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

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Monday, 20 October 2025
8:00 AM - 12:00 PM
HBGCC, 304B

Let’s Work Together: A Large Upper Cambrian Microbial-Lithistid Sponge Bioherm from the Tamarack Canyon Dolomite, East-central California, USA

Paper number: 99-5
Program: Technical Sessions
Time: 9:10 AM – 9:25 AM

Authors

Abstract

The first complex metazoan buildups appear during the lower Cambrian (Terreneuvian, Stage 2; ~525 Ma), and occur as small, isolated mounds (bioherms) constructed by archaeocyathids. Archaeocyathids went extinct towards the end of early Cambrian (Series 2, Stage 4; ~508 Ma), and as a result, the remainder of the Cambrian was dominated by m-scale microbial bioherms that sometimes contain associated lithistid sponges, which later became major framework builders in the Lower Ordovician. This research focuses on bioherms of the Tamarack Canyon Dolomite, which was deposited in the late Cambrian (Furongian), and is located in the Inyo Mountains of east-central California. The Tamarack Canyon Dolomite at the study locality is up to 277 m thick and is primarily made up of laminated to thick-bedded dark gray dolomite that typically weathers to a dull grey. Several mounds are present in the study area, including a large bioherm that is 44 m wide and 16 m in overall thickness. The mound and surrounding rocks were divided into several microfacies based on field and petrographic observations, including: 1) thrombolitic mound facies that comprise the bulk of the mound interior, along with 2) stromatactis-bearing dolowackestone; 3) bedded dolostone that drapes the edges of the mound; 4) oolitic dolograinstone that underlies the mound; 5) fossilferous dolopackstone comprised of peloids and fossil hash made up of a mixture of echinoderm ossicles, trilobite debris, and rare nautiloids that occurs as pockets within the mound, as well as in beds surrounding the mound; and, 6) paleokarst breccias that crosscut the mound and surrounding facies. The bioherms formed in a shallow, high-energy setting based on the oolitic dolograinstone that commonly underlies and drapes the buildups. These observations and previous research suggest that upper Cambrian bioherms were more complex and formed larger structures than previously thought following the extinction of archaeocyathids and represent a transitory period between buildups dominated by microbialites in the aftermath of the extinction of archaeocyathids and the lithistid sponge reefs that were dominant in the Lower Ordovician.

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

doi: 10.1130/abs/2025AM-413710

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