GSA Connects 2022 meeting in Denver, Colorado

Paper No. 168-8
Presentation Time: 9:00 AM-1:00 PM


MARGULIS-OHNUMA, Miranda, Earth and Planetary Sciences, Yale University, Kline Geology Laboratory, 210 Whitney Avenue, New Haven, CT 06511, CHANG, Clara, Lamont-Doherty Earth Observatory, Columbia University, 61 Rte 9W, Palisades, NY 10964, WHITESIDE, Jessica H., Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton, SO14 3ZH, United Kingdom and OLSEN, Paul E., Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964

The Carnian of the early Late Triassic was the setting of major climate events and radiations of many animal groups including dinosaurs and stem mammals (1). Although paleotropical Eastern North American rift basins are well known for their lacustrine Norian (later Late Triassic) to Sinemurian (Early Jurassic) orbitally-paced strata, cyclicity of the rifts’ Carnian strata remains poorly understood. A Carnian astrochronology would provide context for such Earth history events.

The Richmond Rift (VA) has well-developed Carnian lacustrine strata, but previous cyclostratigraphic efforts were based on scant outcrop, one short core, and palynology (2, 3). However, hydrocarbon exploration wells in the basin have nearly untapped potential for astrochronology, including the Horner No. 1 well (3), our focus. We performed geochemical analysis on cuttings collected at 10 ft composite intervals from the lower Horner well (Vinita Fm.) using a handheld XRF spectrometer. Spectral analysis and sedimentation rates were explored using Acycle (4). Multiple elemental proxies reveal a 16 m cycle we interpret as the ~20 kyr climatic precession cycle, equivalent to an accumulation rate of ~0.8 m/kyr. Proxies including K2O, Zr, S, and Zn provide independent estimates of accumulation rate within 0.05 m/kyr of one another. Although initial analysis of natural gamma radioactivity did not produce a clear cyclical signal, a re-examination using parameters determined by the XRF data corroborates the chemistry results. Extrapolating this accumulation rate, the Horner well would represent ~2.4 Myr of the Carnian; analysis of the rest of the well record is underway to test this hypothesis.

XRF analysis of cuttings, along with geophysical logs, can provide paleoenvironmental data in cases where only deep well records are available. Furthermore, understanding chemical cyclostratigraphy will augment our knowledge of environmental responses to orbital forcing. In addition to contributing to a temporal calibration of the Carnian, XRF of well cuttings has the potential to open new horizons in astrochronology.

1) Dal Corso et al. (2020) Science Advances 6(38) eaba0099; 2) Whiteside et al. (2011) PNAS 108(22) 8972; 3) Cornet & Olsen (1990) VMNH Guidebook 1; 4) Li et al. (2019) Comp. Geosci, 127 12.