Carolina Cretaceous: Petrographic Reconnaissance of a Graded Shelf: ABSTRACT

Abstract

More than 140 samples were collected from the transgressive marine strata of the Carolina Cretaceous in order to assess the hypothesis that they were deposited on a size-graded shelf whose sediments were transported by storm-generated wind-drift currents. Calculations of size parameters revealed only a varying ratio of sand (tractive load) to silt and clay (suspensive load worked in by bioturbulation). However, examination of the shapes of the cumulative curves permitted classification of sediments into an evolutionary sequence of nearshore sand, proximal shelf sand, distal shelf sand, and shelf mud. A scatter of modal diameter versus distance from the Cretaceous shoreline has an upper limiting value of 3.5 ^phgr, a spread of 1.0-3.5 ^phgr at distance zero, but an essenti lly constant value of 3.5 ^phgr for the seaward margin of the outcrop zone. The scatter appears to consist of nearshore and shelf segments, perhaps resulting from two distinct dispersal mechanisms. Detailed study of basal (nearshore), central, and upper (offshore) Peedee outcrops shows that the spread of values for nearshore modes corresponds to the presence in the nearshore outcrops of well-defined, size-graded strata of probable storm-current genesis. Offshore outcrops are 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 nearshore sediment, and its movement seaward to replace sediment lost through deposition or bypassing End_Page 744------------------------------ over the shelf edge. Coarser sediment fractions are deposited preferentially nearshore, and the sediment becomes finer and more homogeneous offshore. 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. End_of_Article - Last_Page 745------------