70-13 Towards a detailed record of Pacific Northwest ecosystem change: Middle Miocene phytoliths from Cave Basin, Central Oregon

Session: Paleontology, Biogeography/Biostratigraphy & Phylogenetic/Morphological Patterns (Posters)

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Abstract:

The pace at which global temperatures are currently increasing makes it particularly urgent to understand how vegetation and faunas will adapt to those changes. The Cenozoic offers several instances of climate change during which such biotic responses can be studied. The late Oligocene and Miocene (30–5 Ma) are characterized by major global changes in vegetation as temperatures of the icehouse Oligocene steadily increased into the Miocene Climatic Optimum (17–14 Ma) before cooling and drying in the late Miocene. Although broad trends in vegetation and faunal responses are known from US sites, detailed regional records of coupled floral and faunal information are rare and understudied. The US Pacific Northwest (PNW) preserves a well-resolved fossil record of the climatically volatile Oligo-Miocene suitable for studying the evolution of ecological communities. However, ecosystem evolution is debated based on different lines of fossil evidence. Thus, macrofloras indicate the persistence of forested habitats throughout the Cenozoic while paleosols point to desert shrublands by the Oligocene followed by the spread of forests in the mid-Miocene. In contrast, sparse preliminary phytolith data support the presence of forests in the early and middle Miocene but also show that diverse grass communities might have been locally dominant during this time and again in the late Miocene.

Our project seeks to develop a comprehensive sequence of phytolith assemblages that will test these alternate scenarios for Oligocene-Miocene vegetation change in the PNW. Here, we present new phytolith records from Cave Basin, part of the Crooked River Basin of central Oregon, of the Mascall Formation that fill gaps in the vegetation history of the PNW during the height of the middle Miocene. Results of this study will be compared to phytolith assemblages from Coglan Buttes of south-central Oregon, dated to be earliest Miocene. Phytoliths will be used to infer vegetation structure and composition with two different approaches: reconstructed leaf area index and assemblage composition using plant functional types. Both Cave Basin and Coglan Buttes also contain productive faunal horizons that suggest some key taxonomic and ecological differences from other well-studied records of this time interval, yielding a PNW record of paleoecosystems that reflects the region’s unique history.

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

doi: 10.1130/abs/2025AM-9267

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