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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.

Abstract The final consolidation of southwestern Gondwana during the Ediacaran–Cambrian resulted in the formation of a large landmass originally surrounded by the newborn Panthalassic Ocean. The Río de la Plata Craton is one of the last pieces that complete the geotectonic puzzle along the austral part of this supercontinent. However, the sedimentary record corresponding to the interval between the consolidation of SW Gondwana until the initial deposition in a large Ordovician cratonic basin is apparently missing. In this context, the Ediacaran–Cambrian epiclastic shallow-marine ramp of the Cerro Negro Formation, the uppermost unit of La Providencia Group, Tandilia System, is envisaged as the first known stratigraphic record of the hidden pericratonic basins of Gondwana. Geochronological and provenance data, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with the Ordovician Balcarce Formation than the Neoproterozoic Sierras Bayas Group. The presence of similar Neoproterozoic source areas for both units, the absence of any metamorphism and deformation related to the Brasiliano cycle, and shallow marine deposition strongly suggest that the paleogeography and physiography of the original southernmost continental platform of Gondwana were similar to the Ordovician ones. Thus, the Cerro Negro Formation appears as the missing link between the ultimate stages of the Brasiliano Orogenic Cycle and the final stabilization of the continental platform from the SW part of Gondwana.

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Copyright of Geologia USP: Série Científica is the property of Geologia USP and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Geologia USP: Série Científica is the property of Geologia USP and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)