Abstract
Detailed outcrop and microscopic descriptions and current ideas about factors controlling the origin of oolitic ironstones coming from the Lower Silurian of the western Argentina Precordillera Basin are given. Chamosite and goethite comprise around 20 to 98 % of the oolitic ironstones. Phosphatic nodules are common components in these deposits. Lithofacies arrangement, outcrop architecture and bioturbation patterns allow to interpret them as shallow marine bars, which typically constitute parasequences boundaries. They are associated with widespread detrital facies. Their origin is related to sea-level fluctuations and a good correlation with global events can be established. The origin and distribution of the ironstones in the Precordillera basin is related with cool-water seas in high southern latitudes, during a high stand of sea-level, occurring after the Late Ordovician glacial event.
Highlights
current ideas about factors controlIing the origin of oolitic ironstones coming from the Lower Silurian of the western Argentina Precordillera Basin are given
outcrop architecture and bioturbation patterns allow to interpret them as shallow marine bars
They are associated with widespread detrital facies
Summary
Los depósitos ferríferos de la Precordillera Argentina se distribuyen en la provincia de San Juan (fig. 1) Y están siempre incluidos en las secuencias siliciclásticas con que da comienzo el sistema silúrico 1) Y están siempre incluidos en las secuencias siliciclásticas con que da comienzo el sistema silúrico (Formación Tambolar en el sector sur, Fm. La Chilca en el centro y norte y Fm. Don Braulio en el este). Los aflorantes en la Sierra de Villicúm (Precordillera Oriental) en el techo de la Formación Don Braulio constituyen los más importantes. La Fm. Don Braulio (Baldis el al., 1982) se subdivide en un Miembro inferior de carácter glacigénico (Astini, 1991), de edad Ashgill superior, y un miembro superior constituido por una secuencia de plataforma fangosa del Hirnantiense, coronado por barras oolíticas ferruginosas intercaladas recurrentemente con pelitas (1980) y Peralta (1986) hallaron, respectivamente, faunas de palinomorfos y graptolitos características del Llandovery basal asociadas, en el tramo más superior, a las barras oolíticas estudiadas en la presente contribución En los niveles de fangolitas, Peralta y Baldis (1990) y Sánchez el al. (1991) hallaron Glyplograplus persculplUS (Ordovícico terminal), mientras que Volkheimer el al. (1980) y Peralta (1986) hallaron, respectivamente, faunas de palinomorfos y graptolitos características del Llandovery basal asociadas, en el tramo más superior, a las barras oolíticas estudiadas en la presente contribución
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