FROM THE PIEDMONT TO THE ATLANTIC OCEAN: TI-ZR-REE HEAVY MINERAL SAND DEPOSITS IMAGED BY HIGH-RESOLUTION AIRBORNE RADIOMETRIC DATA

We present initial results from a set of new airborne radiometric surveys over a ~130-km-wide swath of Atlantic Coastal Plain sediments from the Piedmont Province to the modern coastline in South Carolina. These surveys provide source-to-sink imagery of placer deposits up to 1 m deep containing critical minerals Ti (ilmenite and rutile), Zr (zircon) and REE (monazite) through gamma spectrometry for K, U, and Th. Using comparisons to sample data and results from previous coarse-resolution studies as ground truth, the new data have implications for geologic relations and sediment pathways.

Direct correlations between radiometric Th and heavy mineral concentrations are observed and attributed to Th in monazite. Radiometric Th shows marked changes in character over three zones within the transect. The first is within the Upper Coastal Plain, between the Piedmont and the 3.5-3.0 Ma Orangeburg Scarp. Radiometric Th shows the highest values in this area and is correlated with topographic features carved by rivers, especially within river valleys. The second is within Lower Coastal Plain sediments from ~1.2 Ma to the modern shore (Wicomico Fm. to Holocene), marked by sand ridges alternating with clay- and mud-filled areas. Radiometric Th shows local highs within the sand ridges and lower, more distributed values within the finer-grained sediments. Here the Santee River floodplain, exposed downstream of the Santee Dam, shows a broad high over most of its length. The third area, between the Orangeburg Scarp and the Wicomico Fm., has relatively low Th values with exception of areas north of the Santee River, where eolian sediments have been observed.

Radiometric K reflects K-spar and mica, indicating the presence of “immature” sediments, i.e. minerals that are relatively unstable due to K mobility. K is elevated mostly within Piedmont rocks, the Santee/Congaree/Wateree River system floodplain, marine formations younger than ~130 kyr, and urban areas; values elsewhere are generally low. The most probable source of immature sediments in the marine sediments is the Santee River. However, elevated K is observed over 100 km away from the floodplain and is not concentrated near the river system. This suggests that the heavy minerals are delivered offshore, transported by the longshore current, and then re-deposited onshore.