RARE EARTH ELEMENTS IN THE CRETACEOUS OF THE CAROLINA COASTAL PLAIN: DEPOSITIONAL SEQUENCES, PARENT ROCKS, AND SUBSURFACE DISTRIBUTION OF PROSPECTIVE LITHOLOGIES

Rare earth elements play an important role in modern technology and defense systems, and identification of domestic sources is a top priority for both state and federal governments. At least 70% of current rare earth element (REE) production occurs in China, a portion of which is derived from mining and processing of weathered crust elution deposits, also known as ionic or ion-exchangeable clays. These clays formed as lateritic soils and were developed over extended periods of subaerial exposure and intense chemical weathering of felsic parent rocks. Similar lateritic soils are observed in Pennsylvanian cyclothemic strata in the Appalachian and Illinois basins as well as in modern subtropical to tropical climates.

Examination of Upper Cretaceous stratigraphic sequences in cores collected from the Coastal Plain of North and South Carolina provides insight into the stratigraphic and geographic distribution of lithologies with potential to host REEs. These Turonian to Santonian sequences often comprise the first sedimentary units deposited atop crystalline basement rocks and record multiple non-marine to marine transitions across the Carolina Coastal Plain. The Turonian Fort Fisher I sequence, observed in several cores, is characterized by extensive subaerial weathering profiles and is often enriched in limonite, goethite, and hematite. It is interpreted to contain a thick package of sediments deposited under highstand conditions, and its upper sequence boundary is coincident with a hiatal surface observed from South Carolina to New Jersey. The overlying Fort Fisher II sequence records the latest Turonian, but is thin and discontinuous. The Coniacian to Santonian Collins Creek (I and II) and Pleasant Creek (I and II) sequences are characterized by marine sediments in down dip locations and terrestrial sediments up dip, with increasing marine influence through time. Understanding the stratigraphic framework of these Cretaceous non-marine to marine transitions, in conjunction with the distribution of felsic parent rocks, may aid in the evaluation of potential subsurface deposits of REEs.