22. SOURCES OF MIOCENE, PLIOCENE, AND PLEISTOCENE SILICICLASTIC SEDIMENTS IN

The sources of siliciclastic sediments for the upper Miocene through Pleistocene sediments of Unit I (Sites 612 and 613) were determined by grain-shape and surface-texture analyses. Two types of quartz grain shapes were present in these samples. One type, characterized by well-rounded quartz grains with surficial weathering features, represents clas-tic sediments derived from Cretaceous and Tertiary strata on the New Jersey continental margin. The second, character-ized by angular grains with inherited overgrowth features, represents clastic sediments derived from the pre-Cenozoic sedimentary and crystalline rocks of the Appalachian Province. The distribution of these two grain-shape types throughout the sediments of Unit I shows that a drastic change in source for the New Jersey continental margin occurred some time in the Pleistocene. Before this time, the major source of sediment was the Appalachian Province, as indicated by high proportions of the second grain-shape type. After this time, the major source was the Cretaceous and Tertiary sediments of the New Jersey continental margin, as indicated by high proportions of the first grain-shape type. From this pattern, it is concluded that during the late Miocene and Plio-cene a large river which drained the Appalachian Province served as the major conduit for sediment to the New Jersey continental margin. At some time during the Pleistocene, however, the depositional basin for this river shifted away from this area, and most of the sediment supplied thereafter was derived from more local Cretaceous and Tertiary strata