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ABSTRACT

The fine carbonate fraction (less than 63 μ in diameter) in 175 sediment samples collected from the Carolina continental margin has been studied. This fraction, generally under 5% of the total sample, averages 43% CaCO3. By comparison, the CaCO3 fraction of the total samples averages only 25%. On the inner shelf, the percentage CaCO3 in the fine fraction is twenty or more times greater than the percentage CaCO3 in the total sample. With increased distance offshore, the percentages of CaCO3 in the fine fraction and the total sample approach equality. Beyond the shelf break the percentage of CaCO3 in the total sample is greater than that in the fines.

The CaCO3 faunal components are primarily foraminifera, fragments of foraminifera, mollusc-barnacle fragments and echinoid material. Minor constituents include alcyonarian and tunicate spi-cules, Halimeda and other calcified green algae. Many grains are bored and show solution effects. The coarse carbonate fraction shows distinct regional faunal assemblages, but the fines are essentially uniformly distributed. Two exceptions are a zone of mollusc fragments around Cape Hatteras and a foraminifera zone on a portion of the outer shelf. The average carbonate mineral assemblage consists of 44% low-Mg calcite, 38% aragonite and 18% high-Mg calcite. The principal modes of origin of calcareous fines on the Carolina continental margin are probably biological and physical destruction of coarser particles and primary formation in fine size fraction (as in the case of some foraminifera).