--> Environmental Controls, Growth History and Fabrics of Lake Richmond Microbialites, Western Australia

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Environmental Controls, Growth History and Fabrics of Lake Richmond Microbialites, Western Australia

Abstract

The southwestern coast of Australia comprises a series of unique Holocene lakes where the sedimentation is influenced by microbial activity and resulted in the formation of significant microbialite deposits. Whilst Lake Clifton is the most well-studied of these occurrences, many others in the Yalgorup National Park and Rockingham Regional Park remain poorly studied. Lake Richmond (40km south of Perth) is singular among these wetlands in that it combines a low-salinity water body and a relatively deep (15m) basin whose margins are entirely surrounded by microbialites. Growth history, morphologies and fabrics of these microbialites, composition of the benthic microbial communities and links to lake hydrology and geochemistry have been investigated for the first time. Lake Richmond microbialites occupy an area up to 150m wide that extends from a seasonally inundated flat to a permanently submerged platform and slope at least 4m deep. A widespread thrombolitic pavement is present in much of this area and forms the substrate for buildups that present a wide range of morphologies such as gentle domes, sub-spherical, discoidal and flat-topped structures. Internal fabrics are predominantly thrombolytic and coarse laminoid, with fenestral porosity and clots formed by calcified bacterial sheaths interspersed with cryptocrystalline and microcrystalline aragonite, peloids and radial aggregates of aragonite crystals. Microbialite formation began 3700 14C years BP and was subsequently influenced by a gradual water level fall which led to progressive colonization of basinward areas and increased thrombolite exposure in the landward direction, causing erosion and burial of older structures. Growth rates vary from less than 0.1 to 1.2 mm per year. Lithified microbialites are covered by soft microbial mats whose aspect and distribution are greatly influenced by the seasonal variations in water level. Permanently submerged thrombolites are colonized by layered mats up to 5cm thick whereas non-laminated pustular mats occupy the lower part of flats during the dry season. Ephemeral mats form in areas subject to longer term exposure where intense desiccation during summer produces shrinkage cracks and facilitates later erosion by wind or waves when water levels rise again. Thrombolite reservoirs are little known but have attracted attention in current exploration. Data from modern studies may prove useful in interpretation and modelling of ancient microbial reservoir systems.