Abstract

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.

A high-resolution δ¹³C and δ¹⁸O isotope study, in concert with a detailed paleoenvironmental analysis, was carried out to better understand both environmental and chronostratigraphic evolution of the La Laja Formation. Three δ¹³C positive excursions were recorded; the first one at the Glossopleura biozone within the Soldano Member, the second beginning at the base of the Rivadavia Member, and the third during deposition of the Las Torres Member. Comparisons with other Middle Cambrian curves, in the Precordillera and elsewhere (Rocky Mountains and Great Basin, U.S.A., the western Hunan Province in south China, and the Amadeus, Georgina, and Daly basins in Australia) suggest a global control on these excursions. Mechanisms to produce these positive excursions could be related to high bio-productivity and increased burial of C_org (organic carbon) produced by high nutrient influx to the ocean associated with a relative sea-level fall. Local environmental controls could have in part altered the original isotopic signal.