Romain Vaucher, N. Emilio Vaccari, Diego Balseiro, Diego F. Muñoz, Beatriz G. Waisfeld, Luis A. Buatois

This study focuses on upper Cambrian – Lower Ordovician strata containing the lowermost fossil-bearing levels of the basin (Santa Rosita Formation and Guayoc Chico Group). Bounded by two major regional unconformities, this stratigraphic interval was previously considered as a retro-arc foreland basin displaying evidence of westward progradation without tectonic activity during its deposition. Throughout the sedimentary successions, four main facies zones are described, namely fluvial-estuarine, shoreface-foreshore, delta-front, and offshore-shelf environments. Biostratigraphic constrain is provided by trilobite biozones (Neoparabolina frequens argentina, Jujuyaspis keideli, Kainella andina, Kainella meridionalis and Kainella teiichii). Integrating sedimentary facies analysis, biostratigraphy, and sequence stratigraphy information from four selected areas across the Cordillera Oriental (Sierra de Cajas, Angosto del Moreno, Quebrada de Trancas and Quebrada de Moya; Province of Jujuy), a more complex evolution of the basin is proposed. Newly acquired data attest for a northward progradation of the system associated with a partially diachronic basin closure related to a first tectonic pulse induced by the Oclóyic phase. This southernmost tectonic pulse is recorded in various areas at the Cambrian-Ordovician boundary. It is highlighted by large highly erosive wave-ravinement surfaces and depositional geometries suggesting a major change in the basin physiography during the Cambrian-Ordovician transition. This study provides new results helping to constrain the evolution of the western Gondwana margin during the early Palaeozoic.