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57-8 Insights into the stratigraphic correlation, geochemistry, and radiometric ages of Cretaceous bentonites within the Graneros Shale in Central Kansas

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part II

Presenting Author:

Megan Jenkins

Authors:

Jenkins, Megan¹, Andrzejewski, Kate², Moller, Andreas³, McLean, Noah⁴, Andrew, Joseph⁵, Kalbas, Jay⁶

(1) Geology Department, University of Kansas, Lawrence, KS, USA, (2) Kansas Geological Survey, Lawrence, KS, USA, (3) Geology Department, University of Kansas, Lawrence, KS, USA, (4) Geology Department, University of Kansas, Lawrence, KS, USA, (5) Geology Department, University of Kansas, Lawrence, KS, USA, (6) Kansas Geological Survey, Lawrence, KS, USA,

Abstract:

Outcrops of the Graneros Shale in central Kansas preserve important paleontological and sedimentary features that record the beginning of the Greenhorn Cyclothem of the Western Interior Seaway. Additionally, the Graneros Shale serves as the overlying confining unit of the Dakota Aquifer, a significant freshwater resource. Previously, a thick bentonite in the Graneros Shale had been correlated via biostratigraphy to the X-Bentonite dated elsewhere at 95.87 ± 0.10 Ma (Barker et al., 2011). This bentonite has since been used to correlate outcrops throughout central Kansas and into Colorado (Hattin, 1965, Hamilton, 1994). Despite its importance as a stratigraphic marker bed, there is limited geochemical and geochronological data to verify its age for correlative use. The initial U-Pb radiometric age obtained via LA-ICP-MS analysis of zircons from this 30 cm thick bentonite indicates it to be younger than the X-Bentonite, challenging earlier interpretations and correlation of these units (Maresh et al., 2024, Jenkins et al. 2025).

This study aims to establish high-precision radiometric ages and identify the geochemical composition of bentonites from 3 localities in Russell County Kansas, previously identified as the X-Bentonite marker bed by Hattin (1965). Initial analyses extracted and analyzed approximately 300 zircons from each sample using U-Pb LA-ICP-MS. The initial datasets from Maresh et al. (2024) and Jenkins et al. (2025) showed several age populations. Therefore, all samples were reassessed by chemical abrasion (CA)-TIMS, with the youngest and most concordant grains from each sample to estimate eruption ages. To evaluate the geochemistry of the bentonites, bulk samples were powdered and prepared via peroxide total fusion and analyzed via ICP-OES+MS. The results from this study will be used to precisely assess the age of the thick bentonite within the Graneros Shale and determine if these bentonites are in fact time-correlative and useful as marker beds within the complex stratigraphic sequence.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10146

Insights into the stratigraphic correlation, geochemistry, and radiometric ages of Cretaceous bentonites within the Graneros Shale in Central Kansas

Description

Session Format: Oral

Presentation Date: 10/19/2025

Presentation Start Time: 03:45 PM

Presentation Room: HBGCC, 304C

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