Distinguishing Bottom and Turbidity Current Coarse Layers on the Continental Rise

Abstract

ABSTRACT The heavy mineralogy and texture of six sand layers from five cores taken on the lower continental rise off North Carolina were examined. Based upon megascopic criteria, two sand layer types were evident: relatively thick (> 10 cms), visibly graded sand layers and thinner, non-graded layers of fine sand or coarse silt. The former are assumed to be of turbidity current origin and the latter of bottom current origin. Turbidity coarse layers exhibit much greater vertical variability of both grain size and heavy mineralogy than bottom current sand layers. Plots of the percentages of stable versus unstable heavy minerals within sand fractions appear to be a particularly accurate means of determining coarse layer genesis. The results of this study indicate that megascopic criteria, which can be used in shipboard or laboratory core description, can be an adequate means of distinguishing the two types of layers. This should greatly facilitate study of the controversial problem of continental rise origin.