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Cyclicity and Carbonate-Silicate Gel Interactions in Cretaceous Alkaline Lakes


Mg-silicates are a prominent feature of many modern and ancient saline alkaline lake deposits, but whereas models for their distribution are well known, their influence on associated carbonates has not been considered. The cyclic distribution of various types of carbonates and Mg-clays in early Cretaceous lacustrine carbonates from the South Atlantic provides an insight into how evolving lake chemistries in highly alkaline settings control facies development. The cyclothems are typically sub-decametre scale, with component lithofacies arranged both in symmetrical and asymmetrical motifs. Laminated carbonates were deposited linked to short lived pluvial events, causing expansion of shallow lakes developed in rift settings, followed by evaporation which triggered Mg-silicate precipitation and calcite nucleation and growth. Where Mg-silicate deposition dominated textural development, calcite nucleated within gels to produce spherulitic textures. When the rate of gel precipitation decreased or ceased, calcite growth, now less inhibited, produced shrub-like calcites resembling those produced abiotically in modern travertines. Physical reworking of these sediments led to the dispersion of the gels and the concentration of detrital carbonate components. The later behaviour of the Mg-silicates, where deposited and converted to clays, had a profound effect on porosity development. Limited criteria are available to indicate water depths for the lakes, but like modern shallow salt lakes such as the Great Salt Lake, tectonics had a significant control on facies development. Despite earlier proposals, evidence of microbial processes producing carbonates in these Cretaceous lake deposits are rare and the application of facies models based on modern and ancient microbialite analogues maybe be misplaced.