141-2 Problematic cylindrical microbially-influenced sedimentary structures of the Tonian Chichkan Formation, Kishi (Lesser) Karatau, Kazakhstan

Session: New Advances in Geobiology

Presenting Author:

Cecilia Sanders

Authors:

Sanders, Cecilia B¹, Mustapayeva, Sezim², Mamanov, Yerkhozha³, Talgatbekov, Aibolat⁴, Khan, Sabrina⁵, Smith, Emily F⁶

Abstract:

The late Tonian Chichkan Formation of southern Kazakhstan is remarkable for its well-preserved chert-permineralized microfossil assemblage, but it also hosts interesting macro-scale bio-sedimentary features. These include silicified, putatively microbially-influenced sedimentary structures of a unique and problematic laminated cylindrical morphology. In this study, we perform a critical examination of these structures and associated sedimentary features, using sedimentological and stratigraphic observations from outcrop to micro-scales to constrain their depositional context and test hypotheses about their origin and paleoenvironmental meaning. Ultimately, we find that the presence of bent (flexibly deformed) and broken (brittlely deformed) silicified laminae and intraclasts, combined with intercalated silts and fine sands, imply an intertidal-to-peritidal depositional setting. We find that the problematic structures’ 3D and cross-sectional geometry, grouping, orientation, and size distribution are inconsistent with their interpretation as stromatolites, roll-up structures, or oncoids. We instead propose a formation mechanism wherein cohesive microbial mats in an intertidal marine environment, characterized by variable siliciclastic sediment input and episodic changes in wave energy and water depth, accreted sediment and organic material around cylindrical masses whose shoreline-parallel orientation and shoreline-perpendicular motion was maintained by bidirectional flows (tides). We also suggest an alternative nomenclature consistent with this formation model: spitcake structures, named for the coaxially laminated dessert.

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

doi: 10.1130/abs/2025AM-6746

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