Erosion‐initiated stromatolite and thrombolite formation in a present‐day coastal sabkha setting

Abstract

Laminated microbial mats and microbialites have been documented from a variety of coastal marine environments. This study aims to provide the first detailed descriptions of intertidal pools, along with their hosted thrombolite and stromatolite structures, from Abu Dhabi, and to propose a model for their formation and evolution. It is proposed that the development of pools within the upper intertidal zone was initiated by localized erosion of the laminated microbial mats during high energy events. The removal of the protective mats permitted erosion of the underlying unconsolidated sediment to produce erosional scours that continued to develop to create the pools observed today. The margins of the newly-created submerged environment were colonized by a cyanobacteria dominated microbial community. The precipitation of aragonite cement, associated with the cyanobacteria, stabilized the pool walls and cemented the microbial communities to form stromatolitic and thrombolitic fabrics. Syndepositional cementation was further enhanced by the precipitation of marine cements as a result of evaporationdriven Ca 2+ and Mg 2+ supersaturation. Erosion behind and below the cemented pool wall eventually resulted in rim-collapse and the formation of the observed pool margin parallel thrombolite bands. Successive generations of lithification and erosion increased the area of the pool with the earliest thrombolites eroding and becoming increasingly isolated. In summary, the resultant microbialites developed through the complex interplay of erosion, abiotic early lithification and microbially-mediated processes, and represent a continuum between unlithified laminated microbial mats and domal microbialites. These features are most likely produced during a sea-level scenario of stillstand or transgression and, as such, may be useful as a diagnostic tool to elucidate the onset of transgression. The newly proposed model for stromatolite formation has significant implications for the recognition and interpretation of similar structures observed in the fossil record.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

“…The mats can be roughly divided into two zones: 1) a landward, welldeveloped, laterally continuous zone, with mats up to 20 cm thick characterized by the development of polygons >1 m wide (Figure 1B), and 2) a seaward zone where thinner (0.8-6 cm), poorly developed, discontinuous mats with patchy distribution (Figure 1C). Stromatolites (10-25 cm in relief; Figure 1D; Paul et al, 2020) are locally present in shallow tidal creeks (ca 20-40 cm water depth at low tide), which locally undercut the microbial mats and shallow landwards. Beneath the mats, firmgrounds (Figure 1E) containing botryoidal and spherulitic aragonite are laterally separated by unconsolidated bioclastic and peloidal carbonate sediments.…”

Serpulid-microbialitic bioconstructions from the upper Sarmatian (Middle Miocene) of the Central Paratethys Sea (NW Hungary) – witnesses of a microbial sea

“…Typical morphologies of extant microbialites range from domal, discoidal and tabular to ridges and extensive platforms (Siqueiros-Beltrones, 2008;Jahnert and Collins, 2012;Suosaari et al, 2016;Louyakis et al, 2017;Paul et al, 2018Paul et al, , 2021. Numerous Neogene to present-day thrombolitebearing deposits have been described in the literature.…”

“…A modern example containing well-developed thrombolites is reported from the southern shore of the Persian Gulf (alternatively called the Arabian Gulf) close to Abu Dhabi, the United Arab Emirates (Paul et al, 2018(Paul et al, , 2021. The thrombolites occur in a sabkha setting with water salinities of 75-93 psu, and morphologies include domes (5-20 cm in width and a relief of 10-25 cm above a hard ground) and bands.…”

“…Internally, the domes represent stromatolites at the base overlain by thrombolites. Grains are coated by an isopachous acicular aragonite cement with fibrous needles oriented perpendicular to the grain surface (Paul et al, 2018(Paul et al, , 2021. The skeletal assemblage is dominated by foraminifers and ostracods and contains subordinate bivalves.…”

Serpulid microbialitic bioherms from the upper Sarmatian (Middle Miocene) of the central Paratethys Sea (NW Hungary) – witnesses of a microbial sea

“…Typical morphologies of extant microbialites range from domal, discoidal and tabular to ridges and extensive platforms (Siqueiros-Beltrones, 2008;Jahnert and Collins, 2012;Suosaari et al, 2016;Louyakis et al, 2017;Paul et al, 2018Paul et al, , 2021. Numerous Neogene to present-day thrombolitebearing deposits have been described in the literature.…”