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The Kootenai Formation of Western Montana records the Aptian- Albian (121.4Ma-100.5Ma), a significant interval in Earth’s history. The Early Cretaceous is notable for a multitude of changes in both the geologic and biotic realm. Significant events that occurred during this time include the tectonic evolution of the Western Interior Basin (WIB) and the displacement of gymnosperms by angiosperms. Given the significance of this time, previous and ongoing research seek to better understand the timing and interactions between these changes. The focus of this study is to refine stratigraphic constraint of the Kootenai Formation using carbon isotope chemostratigraphy. The depositional age of the lower clastic unit of the Kootenai formation has been debated over the past decade. Detrital zircon U-Pb analyses by Laskowski et al. (2013) indicated an Albian age with a U-Pb detrital zircon maximum depositional age (MDA) of 109Ma. However, more recent studies (Finezl and Rosenblume, 2020 and Rosenblume et al. 2021) using LA-ICP-MS-generated detrital zircon U-Pb analyses indicate MDAs of the lower clastic unit as old as Valanginian to Aptian (MDAs ~135-115Ma) with the upper units of the Kootenai having MDAs from Albian (~105 Ma). Detrital zircon U-Pb analyses have generally been limited in the lower units of the Kootenai particularly because syndepositionally formed zircon grains are not common in the lower units (Quin et al. 2018, Finzel and Rosenblume 2020).Additionally, previous flora in the Kootenai suggests predominately Aptian and older ages(Brown 1946). Given the limited geochronologic constraint of the lower clastic unit of the Kootenai formation, addition data is needed. For this study, approximately 60 samples from just above the basal conglomerate to the top of the lower clastic unit were collected and processed to determine bulk organic carbon isotope values. The prior MDAs suggest C isotope excursions such as those associated with OAE1a and even as old as the Valanginian Weissert event could be preserved in the strata of the lower clastic unit. The new stable isotope data will provide an opportunity to refine the age of these Cretaceous units leveraging the existing U-Pb data.
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This content will become publicly available on November 26, 2026
Radioisotopic age, osmium isotopes, and global correlation of the Albian-Cenomanian boundary
The Valanginian period is marked by significant events, including the earliest perturbations of the global carbon cycle and the initial emplacement of large igneous provinces on land. Recent years have seen the ratification of the global boundary stratotype sections and points (GSSPs) for the Valanginian and Hauterivian stages, reflecting increased stratigraphic and palaeoenvironmental research on the Valanginian. However, paleoceanographic information remains limited for the Early Cretaceous Panthalassa, which was the largest ocean during early Cretaceous. In Northeast Japan, Lower Cretaceous marine sequences, formed in the northwest region of Panthalassa along the northeastern margin of Eurasia, are exposed intermittently. These sequences have been challenging to date accurately due to the limited availability of age-diagnostic fossils. We establish new global stratigraphic correlations using U–Pb zircon ages from tuffs and carbon isotope stratigraphy for the Lower Cretaceous formations of the Karakuwa and Oshima Groups in Northeast Japan. We identify the Berriasian/Valanginian boundary and the Weissert Event within these strata. In addition, we recognize a potential stratigraphic level for the Valanginian/Hauterivian boundary. Notably, the Weissert Event interval within these sequences does not exhibit the significant lithological changes, such as the intercalation of black shales or a decrease in bioturbation observed elsewhere . By contrast, observed lithological changes are indicative of regional tectonics, including large-scale sinistral strike-slip movement and subsequent ridge subduction along the active continental margin of Eurasia.
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- Award ID(s):
- 1951812
- PAR ID:
- 10649814
- Editor(s):
- Rasbury, Troy
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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