174-9 Early Eocene Fossil Floras from the Lower San José Formation, San Juan Basin, New Mexico, USA

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems

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The early Eocene was marked be a series of rapid carbon cycle perturbations and hyperthermal events superimposed on globally warm climate, presenting excellent deep-time analogs for modern anthropogenic climate change. The best-known terrestrial record of these early Eocene hyperthermal events, including the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM2), is from the Bighorn Basin of north-central Wyoming, where mesic temperate plant taxa were temporarily extirpated and replaced by thermophilic, dry-adapted plants, especially legumes (Fabaceae). The rarity of fossil floras from southern North America with good age control inhibits our understanding of the biogeographic origins of these hyperthermal plants, as well as latitudinal differences in floral composition during the Early Eocene. The San Juan Basin in northwestern New Mexico preserves an extensive, but understudied, succession of early Eocene terrestrial deposits making it an ideal region for studying early Eocene floras from southern North America.

The lower San José Formation in the San Juan Basin of northwestern New Mexico consists of over 200 m of fossiliferous fluvial deposits constrained to the first approximately 2.2 million years of the Eocene (56.0-53.8 Ma) via magnetostratigraphy (i.e., magnetochrons C24r-C24n.3n), mammalian biostratigraphy (i.e., early Wasatchian NALMA), and palynostratigraphy. Fossil macro- and microfloras were collected from 35 sampling localities across the southeastern San Juan Basin. Fossil macrofloras are dominated by broadleaf flowering plants, with palms, ferns, and Ginkgo as accessory taxa. Legume foliage and reproductive structures are common within the macroflora, and taxa restricted to hyperthermal intervals in the Bighorn Basin are found outside of hyperthermal events identified in the San Juan Basin. Additionally, the macroflora shifts from a sycamore-dominated to legume-dominated flora through time suggesting increased seasonality and aridity during the early Eocene similar to paleosol derived paleoclimate records for the San Juan Basin. The microflora contains numerous early Eocene indicator taxa, including Platycarya platycaryoides, as well as several taxa indicative of hyperthermal events in the Bighorn Basin. This preliminary floral record suggests the southern Rocky Mountains were a source for taxa restricted to hyperthermal events in the Bighorn Basin and that there were substantial latitudinal differences in early Eocene floral composition.

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

doi: 10.1130/abs/2025AM-7593

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