99-8 Evidence of Burrowing Affecting the Morphology of Oncoids from the Lower-Middle Cambrian Carrara Formation

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

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Abstract:

Oncoids are spherical, microbially induced sedimentary structures that provide a record of the dynamic interplay among microbes, their ecosystem, sediments, and ocean chemistry. The Lower-Middle Cambrian Carrara Formation preserves multiple previously undescribed oncoid-bearing layers that preserve at least eight oncoid morphology types. We sampled and described them from the Echo Shale, Gold Ace Limestone, Pyramid Shale, and Red Pass Limestone members of the Carrara Formation at Emigrant Pass, Inyo County, CA. We collected data on in-situ oncoids (such as orientation in relationship to bedding, shape, and abundance) and their sedimentary context (such as imbrication, sedimentary structures, and ratio of carbonates vs siliciclastic sediments). Samples were polished for analysis under a microscope and for serial sectioning. Six of the morphology types are previously described from Cambrian oncoids from the North China Platform and interpreted as influenced primarily by the sedimentary processes of the environment, while the other two (shell wrap and bioturbated) preserve intricate laminations, cavities, and burrow-like structures that reflect ecological influences. These two morphology types are confined to storm lag deposits in the upper Echo Shale and lower Gold Ace Limestone. The shell wrap and bioturbated oncoids are consistently the largest morphologies, with an average size of 3.3 cm and diameters of up to 6.6 cm. The shell wrap morphology oncoids consist of coatings around shell nuclei, made up of mostly complete hyolith and bivalve shells. Lamina consistently coat the outer shell surfaces and wrap into the openings of the shells where the laminae thin. The bioturbated morphology is influenced by interconnected cavities and burrow-like tubular structures, which have been previously described in oncoids from equivalents of the Carrara Formation in Mexico and the Marble Mountains, CA. The two morphology types can exist within a spectrum, and oncoids can exhibit qualities of both. For example, within the shell wrap morphology, it is common for a burrow-like structure to connect the inside of the shell with the outside. Within serial sections, the cavities and burrow-like tubular show deformation with lamina that is consistent with maintenance of the opening by an organism. Thus, these structures may be the result of cohabitation by a benthic organism as the oncoid grew, providing new insights into benthic ecosystems of the Lower Cambrian.

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

doi: 10.1130/abs/2025AM-5071

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