbonate plus carbonate fluorapatite) content, and grain size data for acid-insoluble sand and gravel fractions (< 4 φ) and for unacidified samples. Heavy-mineral contents of acid-insoluble 3-2 φ fractions range from 0.2 to 2.3%, with hornblende and epidote the most abundant heavy minerals. Individual hornblende and epidote abundances range from 16.2 to 52.0%. Hornblende and epidote contents vary antithetically, with maxima in hornblende content and minima in epidote content at nearshore and offshore ends of transects, and with minima in hornblende content and maxima in epidote content over middle portions of transects. Ilmenite is the most abundant opaque mineral identified, with abundances from 1.3 to 24.5%. Ilmenite maxima are associated with hornblende minima at several stations off Hilton Head and Tybee Islands, apparently due to winnowing. Acid-insoluble residues are very fine to fine (4-2 φ) sands, with very fine sand (Recent) dominant within 5 km of shore and fine sand (relict, or palimpsest) most abundant farther offshore. Acid-soluble contents in our study area are < 25% of total sample weight for nearly all samples analyzed, and are often < 15%. High hornblende contents at seaward ends of transects indicate shoreward transport of hornblende-rich sediments from the adjacent inner continental shelf. Low hornblende contents in middle portions of transects indicate dilution of hornblende-rich sands by hornblende-poor Coastal Plain sediments derived from Coastal Plain rivers, and exposed by shoreface erosion 4–8 m deep, and by tidal inlet erosion. High hornblende contents at nearshore ends of transects reflect contribution of hornblende-rich sediment from Piedmont rivers to the nearshore zone.