Paleoenvironmental and Carbon-Oxygen Isotope Record of Middle Cambrian Carbonates (La Laja Formation) in the Argentine Precordillera

摘要

The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the lower Paleozoic carbonate platform of the Argentina Precordillera. This is a key unit regarding the hypothesis of the Precordillera as a Laurentia-derived allochthonous terrane currently located in the south-central Andes. According to the faunal affinity and stratigraphic development of the thick Cambrian carbonate bank, the Argentine Precordillera would have been attached to Laurentia. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow subtidal unit is internally arranged into several Grand Cycles indicating a complex environmental mosaic, probably with local depocenters related to variable subsidence. This unit records the stabilization of the rifted margin of the Precordillera terrane, prior to the broadening of the carbonate sedimentation during the passive-margin drifting stage.