The western Gondwana margin underwent a complex geodynamic history during the early Paleozoic, and major uncertainties remain as to the role of tectonism in sedimentary dynamics. This study focuses on the lower part Santa Rosita Formation and the coeval Guayoc Chico Group (Cordillera Oriental; Northwest Argentina), ranging from the late Cambrian (Furongian; Age 10) to Early Ordovician (early Tremadocian; Tr1). This stratigraphic interval has been previously interpreted as deposited in an extensional basin to a retro-arc basin without major regional tectonic-induced deformation during its deposition, only recording long-term relative sea-level fluctuations. Four areas (Sierra de Cajas, Angosto del Moreno, Quebrada de Trancas, and Quebrada de Moya) were chosen because they host the most complete and temporally well-constrained stratigraphic sections of the Cordillera Oriental. Throughout the stratigraphic sections, four main facies zones are described and attributed to deposition in estuarine, foreshore-shoreface, delta-front, and offshore environments. Trilobite biozones are used as the biostratigraphic framework. By integrating sedimentary facies analysis, biostratigraphy, and sequence stratigraphy from the four selected sections, a new scenario showcasing the evolution of the basin is proposed. This scenario interprets a tectonically induced deformation during the deposition of the Santa Rosita Formation and the coeval Guayoc Chico Group. The newly acquired sedimentological data show that physiographical changes took place during the Cambrian-Ordovician transition and are expressed in various localities. This major change is recorded in the stratigraphic architecture, where extensive wave-ravinement surfaces and sedimentary hiatus are the result of local, syn-depositional basement uplifts. The initiation of the Puna-Famatinian volcanic arc during the Early Ordovician on the western margin was likely responsible for deformation in the retro-arc basin and the proposed scenario is consistent with the stratigraphic evolution in other areas of the Cordillera Oriental (e.g., Sierra de Mojotoro) and the Sierra de Famatina. Therefore, this study helps to constrain the evolution of the western Gondwana margin during the early Paleozoic, showing changes in the stratigraphic architecture and basin evolution from an extensional to a retro-arc style.
The lower Paleozoic marine siliciclastic succession of the Central Andean Basin, northwestern Argentina, provides a valuable record of the onset of the Great Ordovician Biodiversification Event in western Gondwana. A new ichnospecies of rosette trace fossil, Gyrophyllites cristinae, is documented from lower and upper Tremadocian (Tr1 and Tr2) deposits of this basin. It is characterized by five to six non-overlapping petaloid lobes and can be easily differentiated from the other four formally defined ichnospecies. Gyrophyllites cristinae occurs at the top of hummocky cross-stratified sandstone regularly interbedded with mudstone. These deposits are interpreted as reflecting the alternation of high-energy storm events and low-energy fair-weather conditions immediately below the fair-weather wave base, representing deposition in offshore transition environments. Gyrophyllites has been traditionally interpreted as the product of worms of uncertain taxonomic affinity that mined the sediment in search for food (fodinichnia). The occurrence of Gyrophyllites cristinae in these Ordovician deposits records post-storm colonization. Storms may have increased oxygenation and supplied fresh organic detritus that were exploited by worm-like, surface detritus- or shallow deposit-feeders exploring the uppermost silt-rich fine-grained sediments.
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.