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Advancements in automated mineralogy offer an opportunity to develop new approaches to the study of fine-grained sedimentary lithologies including paleosols and pedogenic minerals that hold valuable paleoclimate information. Automated mineralogy is a non-destructive analytical technique that relies on BSE imaging with spectral data to output multimodal hypermaps. Spatial domains are delineated and assigned mineral phases using whole spectrum best matching to reference spectra, providing quantitative sample composition estimates with high throughput data collection. We targeted a Spodosol in the lower part of the Upper Pennsylvanian Casselman Fm. of the Appalachian basin to evaluate the utility of automated mineralogy in determining paleosol composition. During deposition of the lower Casselman Fm., tropical climate during the Late Paleozoic Ice Age began a return to a more humid regime following the Kasimovian–Gzhelian boundary (~304 Ma) warming event. The Spodosol is a composite paleosol approximately 1.4 m thick that displays redoximorphic mottling, small scale (≤ 3 cm) slickensides and weak angular platy ped development. We performed automated mineralogy analysis on 9 paleosol samples, which were formed into 25 mm polished epoxy mounts of disaggregated peds, and generated complete mineralogical maps of the samples. These results indicate that phyllosilicate clays, mainly illite, formed the dominant mineralogic group (50-85%) with lesser amounts of quartz (~5-23%), feldspar (12-30%), carbonate (0-12%) and Fe-oxides (0-9%). Estimates of Al, Ca, Na and K from were used to determine Chemical Index of Alteration, with values ranging from 59-67. These CIA estimates tend to be quite low compared to CIA estimates determined from previous work using bulk elemental abundances by WDS-XRF (CIA >67). Further interrogation of these preliminary results revealed that interphase quartz-illite analyses were assigned a potassium feldspar interpretation. Ultimately we will combine image analysis (e.g., particle shape/habit) with new reference spectra for paleosol interphase matrix material, which together with WDS-XRF and XRD mineralogy calibration can be used to develop a robust methodology for automated mineralogy analysis of paleosols.
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Outcrop to nano-scale examination of Late Paleozoic paleosols to constrain climate and environmental indicators
Fine-grained sedimentary lithologies can be challenging to characterize but often preserve vital environmental indicators. We used cm-scale outcrop description, bulk geochemistry, and micro- to nano-scale automated mineral identification SEM and characterization analysis to investigate attributes of paleosols in the central Appalachian basin. Paleosols were selected that developed under varying climatic conditions of the latest Pennsylvanian (earliest Permian? Late Paleozoic Ice Age. In the upper Casselman Fm., paleosols exhibit redoximorphic texture (in a suspect spodosol? with cm-scale slickensides and weak ped development, with mineralogy that includes small amounts of gypsum, barite, and pyrite within the predominantly illite matrix. A histosol “underclayˮ in the Casselman shows nodular calcite within an illite matrix that is cross-cut by micro-veins of gypsum. In the overlying Monongahela Fm., within interbedded clastic and carbonate lacustrine deposits is a composite vertisol with large vertic structures and gilgai microtopography. The vertisol contains carbonate nodules in an illite/quartz matrix with disseminated dolomite and rare pyrite. Upward in the Monongahela Fm., lacustrine carbonates of the Benwood Member show evidence for pedogenesis, such as rootlets and auto-brecciation/fracturing, and weak argillan development. These carbonate paleosols show successive upward decrease in illite and quartz content, with carbonate minerals becoming increasingly dolomitic. Microfabrics and mineralogical relationships indicate a primary origin of the dolomite. Status of presenter N/A ⌘K Sign in The integration of soil macro- and micro-morphology along with mineralogy provides more comprehensive climatic and environmental indicators for this succession, and reflects a transition from paleosol interaction with sulfate-rich water toward interaction with more alkaline water over time
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- Award ID(s):
- 2219079
- PAR ID:
- 10617236
- Publisher / Repository:
- SEPM (Society for Sedimentary Geology)
- Date Published:
- Format(s):
- Medium: X
- Location:
- Flagstaff, AZ
- Sponsoring Org:
- National Science Foundation
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