175-1 Exploring the environmental context of a silica-rich fossiliferous Tonian deposit

Session: The Neoproterozoic Earth and Life Co-evolution, Part I

Presenting Author:

Kelsey Moore

Authors:

Moore, Kelsey R¹, Helms, Lucy², Girard-Tsuru, Maelle³, Pavia, Frank⁴, Khan, Sabrina⁵, Sanders, Cecilia⁶, Talgatbek, Aibolat⁷, Mustapayeva, Sezim⁸, Mamanov, Yerkhozha⁹, Smith, Emily F¹⁰

(1) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (2) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA; Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA, (3) Baltimore Polytechnic Institute, Baltimore, MD, USA, (4) School of Oceanography, University of Washington, Seattle, WA, USA, (5) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA, (6) Department of Geology, University of Maryland, College Park, MD, USA, (7) Department of Geological Survey, Satbayev University, Almaty, Kazakhstan, (8) Department of Geological Survey, Satbayev University, Almaty, Kazakhstan, (9) Department of Geological Survey, Satbayev University, Almaty, Kazakhstan, (10) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA,

Abstract:

The Tonian is a key interval in the evolution of life and the environment, characterized by potential evolutionary and ecological shifts among eukaryotes. However, there remain questions surrounding the drivers of these biological and ecological shifts as well as the environmental context in which they occurred. This is due, in part, to the Tonian fossiliferous strata being biased towards low temperature shallow marine depositional environments. One fossiliferous Tonian deposit, the >759 Ma Chichkan Formation in southern Kazakhstan, differs from other Tonian sites globally in that it is pervasively silicified, likely the result of a distinct depositional environment. The classification and genesis of chert within the Chichkan Formation – and the implications that these hold for paleoenvironmental conditions, paleoecology, and microfossil preservation – have not been previously studied.

Here, we integrate field, petrographic, and geochemical data from the major silica-rich facies of the Chichkan Formation to classify the different types and generations of silica, explore silica formation mechanisms, and constrain the timing and conditions under which different types of silica formed. Together, our results suggest that the Chichkan Formation contains a combination of primary and diagenetic silica, indicating that this unit formed in an environment with high concentrations of dissolved silica. With our integrated petrographic and geochemical data sets, we explore alternative depositional environments for the Chichkan Formation, including hydrothermally influenced environments. By characterizing the environmental context of this Tonian deposit and the diverse microfossil assemblage that it preserves, we expand our understanding of the environmental context for Tonian eukaryotic and microbial ecosystems and the processes that facilitated fossil preservation.

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

doi: 10.1130/abs/2025AM-5785

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