A Multi-Scale Characterization of Thrombolites from the Late-Ediacaran Nama Group, Namibia: Towards Systematic Identification in the Sedimentary Record

Microbialites are organosedimentary carbonate structures formed by the trapping, binding, and precipitation of minerals by microbial communities. They are critical tracers of life in Earth’s history, representing the oldest evidence for life (~3.45 Ga) and persisting today in the sedimentary record as important indicators of environmental conditions and biological activity. Despite their significance, some classes of microbialites remain poorly understood. Thrombolites, which are generally characterized by clotted textures on the mm- to cm-scale, are an enigmatic class of microbialites for which the formative processes, environmental controls, and diagnostic criteria remain inadequately constrained. As a result, thrombolites are commonly misidentified as texturally similar facies, such as grainstones, or overlooked altogether. Therefore, further detailed studies focusing on thrombolite characterization are imperative for the development of a systematic identification system and to ensure their reliable identification in the rock record. In southern Namibia, the late-Ediacaran middle Nama Group features abundant thrombolite-dominated patch reefs, and other thrombolite-like facies, deposited in a restricted foreland basin. Often, these thrombolites can be found in association with fossils of the earliest carbonate biomineralizing metazoans, such as Cloudina, causing their particular interest in recent literature. In particular, the Huns Member of the Witputs sub-basin hosts abundant thrombolite facies that can be defined on the outcrop-scale according to their clot size. We aim to provide a detailed, multi-scale (i.e., micro- to outcrop-scale) characterization of the diverse thrombolitic textures observed in the Huns Member of the Nama Group. The overarching aim of this project is to integrate high-resolution sequence stratigraphy alongside laboratory techniques such as optical microscopy, scanning electron microscopy (SEM), trace element analysis by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICPMS), and other isotopic analyses to characterize the textures, associated inorganic features, and geochemical signatures of these thrombolites. These signatures will then be used to infer paleoenvironmental parameters such as sea level, temperature, and redox conditions during thrombolite growth, and possibly even the metabolisms of the microbes responsible for their formation. Here, we present optical microscopy and SEM analyses of thrombolitic textures from the Huns Member within a sequence stratigraphic context. Based on preliminary data, we hypothesize that the morphology of thrombolitic textures is controlled by three factors: microbial influence, sedimentation rate, and cementation. We hope to quantify how thrombolite morphology is affected by these factors and ascribe facies accordingly. Ultimately, this work will establish a consistent, process-based framework for defining thrombolites and interpreting their paleoenvironmental significance.