Timeline of events in the Ordovician–Silurian Transition of the Precordillera (Argentina): Paleoenvironmental, paleoclimatic and paleobiologic implications

Abstract

This paper aims at analyzing the Ordovician–Silurian (Hirnantian–early Llandoverian) Transition in the Centraland Eastern Precordillera of San Juan Province based on stratigraphic relationships, diagnostic deposits andsedimentary, paleobiologic and isotope data. At the Central Precordillera, three sections have been examined andsampled: (a) Los Ba˜nos de Talacasto, (b) Poblete Norte in the Talacasto area, and (c) northward the Cerro LaChilca section. At the Eastern Precordillera, only the Don Braulio section at the Villicum Range has beenconsidered. These four sections have been selected because they include significant brachiopod assemblagesrelated to the three Transitional Benthic Faunas (TBF 1–3), Hirnantian and Rhuddanian graptolites zones andpalynomorph assemblages, in addition to isotope data as a useful tool for dating and correlations. The analysis ofthese sections has allowed recognizing the timeline of four events that span early Hirnantian–early Llandoverian.Event 1 is linked to the early Hirnantian postglacial transgression, in accordance with TBF 1 and Metabolograptusextraordinarius Zone correlation. Event 2 developed in the middle-upper Hirnantian, is witnessed in a transgressiveshallow-water succession, coeval with the Hirnantia Fauna and Metabolograptus persculptus Zone. Event 3is widely represented by the basal cherty pebbly conglomerate of La Chilca Formation and its correlatives LosBretes and lower Tambolar formations in the Central Precordillera. This conglomerate is thought to be a lagdeposit, which erosively overlies Ordovician strata, as a result of the Hirnantian postglacial transgression, but itis absent in the Eastern Precordillera. Finally, Event 4, at the Villicum section, spans the late Hirnantian–earlyLlandoverian, standing out a shallow-water, bioturbated mudstone succession, of 12 m thick with no diagnosticfossils, indicating upwelling processes and probably including the Ordovician–Silurian Boundary. However, inthe Los Ba˜nos de Talacasto and Poblete Norte sections, this succession is composed of a graptolite-rich pelitesuccession 12 m thick, exhibiting at the base Fe-phosphate strata, and bearing graptolite assemblages of theHirnantian Metabolograptus persculptus Zone, and the Rhuddanian Parakidograptus acuminatus Zone, indicatingthe Ordovician–Silurian boundary. It should be noted that the A. ascensus Zone has not been registered to date inthe Precordillera.