March 2023 Virtual Meeting

Monday, March 13, 2023
6:00 - 7:30 pm

Note: ETGS members will receive an email with info for logging into the meeting.

March Presentation

Miocene or Pliocene, Hemphillian or Blancan? Dating the Gray Fossil Site with Cosmogenic Nuclides to test Existing Biochronology



William Odom, PhD
Geologist, USGS Florence Bascom Geoscience Center



The Gray Fossil Site, discovered in 2000, is a filled sinkhole complex that hosts one of the most diverse fossil assemblages in the eastern U.S. In addition to many others, the site notably includes tapir, red panda, short-faced bear, and rhinoceros fossils. Using the latter two fossils, Wallace and Wang (2004) proposed that the sinkhole filled between 7-4.5 Ma, an age range that straddles the boundaries of the Miocene and Pliocene epochs. Subsequent age estimates based on fossil pollen inferred separate depositional episodes in the early and late Cenozoic (Zobaa et al., 2011). Most recently, a Pliocene age of 4.9-4.5 Ma was proposed by Samuels et al. (2018) based on the presence of micromammal fossils. Obtaining a precise constraint on the age of the Gray Fossil Site has important implications for interpreting mammal occurrence and migration in addition to characterizing regional erosional trends, but no previous radiometric dating has been done on this site.

Given the paleontological significance of this site and the variable biochronologic estimates for its filling, we sampled the deposit for cosmogenic nuclide dating to provide an independent age. Measuring in-situ cosmogenic 26Al and 10Be at eight intervals within a 35-m-deep core, we were able to model the burial history of the sinkhole fill material. Depth-concentration trends of 26Al and 10Be were consistent with a single-stage burial history that occurred no earlier than ~4.5 Ma, and most likely during the latest Miocene. In this talk, I will discuss ongoing efforts to further constrain the age of the Gray Fossil Site and consider the geologic implications of this work.


William Odom is a geologist at the USGS Florence Bascom Geoscience Center in Reston, Virginia. He joined USGS in 2020 upon completing his PhD at Purdue University, where he investigated the Cenozoic history of the southern and central Appalachian Mountains using cosmogenic nuclides and 40Ar/39Ar geochronology. Since joining USGS, he has worked with the Bascom 40Ar/39Ar Dating (BARD) Lab and established the Reston Cosmogenic Nuclide Lab, which has collaborations in the eastern and central US with several ongoing mapping projects.


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Page updated May 10, 2023