Middle Eocene bentonite in the North Carolina Coastal Plain and its relationship to the volcanic swarm in western Virginia
Published:April 13, 2020
W. Burleigh Harris, Paul A. Thayer, 2020. "Middle Eocene bentonite in the North Carolina Coastal Plain and its relationship to the volcanic swarm in western Virginia", The Appalachian Geology of John M. Dennison: Rocks, People, and a Few Good Restaurants along the Way, Katharine Lee Avary, Kenneth O. Hasson, Richard J. Diecchio
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A local clay lens up to 60 cm thick in the Eocene Castle Hayne Limestone at the abandoned Fussell Quarry, Duplin County, North Carolina, is identified as a bentonite. It is composed of authigenic smectite with sparse euhedral biotite and apatite. Scanning electron microscope examination shows that the bentonite consists of relic bubble-wall shards altered to smectite. Smectitic columnules, rod-shaped casts of elongate pipe vesicles in pumice fragments derived from early dissolution of nearby small glass shards, also occur. This association is considered diagnostic of a silicic air-fall ash. K-Ar and Rb-Sr biotite dates from the bentonite are 46.2 ± 1.8 Ma and 45.7 ± 0.7 Ma, respectively, and a fission-track age of apatite is 51.0 ± 2.0 Ma; this later date is considered to be incorrect.
Biotite compositions determined from electron microprobe analyses on 100 crystals suggest derivation from a single volcanic source no more than 4000 km from the bentonite. Possible sources of the ash include Bermuda; Highland County, Virginia; and the Caribbean; however, because of distance, prevailing wind direction, and similarity in age and composition, the volcanic swarm in Highland County, Virginia, is the suggested source.