MOTTLED MUDSTONE OF THE BOLADO PARK FORMATION, CALIFORNIA, AND EARLY EOCENE HYPERTHERMAL EVENTS

Continental margins are an ideal setting to study the environmental impact of global warming, only a few studies have focused on the Eocene Thermal Maximum 3 (ETM3) or the Early Eocene Climatic Optimum (EECO). The Bolado Park Formation exposed near Hollister, California, is a geographically restricted formation, characterized by red and green mudstones. Mottled mudstones are often associated with deposition below the CCD in areas affected by intensified hydrologic cycles.

Based on microfossil assemblages, the basal Bolado Park Formation is early Eocene in age and includes rare reworked Cretaceous calcareous nannoplankton and a few longer ranging agglutinated foraminifers. The agglutinated foraminifers have dissolution-resistant tests that suggest deposition at lower bathyal to abyssal depths, below the carbonate compensation depth (CCD). Assemblages above this basal interval become more abundant and diverse and are assigned to early Eocene calcareous nannofossil Zones NP11 and NP12. Foraminiferal assemblages alternate between 1) an assemblage with low foraminiferal numbers, common dissolution resistant agglutinated foraminifers, and reduced numbers of calcareous foraminifers, and 2) an assemblage with higher foraminiferal numbers, common calcareous species, reduced agglutinated species, and increased opportunistic species that require a higher food supply. This indicates deposition occurred below the CCD in the basal part of the Bolado Park Formation under dysoxic conditions. Stratigraphically higher assemblages suggest more suboxic conditions as the CCD fluctuates and organic input increases.

Based on the age of the Bolado Park Formation, deposition occurred during the ETM3 through the onset of the EECO and represents a peak in Early Cenozoic global warming. This idea is supported by the rapid increase in NP11 of Sphenolithus spp., a calcareous nannofossil genus thought to be indicative of warmer water. This peak is followed by an increase in Zygrhablithus bijugatus, a species possibly indicative of rapidly increasing nutrient availability. Thus, the mottled mudstones of the Bolado Park Formation and associated microfossil assemblages support deposition in a rapidly warming environment in which climatic conditions were strongly affected by intensification of hydrologic cycles and an increased supply of organic matter during the EECO.