148-8 Microfossil biostratigraphy and sedimentology of the lower Eocene Bolado Park Formation, California: an update

Session: Insights from Microfossils and Their Modern Analogs: From Traditional to Emerging Approaches (Posters)

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In recent years, interest in the study of and early Eocene sediments has increased, because these sediments were deposited during a peak in global Cenozoic warming that may be similar to projected future climate change.  However, comparatively few analyses have examined the Eocene from the West Coast of North America.  These sediments occur throughout California in fault blocks on either side of the San Andreas Fault system.  To better understand the global nature of Eocene hyperthemal events, modern taxonomy, sedimentology, and geochemical analysis are being applied to individual sections then correlated to the modern Geologic Time Scale. This requires re-examination of outcrops and formations from California that have not been studied in decades with the goal of correlation to previously published global sections.

Thirty-seven samples from the Bolado Park Formation, Tres Pinos section, located near Hollister, California, were examined for calcareous nannofossil biostratigraphy and paleoecology, and grain size analysis. The basal Bolado Park Formation (~15 m thick) consists predominantly of 80% blue-gray clay and silt that grades up to an ~80 m thick mottled mudstone with varying amounts of very fine to medium sand (10-60%). Samples contain abundant calcareous nannofossils and assemblages are assigned to calcareous nannofossil Zone NP12 based on the occurrence of Discoaster lodoensis with rare occurrences of Tribrachiatus lunatus.  Assemblages are diverse, allowing for the identification of species not previously recorded from California sediments. They provide information regarding the global reproducibility of acme events for biostratigraphy, timing on the evolution of stellate Discoasters, and documentation of radiation of the genus Blackites during the early Eocene.   Additionally, high abundances of Zygrhablithus bijugatus and Sphenolithusspp. indicate that deposition occurred in warm water with rapidly increasing nutrient availability.  Foraminferal assemblages indicate deposition below the CCD with high organic input and considerable downslope transport, which may account for the sands present in the mudstone and supports  the hypothesis that this site records intensification of the hydrologic cycle during the early Eocene.

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

doi: 10.1130/abs/2025AM-11335

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