Hydraulic Fractionation of Heavy Mineral Suites on an Unconsolidated Retreating Coast

Abstract

ABSTRACT A study of sediments of the North Carolina-Virginia coast has been undertaken in order to determine the genesis of the coastal sands. This paper reports the results of a portion of this study, namely an examination of the 85-177 micron heavy mineral fraction. Within this fraction, we found three well-defined heavy minerals provinces. Au amphibole-garnet-kyanite suite exists on the beach and offshore. An amphibole-epidote-kyanite suite in the nearshore zone separates the other two. There three provinces correspond to three well-defined grain size provinces generated by the contrasting hydraulic regimes of the surf, zone of shoaling waves, and the shelf floor. Coastwise variation in heavy mineral content is also apparent, but is weaker. The most important trends are an increase in garnet and opaque minerals and a decrease in amphibole on the beach from Cape Henry towards Cape Hatteras. These trends are not easily correlated with the reported pattern of littoral drift, and may reflect instead the effect of southward-increasing mean annual wave heights on sediment eroded from the underlying Pleistocene. Interpretation of the coastwise trend is complicated by the fact the Pleistocene substrate is itself a beach ridge sequence with its own pattern of heavy mineral variation. The simplest interpretation of the heavy mineral data is that the respective provinces consist of autochthonous fractionates of the Pleistocene substrate, generated by shore face retreat.