147-4 Paleoenvironmental and Paleoclimate Interpretations of a Historic Two Medicine Formation Site

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

Presenting Author:

Authors:

Abstract:

The Upper Cretaceous Two Medicine Formation’s Willow Creek Anticline (WCA) outcrop is famous for its dinosaur and trace fossils and has been historically interpreted as a primarily fluvial setting in a semi-arid paleoclimate. To refine our understanding of the origins and formation mechanisms of WCA carbonates with the intention of using their stable isotopes as climate proxies, we created stratigraphic columns for two areas: 1) an outcrop ~30m south of a hadrosaur hatchling nest discovered in 1979 (HN Strat, ~12m), and 2) an outcrop stratigraphically up-section from the nesting horizon and colloquially called ‘Rainbow Mountain’ (RM Strat, ~17m).  The HN Strat contains evidence of at least 4 paleoenvironments/climates: fluvial (indicated by sandstones with trough cross beds and contorted bedding), floodplain (a period of nondeposition indicated by a paleo-Vertisol with redoximorphy and pedogenic slickensides, which are consistent with seasonally varying soil water surplus and deficit), subhumid palustrine (indicated by carbonaceous shale with possible leaf impressions), and evaporative palustrine (indicated by massive limestones with evidence of bioturbation and pedogenic overprinting). Traditional carbon and oxygen stable isotope analyses of HN Strat limestones suggest similar formation timing, mechanisms, and DIC sources, and “clumped” stable isotope analyses of some limestones and a rhizolith from the paleosol yield mineral formation temperatures that represent reasonable Earth surface temperatures for the Late Cretaceous (~28-38°C). Results from stable isotope analyses of vein calcite from the HN Strat hillside indicate distinct formation mechanisms, timing, and DIC sources and yield a higher mineral formation temperature (~100°C). The RM Strat section contains sandy facies interbedded with limestones and fissile muddy units, with at least one paleosol profile. We interpret some RM sandy facies as fluvial/channel deposits due to their discrete bedding structures, while we interpret the limestone units as evaporative palustrine due to their similarities with the limestones analyzed in the HN Strat section. The paleosol represents an extended period of nondeposition, likely on a floodplain. Paleopedological analyses of this paleosol will be conducted during a future field season, and stable isotope analyses of limestone and vein calcite samples collected from the RM Strat are forthcoming. In this presentation, we will primarily discuss carbonate formation mechanisms for the carbonates in these sections and present a comparison of our preliminary environmental interpretations to previous interpretations.

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

doi: 10.1130/abs/2025AM-4991

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