First documentation of the Polygnathoides siluricus conodont Zone (Ludfordian) in South America (Argentina) and the stratigraphic significance of the younger species of Kockelella (Conodonta)

The coquinoid beds from the middle part of the Los Espejos Formation at the Poblete creek section (Talacasto Creek) yielded abundant conodonts. The genus Kockelella (Walliser) represents the most relevant biostratigraphical genus in this conodont fauna. The co-occurrence of Kockelella maenniki Serpagli and Corradini, Kockelella variabilis ichnusae Serpagli and Corradini, K. variabilis Walliser, Kockelella ortus sardoa (Serpagli & Corradini), and Kockelella ortus absidata (Barrick & Klapper) allow us to record for the first time the Polygnathoides siluricus Zone in South America, which suggests the Ludfordian Stage (late Ludlow). We also propose an accurate correlation of the Los Espejos Formation with the lower Ludfordian deposits from the Carnic Alps, Sardinia, Morocco, Czech Republic, Gotland, and North America.Fil: Gomez, Maria Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Mestre, Ana Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Garcias Paez, Yanina Vanesa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Corradini, Carlo. Università degli Studi di Cagliari; Itali

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggests a tide‐dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supratidal to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The intertidal–supratidal depositional environments are represented by tidal channels, scour and fill, tidal dunes, and hummocky cross‐stratified sandstones. The estimated tidal conditions for the tidal plain were mesotidal to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low‐velocity waves. The identification of microbially induced sedimentary structures (MISS) in these deposits, such as petee polygonal structures, pitted texture, imploded sand volcanoes, Astropolithon sp., Manchuriophycus sp., domal build‐ups, biofilms, elephant skin, and trace fossils such as Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp., and gastropod traces, may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation deposits. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression–regression cycles in shallow‐water environments under stable base‐level conditions. The middle‐upper section of the La Chilca Formation shows non‐cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two third‐order transgressive events and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of Fe‐oolites, while the shallowing event is determined by the presence of MISS.Fil: Asurmendi, Estefania. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Sanchez, Maria Lidia. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; ArgentinaFil: Heredia, Susana Emma. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentin

New age constraints for the Cenozoic marine transgressions of northwestern Patagonia, Argentina (41°-43° S): Paleogeographic andtectonic implications

In this contribution we address the current controversial issues related to the age, correlation, tectonic setting and paleogeographic links of the Cenozoic fossiliferous marine strata that crop out in the eastern sector of the North Patagonian Andes between 41° and 43°S based on new field (detailed geologic and structural mapping), geochronological (U-Pb LA-ICPMS analyses in zircons) and biostratigraphic (calcareous nannofossils studies) data. The marine strata form part of the volcano-sedimentary infill of several depocenters included in the Ñirihuau basin. We constrained the age of the main Cenozoic fossiliferous marine successions within the Early Miocene (23-16Ma). In addition, strata previously interpreted as deposited in marginal-marine environments suggest that the marine influence in the area could have lasted until the Middle Miocene. Our results indicate that the main depocenters of the basin evolved simultaneously during the Late Oligocene? to Miocene, and also give some clues about possible connections between them during distinctive stages of their evolution. We interpret that the marine transgressions registered in the Ñirihuau basin were related to a regional extensional tectonic stage that took place during the Oligocene to Early Miocene. The ingression of the sea occurred before the main contractional phase that gave place to the uplift of the North Patagonian Andes between the Early-Middle Miocene and the Pliocene, and the marine influence probably lasted until the early stages of the fold and thrust belt development. The marine strata can be correlated with one or more pulses of a major transgression that flooded a wide area of Patagonia between the Late Oligocene and the Middle Miocene. Considering the currently available data, a direct link of the Ñirihuau basin with the Atlantic Ocean on its northern, eastern or southern sides is unlikely. Marine connections would have taken place most likely with Pacific marine basins located to the west.Fil: Bechis, Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Diversidad Cultural y Procesos de Cambio. Universidad Nacional de Río Negro. Instituto de Investigaciones en Diversidad Cultural y Procesos de Cambio; ArgentinaFil: Encinas, Alfonso. Universidad de Concepción; ChileFil: Concheyro, Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos ; ArgentinaFil: Litvak, Vanesa Dafne. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos ; ArgentinaFil: Aguirre-Urreta, Maria Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos ; ArgentinaFil: Ramos, Victor Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos ; Argentin