Gualcamayo Formation olistoliths (Middle Ordovician) embedded in the Rinconada Formation (Silurian-Devonian): paleontology, morphology and evolution remarks of the Ordovician and Silurian basins of Precordillera, Argentina

his contribution provides new paleontological and stratigraphic data for the Eastern Precordillera (San Juan province, Argentina) Ordovician basin. This is based on the description of a group of olistoliths, interpreted herein as coming from the lower beds of the Ordovician Gualcamayo Formation, found embedded within the matrix of the Silurian-early Devonian Rinconada Formation. These olistoliths are characterized by variable dimensions and uniform lithologies, composed of black shales and marlstones or following hectometric limestone blocks related to the San Juan Formation. The Gualcamayo Formation blocks show a graptolite fauna characterized by Holmograptus spinosus, H. bovis, Cryptograptus schaeferi, Paraglossograptus tentaculatus, Archiclimacograptus spp., Atopograptus sp., Pseudophyllograptus sp., Tetragraptus sp., and Xiphograptus sp.; whereas on the bedding plane surfaces, conodont elements of Histiodella sp., Paroistodus horridus, Periodon macrodentatus, and Protopanderodus sp. are found. In the interbedded black shales-marlstones, the trilobites Annamitella sp., Mendolaspis sp., and Carolinites? sp. are observed. The presence of this graptolite fauna allows recording the middle Darriwilian H. spinosus Zone, representing the third record in South America, and enabling a correlation with other outcrops of Precordillera and North America, Australasia, and China. These middle Darriwilian deposits at the La Rinconada section represent the youngest age ever recorded for the drowning phase of the Ordovician carbonate platform (locally referred to as transfacies). This is coherent with a progressive drowning model from north to south and agrees with the diachronism of the base of the Gualcamayo Formation and equivalent units. In addition, the difference between the underlying units of the Rinconada Formation in different sections, together with contrasting thickness measurements, might indicate evidence of the Alto del Tambolar tectonic activity during the Rinconada Formation accumulation in Eastern Precordillera. These processes were also previously recognized for the deposition of the Tucunuco Group in the Central Precordillera. This study expands the knowledge regarding the carbonate drowning process of the Precordillera basin that occurred during the Middle Ordovician and the effects of the Guandacol Tectonic Phase in the La Rinconada area, never mentioned until this moment

First record of Pridolian graptolites from South America: Biostratigraphic and paleogeographic remarks

Graptolites were planktonic organisms that constitute one of the essential biostratigraphic tools used in Paleozoic successions worldwide. Notably, Pridolian (upper Silurian) graptolites have been globally recognized in several geological units, but completely absent from South American sections until now. This study introduces the first Pridolian graptolite assemblage in this subcontinent. The Rinconada Formation, from the Eastern Precordillera of San Juan Province, Argentina, has an abundant fossil content that includes: graptolites, brachiopods, bivalves, trilobites, corals, plant remains, and microfossils. Two graptolite taxa have been identified and described, including Skalograptus parultimus (Jaeger) and cf. Enigmagraptus. Both genera are mentioned for the first time in South America. As a result, the S. parultimus Biozone is recorded from the upper outcropping levels of the Rinconada Formation at its eponymous section, indicating an early Pridolian age. This discovery represents the first mention of Pridolian graptolites for South America, and increases the knowledge about the graptolite content of the studied formation as well. In addition, the assemblage might represent the faunal recovery interval after the Kozlowskii-Lau Event, a globally recorded episode of biotic crisis. The co-occurrence of graptolites and plant remains within the same stratigraphic levels in the Rinconada Formation represents the first and oldest record in western Argentina. Finally, the age of the Rinconada Formation is reviewed, and a local, regional, and global correlation is proposed with several Pridolian sections around the globe.Fil: López, Fernando Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; ArgentinaFil: Kaufmann, Cintia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Geología "Dr. Emiliano Aparicio"; ArgentinaFil: Drovandi, Juan Martín. 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: Conde, Osvaldo Agustin. 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: Braeckman, Alejandro R.. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; ArgentinaFil: Arnol, Jonatan Ariel. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Estrada, Lautaro. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; ArgentinaFil: Pedernera, Fernando. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; ArgentinaFil: Abarca, Ulises. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; Argentin