The Carolina Cretaceous: Petrographic Reconnaissance of A Graded Shelf

Abstract

ABSTRACT It has been hypothesized that the transgressive marine strata of the Carolina CoastaI Plain were deposited on a size-graded shelf whose sediments become finer grained with distance from shore. In order to test the hypothesis, over 140 samples were collected and subjected to size analysis. Examination of the shapes of cumulative size distribution curves has permitted classification of the samples into sands, proximal shelf sands, distal shelf sands, and shelf muds. A plot of modal diameter (measure of the central tendency most sensitive to tractive lead behavior) against distance from the Cretaceous shoreline results in a triangular scatter with a well-defined upper limit of 3.5 phi. Nearshore samples range from 1.0 to 3.5 phi, but the variance diminishes in a s award direction until an essentially invariant value of 3.5 phi is attained. The scatter appears to consist of and shelf segments, perhaps resulting from two distinct dispersal mechanisms. Detailed study of a basal (nearshore), central, and upper (offshore) Peedee outcrop shows that the spread of values for modes corresponds to the presence in the nearshore outcrop of well defined, size graded strata of probable storm-current genesis. Offshore outcrops are more fine-grained, more homogeneous, and do not have well defined meteorologic stratification. A model is proposed whereby resuspension of bottom sediments by storms results in textural differentiation of sediment, and its movement seaward to replace sediment lost through deposition or bypassing over the shelf edge. Coarser sediment fractions are preferentially deposited nearshore, and the sediment becomes more homogeneous and finer grained in an offshore direction. The model conforms to the available data, but more detailed testing of the genesis of the strata is suggested in order to substantiate the hypothesis.