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1. ASSESSMENT OF BASIN EVOLUTION FROM INTEGRATED CHEMICAL MODELS AND LITHOSTRATIGRAPHY OF THE LOWER WILKINS PEAK MEMBER OF THE GREEN RIVER FORMATION, SOUTHWEST WYOMING, USA

   ARNUK, W; LOWENSTEIN, TK; WALTERS, A P; CARROLL, A R; SMITH, M E (October 2023, Geological Society of America)

   Chemical sediments from the Early Eocene Green River Formation can be used for assessing hydroclimate and basin evolution during their deposition. The Wilkins Peak Member (WPM) of the Green River Formation contains a relatively continuous record of perennial closed-basin saline lake deposition in the Bridger Basin, southwest Wyoming, from approximately 51.6 to 49.8 Ma. The volumes and paragenesis of authigenic chemical sediments in the WPM are intrinsically related to the chemical evolution of basin brines. The geographic distribution of those chemical sediments across the Bridger Basin relates to the syn- and post-depositional tectonic history of the basin. In this study, we integrated thermodynamic modeling of chemical evolution of lake brines with chemostratigraphic and lithostratigraphic interpretations of the basin-center Solvay S-34-1 core to evaluate physical and chemical changes to and within ancient Lake Gosiute during the Early Eocene. Fine-scale X-ray fluorescence (XRF) scanning along the length of the core provides a high-resolution chemical stratigraphy of the WPM. Thermodynamic modeling of the evaporation of hypothetical inflow waters and lake brines yield predicted sequences of evaporite minerals, allowing estimation of the salinities and evaporated volumes of water required to reach saturation with respect to observed mineral deposits from the basin. The spatial distributions of bedded evaporites allow us to investigate tectonic changes to the basin during and after the deposition of the WPM. Here, we integrate these data to interpret changes in lake-level, salinity, and hydroclimate of ancient Lake Gosiute during the Early Eocene. 

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2. Spatiotemporal quantification of organic matter accumulation in the Eocene Green River Formation, Bridger Basin, Wyoming

   https://doi.org/10.31711/ugap.v50i.117  

   Tofte, Marshal; Carroll, Alan (September 2022, Geosites)

   It has long been recognized that lakes can bury large amounts of organic carbon (CORG) in their sediment, with important consequences for conventional and unconventional petroleum resources and potentially for the global carbon cycle. The detailed distribution of lacustrine organic carbon through space and time is important to understanding its commercial and climatic implications, but has seldom been documented in detail. The Green River Formation offers a unique opportunity to improve this understanding, due to extensive Fischer assay analyses of its oil generative potential and to recently published radioisotopic age analyses of intercalat ed volcanic tuffs. Fischer assay analyses reveal distinctly different patterns of organic matter enrichment that correlate with different lacustrine facies associations. Histograms of oil generative potential for evaporative facies of the Wilkins Peak Member exhibit an approximately exponential distribution. This pattern is interpret ed to result from episodic expansion and contraction of Eocene Lake Gosiute across a low-gradient basin floor that experienced frequent desiccation. In contrast, histograms for fluctuating profundal facies of the upper Rife Bed of the Tipton Member and the lower LaClede Bed of the Laney Member exhibit an approximately normal or log normal distribution, with modes as high as 16–18 gallons per ton. This pattern is interpreted to reflect generally deeper conditions when the lake often intersected basin-bounding uplifts. Within the Bridger basin, burial of CORG was greatest in the south during initial Wilkins Peak Member deposition, reflecting greater rates of accommodation near the Uinta uplift. The locus of CORG burial shifted north during upper Wilkins Peak Member deposition, coincident with a decrease in differential accommodation. CORG burial during deposition of the upper Rife and lower LaClede Beds was greatest in the southeast, due either to greater accommodation or localized influx of river-borne nutrients. Average CORG burial fluxes are consistently ~4-5 g/m2 yr for each interval, which is an order of magnitude less than fluxes reported for small Holocene lakes in the northern hemisphere. Maximum rates of CORG burial during deposition of organic-rich mudstone beds (oil shale) were likely similar to Holocene lakes however. Deposition of carbonate minerals in the Bridger basin resulted in additional, inorganic carbon burial. Overall it appears that carbon burial by Eocene lakes could have influenced the global carbon cycle, but only if synchronized across multiple lake systems. 

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3. Paleoclimate reconstructions of the Early Miocene hominoid fossil sites from Chamtwara, Legetet and Koru, Tinderet, western Kenya

   PEPPE, Daniel; MCNULTY, Kieran P; COTE, Susanne; HEAD, Jason; MUNYAKA, Venanzio; OGINGA, Kennedy Ogonda; KINYANJUI, Rahab N; LUKENS, William E; LUTZ, James A; TEGART, Amanda (October 2023, Geological Society of America Abstracts with Programs)

