Structure and formation of rock coast microbilites

Abstract

Tufa microbialites, traditionally regarded as a terrestrial carbonate facies, are increasingly recognised from the marginal-marine realm, particularly on supratidal rock coasts at the cooccurrence with groundwater springs. This thesis represents the first detailed and systematic investigation of the biogeomorphology and sedimentology of rock coast microbialites and associated tufa facies from coast of Co. Sligo, Ireland, with comparative examples in the wider north-east Atlantic region, Western Australia and south Africa.

Following a review and refinement of the current terminology applied to rock coast microbialites, multi-scale analyses, including UAV-derived orthomosaic and elevation models, sedimentological and petrographic analysis, and SEM-EDX, are undertaken to characterise supratidal tufa deposits. Bryophyte phytoherm tufa, tufa microbialite, infaunal metazoanbioturbated tufa and the newly identified Ulva tufa, alongside associated spring-carbonate facies, including associated supratidal beach cementation, are described.

Facies distribution and geomorphology are controlled by environmental parameters (elevation, slope and rock coast topography) and biological-mediator ecological requirements. The first quantified elevation distributions of supratidal tufa facies shows precipitation is confined to the shore platform: bryophyte phytoherm tufa marks the terrestrial limit (> 6 m AMSL); tufa stromatolite and infaunal metazoan-bioturbated tufas are deposited < 3 m AMSL; and Ulva tufa is restricted to < 4 m AMLS to ca. 1.6 m AMSL at the high tide, where precipitation ceases. Postdepositional modification observed in relict deposits is substantial, with infaunal metazoan bioretexturing and early diagenesis (aggrading neomorphism, cementation, dissolution and micritisation) substantially altering structure.

These studies permit the assessment of such deposits as coastal and shore-line indicators. This potential is demonstrated through sedimentological analysis of relict deposits at Cape Freycinet, Western Australia that defines a Pleistocene palaeo-shoreline. The findings of this study also highlight important distinctions between marginal-marine and terrestrial tufa deposits, further extending their global recognition; and collectively demonstrate the utility of such deposits as coastal archives.

Date of Award	Oct 2025
Original language	English
Supervisor	Joerg Arnscheidt (Supervisor) & Andrew Cooper (Supervisor)