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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. 

ABSTRACT Although intensified work on the volcaniclastic-rich sediments of the fossil-bearing Mussentuchit Member (uppermost Cedar Mountain Formation, Utah) has provided a refined chronostratigraphic framework, paleoenvironmental interpretations remain cryptic. To resolve this, we performed facies analysis and architectural reconstruction on exposed Mussentuchit Member outcrops south of Emery, central Utah, USA. Contrary to previous interpretations (fluvial, lacustrine), we identified a broad suite of facies that indicate that deposition occurred on the landward part of a paralic depocenter, influenced by both distal alluvial and proximal coastal systems. We conclude that the Mussentuchit Member was a sink for suspension-settling fines with most undergoing pedogenic alteration, analogous to the modern coastal plain of French Guiana (Wang et al. 2002; Anthony et al. 2010, 2014). However, this landward paralic depocenter was not uniform through time. Sedimentological evidence indicates landscape modification was ongoing, influenced by an altered base-level (high groundwater table, long residency of water in sediments, shifts in paleosol types, heavier to lighter δ18O, and distinct shifts in relative humidity (ε); common in coastal settings). If the above data is coupled with recent age data, we interpret that the Mussentuchit Member correlates to the S.B. 4 Greenhorn Regression (Thatcher Limestone) of the adjacent Western Interior Seaway to the east. As a landward paralic depocenter, the Mussentuchit would have been sensitive to base-level conditions in response to ongoing tectonic processes pushing the foredeep east, and lower paleo-CO2 levels coupled with a minor global sea-level fall (brief glacial phase) just before to the Cenomanian–Turonian Thermal Maximum. Altogether, our results not only strengthen linkages in the central Western Interior Seaway, but simultaneously results in novel linkages to near-coeval paralic depocenters across mid-Cenomanian North America. 

We present a previously discovered but undescribed late Early Cretaceous vertebrate fauna from the Holly Creek Formation of the Trinity Group in Arkansas. The site from the ancient Gulf Coast is dominated by semi-aquatic forms and preserves a diverse aquatic, semi-aquatic, and terrestrial fauna. Fishes include fresh- to brackish-water chondrichthyans and a variety of actinopterygians, including semionotids, an amiid, and a new pycnodontiform, Anomoeodus caddoi sp. nov. Semi-aquatic taxa include lissamphibians, the solemydid turtle Naomichelys , a trionychid turtle, and coelognathosuchian crocodyliforms. Among terrestrial forms are several members of Dinosauria and one or more squamates, one of which, Sciroseps pawhuskai gen. et sp. nov., is described herein. Among Dinosauria, both large and small theropods ( Acrocanthosaurus , Deinonychus , and Richardoestesia ) and titanosauriform sauropods are represented; herein we also report the first occurrence of a nodosaurid ankylosaur from the Trinity Group. The fauna of the Holly Creek Formation is similar to other, widely scattered late Early Cretaceous assemblages across North America and suggests the presence of a low-diversity, broadly distributed continental ecosystem of the Early Cretaceous following the Late Jurassic faunal turnover. This low-diversity ecosystem contrasts sharply with the highly diverse ecosystem which emerged by the Cenomanian. The contrast underpins the importance of vicariance as an evolutionary driver brought on by Sevier tectonics and climatic changes, such as rising sea level and formation of the Western Interior Seaway, impacting the early Late Cretaceous ecosystem.