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The record of the post-Jurassic transgression which caused the formation of the northern Atlantic Coastal Plain begins with the Potomac group, non-marine sediments of early Cretaceous age. Lithologically similar non-marine and transitional sediments of early late Cretaceous age, the Raritan and Magothy formations, overlie the Potomac group. The lithologic similarity of the several formations, and disagreement as to the significance of their contained plant fossils, which are the major means of dating the exposed deposits, have caused some recent confusion in their stratigraphic interpretation.
Recent laboratory and field studies show that heavy minerals are useful for local correlation, and permit the following conclusions.
1. A considerable wedge of Lower Cretaceous sediments is present in the shallow subsurface of southern New Jersey, and was derived principally from the crystalline Piedmont of Pennsylvania and Delaware.
2. During Raritan time the Piedmont was covered by Cretaceous sediments and the materials which make up the Raritan formation were supplied from older sedimentary terranes northwest of the Coastal Plain except in the northern New Jersey-Long Island area which received sediments derived from crystalline rocks exposed in southern New York and New England. Similar conditions prevailed during the deposition of the Magothy formation, but during Magothy time, the Cretaceous sediments were stripped from the Piedmont and it again became a site of active erosion.
3. The Potomac group and the Raritan formations are dominantly fluvial sediments at the outcrop and in the shallow subsurface, but marine facies of the Raritan are present only a few miles downdip from the outcrop areas. No marine Lower Cretaceous has been reported from the northern Atlantic Coastal Plain. The Magothy formation is transitional marine at the outcrop.
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