Refining the Meaning of K/Ar Ages of Glaucony at the Single Pellet Level in the Austin Chalk Formation of Texas

Lead PI: Sidney Rasbury Hemming

Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)

July 2021 - August 2022
North America ; United States ; Texas
Project Type: Research

DESCRIPTION: This project is a detailed study of the chemistry, mineralogy and K/Ar geochronology of individual pellets from glauconite layers from the Austin Chalk, from a section in Bexar County Texas. The age range of the proposed section is estimated to be 89.8-83.6 Ma, and they are bracketed by two prominent and regionally known ash beds, so the three glauconite beds within is were deposited in a relatively narrow window of time. The glauconite is found associated with flooding surfaces that have been previously connected with stratigraphic boundaries related to relative sea level fluctuations. A long history of the study of glauconite, most notably by Gilles Odin, has demonstrated that the K/Ar ages of carefully selected samples are in close agreement with ages from other methods, although there may be a ~2% offset. Most of these previous studies were done on large samples, and we plan to take this to the single pellet level to examine reasons for variability such as degree of maturity. The work is exciting because of the common occurrence of glauconite rich layers and their intimate connection with sedimentary processes such as responses to sea level change. Although volcanic ash layers are a preferred means of dating strata, they are not always present and other direct dating methods provide added value. To harness the value requires detailed study of the systematics and a workflow to select the best samples and to evaluate geologic scatter.