147-5 The Last Vestiges of the Western Interior Seaway: KPg Microfossils as Palaeoceanographic Clues.

Session: Climate, Ocean and Environmental Changes Through Earth History: From Marine and Terrestrial Proxies to Model Assessments (Posters)

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Abstract:

The fossil record consistently demonstrates a strong link between the rapid decline of marine ecosystems and mass extinction events, often paralleled by the deterioration of terrestrial ecosystems. Among global mass extinctions, the Cretaceous-Paleogene (KPg) event stands out as the most recent in Earth’s history. Its swift onset and worldwide ecological impact closely mirror current global ecological disruptions, making it a critical area of study.

While terrestrial strata within the U.S. Western Interior offer prime locations to study the KPg boundary, complete erosion of coeval marine strata has long prevented direct understanding of the paleoceanography and paleoecology of the Western Interior Seaway (WIS) during the Terminal-Cretaceous. This critical gap of data hinders our ability to fully determine marine conditions such as the seaway's extent, salinity, temperature, depth, and fauna during this pivotal geological period.

A unique KPg "surge" or seiche deposit in Southwestern North Dakota contains marine microfossils that can help to bridge this knowledge gap. The deposit itself, found within the terrestrial Hell Creek Formation, was rapidly emplaced—within hours of the Chicxulub impact—by an inundation surge originating from the WIS, which facilitated remarkable temporal resolution and fossil preservation. This offers an unparalleled opportunity to sample fossilized terrestrial and contemporaneous marine organisms together.

Our study focuses on a taxonomic and taphonomic analysis of the marine microfauna from this surge deposit, aiming to understand its relationship to slightly older assemblages within the WIS and those from the later Paleogene. Marine foraminifera, ostracods, and microscopic fish remains occur within several horizons throughout the impact ejecta-bearing deposit. The microfossil specimens are remarkably well-preserved, including delicate planktonic and benthic species with intact and empty chambers, strongly supporting rapid deposition simultaneous with the KPg event. The presence of foraminifera genera such as Lenticulina, Anomalinoides, Racemiguembelina, Planoheterohelix, Globogerina, and Globotruncana, alongside partial and nearly whole ammonite shells (Spenodiscus lobatus), indicates a strong open marine connection for the WIS at the time.

A robustly supported reconstruction of the Terminal-Cretaceous WIS can ultimately be achieved by integrating the approaches employed here with additional analyses including isotope geochemistry, bone histology, and synchrotron-based techniques. By establishing this ecological baseline for the WIS immediately preceding the KPg boundary, we can significantly improve our understanding of the profound biotic effects of the Chicxulub impact. 

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

doi: 10.1130/abs/2025AM-7886

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