   Hominoids exhibited high diversity in the early Neogene. The Early Miocene, in particular, is the inferred timing of the origin of the crown Hominoidea. Thus, understanding the paleoecology and paleoenvironments of the Early Miocene is critical for understanding the selective pressures that led to the evolution and diversification of hominoids. The Early Miocene fossil sites of Koru, Legetet, and Chamtwara occur on the southwestern flank of the now-extinct Tinderet volcano in western Kenya. While not as well- known as the Songhor site on the northwestern flank of the same volcano, the Chamtwara, Legetet, and Koru sites surrounding the village of Koru document surprising taxonomic diversity of Early Miocene hominoids; yet relatively little paleoenvironmental work has been conducted to contextualize this taxonomic diversity. Our recent geological, paleontological, and paleoenvironmental work has focused on reconstructing the paleoclimate and paleoecology of these fossil sites using a variety of proxies. Sedimentological analyses of the fossil sites indicate periodic disturbance of the landscape due to volcanic activity and that most of the fossiliferous strata are moderately to poorly developed paleosols and fluvial channels. Paleosol features are nearly identical across sites and demonstrate relatively wet and well-drained conditions with some evidence of seasonality and/or periodic water deficit. Paleosol based proxies for paleoclimate indicate wet conditions with mean annual precipitation greater than 175 cm/year. Paleobotanical proxies from fossil leaves and fossil tree stump casts indicate a warm and very wet climate indicating a tropical seasonal forest to tropical rainforest biome. Paleoclimate estimates based on habitats of extant relatives and vertebral ecomorphology of fossil snakes similarly indicate very warm and wet conditions consistent with tropical rainforests. Faunal analyses of the mammal community composition and dietary ecology also indicate forested environments. Taken together, our multi-proxy reconstructions of paleoclimate and paleoenvironment indicate that the Chamtwara, Legetet, and Koru sites were warm and very wet forested habitats connecting early hominoids to closed habitats. 

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4. Staying Grounded: Establishing Baseline Critical Zone Conditions in Mississippi Prior to the Onset of Paleocene-Eocene Hyperthermals

   Rathbun, S.; Platt, B.F. (January 2021, AGU Fall Meeting)

   Paleocene-Eocene hyperthermals are viewed as some of the best ancient analogs for projected future anthropogenic climate change. In order to fully evaluate the magnitude of these climactic perturbations, however, a more complete understanding of prevailing background conditions is necessary. The Mississippi Embayment, a major southwest-dipping sedimentary basin in the Gulf of Mexico coastal region of North America, contains an extensive record of Paleocene strata deposited prior to the onset of the Paleocene Carbon Isotope Maximum (PCIM), a gradual warming trend upon which the Paleocene-Eocene Thermal Maximum (PETM) was superimposed. In order to evaluate pre-PCIM paleoclimate, we focus on paleosols in the Upper Paleocene Naheola Formation. A continuous section of the Naheola is available in archival core collected by Mississippi Minerals Resources Institute from Tippah County, Mississippi, USA. We performed a suite of initial core description methods, including logging of visual observations (e.g., grain size and Munsell colors), gamma density, magnetic susceptibility, smear slide analysis, and scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS). Results indicate a > 8-m-thick interval of 5 stacked paleosols associated with 4 lignite seams. The paleosols range in thickness from 0.6 m to 1.9 m, while the lignite seams range in thickness from 0.3 m to 1.3 m. Paleosols are characterized by low chroma matrix colors, mottling, and abundant carbonized roots. The thickest paleosols each exhibit an interval that coarsens and then fines upward; these are likely composite paleosols. Applying SEM-EDS results from all paleosols to the chemical index of alteration minus potash (CIA-K) yields preliminary mean annual precipitation estimates between 1200 and 1300 mm. The oldest paleosol contains abundant kaolinite and future stable isotope analysis will be used to reconstruct paleotemperature. Ongoing work will evaluate the relative influence of each of the five soil-forming factors on Naheola paleosol development and reexamine Paleocene- Eocene hyperthermals within the context of our results. Future work will include pollen analysis to improve chronostratigraphic control and evaluate paleoecological response to the Paleocene- Eocene climate change. 

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5. Reconstructing the climate and ecology of an early Miocene tropical forest on the flanks of the Tinderet Volcano, Nyanza Province, Western Kenya

   Munyaka, Venanzio; Oginga, K; Cote, Susanne; Head, Jason J; Lukens, William; Lutz, James; Kinyanjui, Rahab N; Manthi, Fredrick Kyalo (July 2024, Eastern African Association of Palaeoanthropology and Palaeontology)

   Tectonically driven physiographic evolution in early Miocene of eastern Africa significantly shaped landscapes, climates, and vegetation, resulting in habitat heterogeneity. Early hominoids inhabited these landscapes, and their evolutionary history was likely influenced by these heterogenous environments. In western Kenya, around the extinct Tinderet Volcano (ca. 19-21Ma), fossil-rich exposures offer crucial insights into this history with evidence of early hominoids. Here we use analyses of sedimentology, paleosol paleoclimate proxies, fossil leaves, and forestry metrics, to reconstruct the paleoclimate and paleoecological reconstruction of the Koru-16 fossil site. Sedimentological and stratigraphic analyses at Koru-16 reveal a landscape marked disturbance created by periodic volcanic eruptions and stable intervals marked by moderately to poorly developed paleosols. Paleoclimate reconstructions based on paleosol geochemistry indicates warm and wet conditions. Over 1000 fossil leaves were collected from the Koru-16 site, representing 17 morphotypes across two stratigraphic intervals. Mean annual precipitation estimates based on leaf size of shape indicate >2000mm/yr. Leaf lifespan reconstructions reveal predominantly evergreen taxa with a distribution leaf lifespan, similar to modern equatorial African rainforests. Fossil tree stump casts suggest an open forest, similar to contemporary tropical forests supporting large-bodied primates. Importantly, fossil leaves, the tree stump casts, a medium-sized pythonid, and multiple specimens of large-bodied primates occur in the same stratigraphic layer demonstrating their cooccurrence in the Koru-16 ecosystem. The multi-proxy paleoclimate and paleoecological reconstructions for Koru-16 converge on a very wet and warm climate supporting a closed, tropical seasonal forest to rainforest biome. This environment likely provided an ideal habitat for early hominoids, emphasizing the role of forested habitats in their early Miocene evolution. Additional work is ongoing on refining the paleosol paleoclimate estimates with a more recent model and δ13C analysis of soil organic matter will help to further refine these reconstructions. 

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