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Mesoproterozoic lacustrine clay-mineral record of weathering prior to greening of the continents

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posted on 2022-11-11, 00:59 authored by Cassandra A. Wheeler

While the evolution of land plants has long been considered a step-change to the continental weathering regime, the bulk composition of sedimentary sequences has remained largely unchanged since the emergence of the first continents. However, a key shortcoming of conventional characterisation techniques is the inability to differentiate sedimentary constituents and geochemical signals that are products of weathering versus post-depositional alteration. This research investigates alluvial and lacustrine mudrocks from the Precambrian Wester Ross Supergroup (NW Scotland), which record a snapshot of weathering prior to the greening of the continents. I employ a novel petrographic approach, combining SEM-EDS mineral mapping and TEM imaging, permitting the quantitative differentiation of detrital phases, overcoming the limitations of previous studies. I find that detrital clays (illite, chlorite) are consistently of physical weathering origin, anomalously abundant μm-size feldspar grains, and an absence of chemically weathered clays. This indicates subaerial weathering was controlled by physical mechanisms at this time, in contrast to mudstones deposited in vegetated Phanerozoic environments, arguing for a weak chemical weathering regime in the Proterozoic. I conclude that the lack of a plant weathering signal in bulk rock studies is due to the pervasive post-depositional alteration of chemically immature sediments during ‘subsurface weathering'.


Table of Contents

1. Introduction -- 2. Geological context and background -- 3. Methods and materials -- 4. Results -- 5. Discussion -- Conclusion -- 6. References -- 7. Supplementary material


A thesis submitted in partial fulfillment of the requirements for the degree of Master of Research

Awarding Institution

Macquarie University

Degree Type

Thesis MRes


Thesis (MRes), Macquarie University, Faculty of Science and Engineering, 2022

Department, Centre or School

Department of Earth and Environmental Sciences

Year of Award


Principal Supervisor

Stefan Löhr

Additional Supervisor 1

April Abbott


Copyright: Cassandra A. Wheeler Copyright disclaimer:




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