175-2 A Late Tonian Lagerstätte: Pickled Vase-Shaped Microfossils From the Chuar Group, Grand Canyon, USA

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

Presenting Author:

Authors:

Abstract:

Vase-shaped microfossils (VSMs) occur prolifically in late Tonian (ca. 789–729 Ma) cherts, phosphatic nodules, carbonates and shales. Though tests are inferred to have been originally organic in life, they are typically preserved as siliceous, phosphatic, calcareous or pyritic casts. Recent work has confirmed the presence of organically-preserved VSMs from mudstones of the upper Awatubi and lower Walcott members of the Chuar Group, Grand Canyon, Arizona, suggesting special preservational conditions at work. At macroscopic scale, these organic VSMs are co-preserved with jarositized VSMs, which were initially pyritized and subsequently oxidized to jarosite—a ferric-potassium hydroxide sulfate [KFe₃(SO₄)₂(OH)₆] typically formed under evaporative, oxic, and limited-water environments with a pH range of 1.0–3.0. Associated evidence for hypersaline environments includes halite casts, mudcracks, and elevated levels of gammacerane—a biomarker derived from ciliates living at redox boundary in stratified water column. Bulk clay analyses of shales preserving organic VSMs reveal a high amount of illite/montmorillonite, which tends to form in evaporative, dry and cold environments. Some jarositized VSMs walls retain a central organic layer, with the thicknesses of inner and outer jarosite layers strongly correlated (R² = 0.88), suggesting that the organic walls of VSMs were replaced at comparable rates both inward and outward. In addition, automated mineralogy mapping and Raman spectroscopy of jarositized VSMs reveal anhydrite surrounding the outer test wall and coating the organic blob within the fossil chamber. Energy dispersive X-ray spectroscopy (EDS) spot analyses reveal high silicon and aluminum contents on the test wall and clay minerals within organic VSMs, highlighting the role of aluminosilicification in preserving organic material. Based on these observations, we propose these organisms lived in or were transported into mat-rich ponds, where their tests were immersed in an organic-rich fluid. Where iron was present, the walls were partly or fully pyritized. Subsequent evaporation of the pond and oxidation of pyrite created a highly acidic and hypersaline microenvironment, promoting the shrinkage of organic material within the test and concomitant deposition of silica and anhydrite within the fossil chambers. Finally, illite filled the remaining voids within the chambers. These findings support the hypothesis that VSMs may have inhabited terrestrial environments during the late Tonian Period, and support hypotheses that suggest arid and possible icy conditions at equatorial latitudes during this time.

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

doi: 10.1130/abs/2025AM-9048

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