Metazoan activity facilitates passive sediment trapping in modern supratidal microbialites: Revealed using µ‐CT‐scanning and microscopy

Abstract

Microbialites are formed through two processes, the trapping and binding of sediment grains and mineral precipitation. Sediment trapping and binding result in coarse sandy textures, whereas fine micritic textures are produced by mineral precipitation.Although well-studied modern microbialites (e.g. Bahamas and Shark Bay) are formed through the former process, purely trapped-and-bound examples are rare throughout the geologic record and limited to shallow-marine environments. Through the use of conventional microscopy and 3D micro-computed tomography (µ-CT) scanning, this study found that South African coastal microbialites have a primarily mineral precipitated texture, although detrital material is included sporadically. Furthermore, these modern microbialites exhibit both micritic and fibrous layering with high porosity. The novel use of 3D microtomography rotational scans has revealed that microbialites are extensively burrowed by metazoan activity and was also able to distinguish the occurrence and distribution of heavy minerals and detrital shell material in the samples. Some of the detrital grains appeared to be accidental/random inclusions, while in other cases the metazoan burrows provided space for the accumulation of sediment once abandoned. In both cases, sediment was incorporated as a product of intermittent accumulation, rather than systematic trapping and binding. The microfabric texture of South African microbialites is therefore a function of both biological (e.g. microbially mediated precipitation) and environmental (e.g. sporadic sediment deposition and inorganic cementation) influence. Overall, the findings presented here highlight the importance of these systems regarding microbialite formation, ichnology and taphonomy.

Reviews and syntheses: Tufa microbialites on rocky coasts towards an integrated terminology

“…10). Blister mats accrete on low gradient substrates subject to frequent subaerial exposure (Dodd et al, 2021;Edwards et al, 2017) and Edwards et al (2017) suggest that their formation is a result of subaerial exposure of microbial mats, splitting domal structures and releasing trapped gases produced by the mat decomposition, as described by Gerdesi and Krumbein (1994).…”

“…This thrombolitic internal organisation has multiple geneses related to the trapping and binding of peloids (Castro-Contreras et al, 2014); the morphology of the microbialite builders (Kennard and James, 1986), and the disruption, destruction, and alteration of stromatolitic internal organisation by biota: bioturbation (Walter and Heys, 1985). The latter has been suggested as a mechanism for the formation of thrombolites in South African rock coast microbialites, predominantly by metazoans and potentially by foraminifera and coccoids (Dodd et al, 2021;Weston et al, 2018). This may also be involved in the genesis of leiolitic (sensu Braga et al, (1995)), or structureless, mesofabrics (Dodd et al, 2021).…”

“…The latter has been suggested as a mechanism for the formation of thrombolites in South African rock coast microbialites, predominantly by metazoans and potentially by foraminifera and coccoids (Dodd et al, 2021;Weston et al, 2018). This may also be involved in the genesis of leiolitic (sensu Braga et al, (1995)), or structureless, mesofabrics (Dodd et al, 2021). While stromatolitic, thrombolitic and leiolitic internal organisation has been observed in the rock coast setting, future work is required to describe these structures, and further subsets (e.g., dendrolites), before they can be confidently attributed to process.…”

“…These structures are similar to the gas domes described by Noffke (2010) which can blister or collapse, releasing the trapped gas and resulting in a crater-like morphology. These structures may be recolonised by microbial mats (Lakhdar et al, 2020) potentially resulting in laminoid and irregular fenestrae in section as suggested by Dodd et al (2021). Smith et al (2020) describe wave rip-up clasts of stromatolite within rock pools and areas stabilised by vegetation on a shore platform setting on the South African coast, that form due to increased wave energy and/or water level during storm conditions.…”

Reply on RC1

“…This thrombolitic internal organisation has multiple geneses related to the trapping and binding of peloids (Castro-Contreras et al, 2014); the morphology of the microbialite builders (Kennard and James, 1986), and the disruption, destruction, and alteration of stromatolitic internal organisation by biota: bioturbation (Walter and Heys, 1985). The latter has been suggested as a mechanism for the formation of thrombolites in South African rock coast microbialites, predominantly by metazoans and potentially by foraminifera and coccoids (Dodd et al, 2021;Weston et al, 2018). This may also be involved in the genesis of leiolitic (sensu Braga et al, (1995)), or structureless, mesofabrics (Dodd et al, 2021).…”