184-5 Petrographic and Isotopic Constraints on the Processes and Environments of Miocene Lacustrine Carbonate Formation on the Colorado Plateau

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources (Posters)

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Abstract:

Lacustrine carbonates are valuable sedimentary archives of paleoenvironmental conditions such as climate, hydrology, and biogeochemical cycles. However, interpretations can be complicated by physical, chemical and biological influences on carbonate formation, deposition, and post-depositional alteration. Here we investigate the origins and isotopic variability of fine-grained carbonates in well-preserved Miocene lacustrine marls from the Bidahochi Formation of the Colorado Plateau. Using detailed petrography alongside δ¹³C, δ¹⁸O, and dual clumped isotope (Δ47, Δ48) thermometry, we assess how biological processes and early diagenesis may affect geochemical data for paleoenvironment reconstructions. 

Collected from the profundal part of the basin, we selected four fine-grained carbonate samples for isotopic analyses, each lacking diagnostic features in hand sample but exhibiting textural diversity at the microscale. Petrographic analysis revealed abundant micritic cement or carbonate mud, peloids, and biologically derived structures such as cyanobacterial filaments.  Micrite is commonly found in fine-grained lacustrine carbonate rocks and is proposed in recent work to form through both inorganic and biogenic mechanisms. Chemically or biochemically mediated micrite may form in the water column during “whiting events”, or at the sediment-water interface and below, as microbial degradation of EPS (Extracellular Polymeric Substances) or algal ooze triggers carbonate precipitation independently of broader climate or hydrologic factors. Although some observed micrite could have precipitated in the water column, the peloids and biologically derived components suggest significant carbonate precipitation at or below the water-sediment interface and in pore spaces after deposition–lake-bottom environments that may differ significantly from lake surface water and air temperatures. Understanding these factors and their impact on carbonate proxy measurements is crucial for accurate paleoenvironment interpretations from stable isotope records. This significant presence of micrite and biologically derived structures motivated us to investigate these samples further for biologically mediated origins that may cause carbonate to precipitate out of isotopic equilibrium as shown by Δ47-Δ48 data and alter paleoclimate and hydrologic interpretations of this basin. Coupled with our observations of microbial micrite and peloids, bulk isotope analyses for carbonate δ¹³C and δ¹⁸O revealed both subtle and substantial intra-sample variability. One sample had an intra-sample standard deviation for δ¹⁸O of >4‰, suggesting significant heterogeneity in carbonate formation and early diagenetic conditions. These results underscore the importance of evaluating microbial and diagenetic processes when interpreting isotopic signals in lacustrine carbonate paleoenvironment records.

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

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