CABALERI nora Graciela
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Genesis of siliceous stromatolites in the Jurassic Cañadón Asfalto Basin, Chubut, Argentina.
Museo Argentino de Ciencias Naturales "B. Rivadavia"
Simposio; VII International Symposium on Mesozoic Terrestrial Ecosystems; 1999
Institución organizadora:
Museo Argentino de Ciencias Naturales "B. Rivadavia"
The Cañadón Asfalto Formation (Callovian-Oxfordian), crops out in the ChubutMedio River area, near to Cerro Cóndor (69º 07´- 69º 10´ W; 43º 17´-43º 26´S), central Patagonia, Argentina. This Formation is divided in two informalsections. An environmental study of the lower section of the Formation hasbeen performed and the origin of siliceous stromatolites and nodules isproposed. The lower section is represented by a volcanic sedimentaryassociation constituted by cycles of basaltic eruptions, and lacustrinecarbonate facies interbedded with pyroclastic rocks, developed in theCañadón Asfalto-Somuncura extensional Basin. The carbonate facies correspondto a littoral lacustrine environment, and it can be differentiate in theeulittoral, infralittoral, and supralittoral subenvironments. The eulittoralsubenvironment is characterized by bioherms composed of: a) planar micriticstromatolites with densely packed algal flabelliform filaments, b)stratiform stromatolites represented by silica laminae with spherultic andbanded microgranular textures alternating with micritic carbonate laminae,and c) hemispherical stromatolites interbedded with thin chert levels. Themicrofacies associated with the eulittoral bioherm (intraclasticgrainstone/rudstone; peloid mudstone/wackestone) present voids filled withsilica, showing spherulitic, radial and drusy textures. Banded chertynodules with mammilated surfaces are very conspicuous in this facies.Stratiform stromatolites has been observed in the infralittoralsubenvironment, related to fluvial channels. These stromatolites arecomposed by silica levels of 5 cm thick with concentric, and spheruliticmicroestructure. Supralittoral subenvironment presents microbialiticmudstones with storm levels and evaporitic facies typical of dry climate.The three subenviroments are associated with shard deposits.Abundant amount of silica has been observed in the stromatolites of theeulittoral and infralittoral.