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The Lower Permian Vrchlabí Formation was deposited in the Krkonoše Piedmont Basin of the Czech Republic during theAutunian (297–299 Ma) when global climate was characterized by a prolonged cold phase of the Late Paleozoic Ice Ageand when terrestrial environments shifted toward more arid conditions as atmospheric pCO 2 decreased. The RudníkHorizon of the lower Vrchlabí Formation is a well studied interval that records extensive lacustrine deposition. However thefine-grained redbed interval that underlie this interval are less well studied. In order to contrast the geochemistry of theseredbed arid climate facies with the deposits formed during inundation by the Rudník lacustrine system, we analyzed 17 fine-grained samples of varying lithology using XRF and LOI analysis from a detailed section through ~100m outcrop exposureof red beds near the down of Vrchlabí. These samples represent 5 different facies: 1) featureless maroon reddishmudstone, 2) pedogenic red-brown-orange mudstone, 3) gray laminated shale, 4) pedogenic gray mudstone and 5) yellowlaminated siltstone. Facies 5 produced substantially different results from the others, having the highest LOI (~21%)compared to 8.8-12.3% for all other facies, and lower Al overall than the other facies (13% compared to 18-20%). Facies 5is interpreted to represent a paludal mudstone that experienced calcite precipitation during deposition. Chemical Index ofAlteration (CIA) was highest for Facies 1 (70.1) and this facie also exhibited the highest Ti/Al (0.04) and is consistent withhighly weathered sediment influx and could represent a loessite. Facies 3 and 4 show similar average CIA values (63 and69, respectively); this is interpreted to indicate that Facies 3 formed from less chemically weathered sediment that servedas the parent material which was then subject to weak pedogenesis to form Facies 4. In contrast, Facies 2 showed muchlower average CIA value (57). This is interpreted to suggest pedogenic carbonate was precipitated, which also would havecaused the overall higher Ca/Al (0.38) for Facies 2 compared to the other facies, including the red beds of Facies 1 (0.09).Altogether these results suggest variable sediment influx under variable environmental conditions prior to the developmentof the Rudník lacustrine system.
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This content will become publicly available on January 23, 2026
Carbonates and microbialites record a dynamic lake basin evolution in the Late Cretaceous to Eocene Sheep Pass Formation, Nevada, USA
ABSTRACT Ancient terrestrial sediments provide critical information about the responses of continental environments to global scale climate and tectonic perturbations, which are vastly understudied relative to marine archives. The >1 km thick Sheep Pass Formation type section in east‐central Nevada preserves non‐marine carbonates, including microbialites, and lesser siliciclastics, deposited in a tectonically active, high‐elevation basin during the latest Cretaceous through to middle Eocene time, an interval spanning major global greenhouse climate states and warming events. This study combines outcrop and hand‐sample observations, thin section petrography and X‐ray diffraction mineralogical analyses to create a facies framework and interpreted environmental evolution for the Sheep Pass Basin. Together, these observations portray the Sheep Pass Formation type section as a dynamic and highly sensitive basin due to its small size. The dominance of thrombolite boundstones compared to metazoan fossils, which sets the Sheep Pass Formation type section apart from other Palaeogene‐aged lake basins in the western United States, reflects the resilience of microbial mats compared to metazoans in this dynamic setting. The major lacustrine phase of the Sheep Pass Basin records three intervals: a shallow lake with few microbialites, followed by abundant microbialites, before the transition to a marginal setting with evaporative conditions, marking the culmination of this major lacustrine phase. The transition to a microbialite‐dominated interval was likely driven by physicochemical conditions (for example, higher alkalinity), paired with lower competition from metazoan grazers. Although the Sheep Pass Formation type section preserves environmental change in response to both tectonics and climate, similar trends in facies, mineralogy and invertebrate abundance compared to other sedimentary basins from this time suggest that global climate influenced distinct environmental shifts in the Sheep Pass Basin. This work provides a detailed sedimentological framework for a new, high‐elevation palaeoclimate record during a pivotal geological climate interval.
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- Award ID(s):
- 1826850
- PAR ID:
- 10567765
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Sedimentology
- ISSN:
- 0037-0746
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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