99-1 Taphonomy and Paleoenvironmental Significance of Silicified Cyanobacteria Fossils in the Early Cambrian Harkless Formation, Nevada, USA

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part I

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Constraining the diversity, abundance, and spatial distribution of marine primary producers through time is critical for understanding past marine biogeochemical cycling and ecology. Biomarker, geochemical, and some fossil evidence suggests that primary producers in deep time experienced a transition from cyanobacterial- to algal-dominated ecosystems from the Neoproterozoic into the early Phanerozoic. However, the exact timing, dynamics, and drivers of this transition are not well understood. This is due in part to the limited preservation of soft-bodied primary producers in the fossil record. Here we document a new Cambrian (Stage 4, ~515 million years ago) occurrence of abundant silicified cyanobacteria fossils from the mixed carbonate-clastic succession of the Harkless Formation of southern Nevada, USA. These fossils provide a rare glimpse into Cambrian primary producers and the paleoenvironmental conditions that can facilitate soft-bodied cyanobacteria preservation.

The upper Harkless Formation exposed near Gold Point, NV has been the subject of many paleontological studies since it contains well-exposed archaeocyathan reefs and fossil assemblages representing diverse benthic communities. Our examination of two limestone lenses located below the reefs within this predominantly shale succession revealed the presence of over 500 examples of Obruchevella, a helically coiled cyanobacteria, and Girvanella, a filamentous cyanobacteria. Dissolution of limestone samples in dilute acetic acid yielded examples of these cyanobacteria in residue. Analysis of these fossils and their thin sections using scanning electron microscopy (SEM-EDS) and Raman spectroscopy indicates that they are silicified in a carbonate matrix and that they preserve remnant organic matter within the cyanobacterial sheath. We suggest that the remarkable and abundant preservation of Obruchevella and Girvanella occurred in mass flows that transported filaments to deeper waters and produced deposits with relatively higher porosity. Once transported, low oxygen and Fe- and Si-rich bottom waters slowed decay of the filaments and facilitated early silicification of these organisms. Given that no other Harkless Fm fossils in nearby beds appear to have experienced early silicification, it is possible that the specific biochemistry of cyanobacterial sheaths made this style of preservation exclusive to cyanobacteria. This unique taphonomic window in the Harkless Fm allows us to infer that cyanobacteria were key primary producers in the Harkless ecosystem and perhaps early Cambrian ecosystems more generally.

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

doi: 10.1130/abs/2025AM-8631

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