El Darriwiliense inferior (Ordovícico Medio) en el extremo norte de la Precordillera de Mendoza, Argentina: implicanciones estratigráficas y bioestratigráficas

Abstract

En esta contribución se da a conocer una nueva localidad fosilífera en la Cantera Los Guanacos del extremo norte de la sierra de Las Higueras, a 10,5 km de la localidad de Salagasta, Precordillera de Mendoza, oeste de Argentina. Se trata de un nivel carbonático lenticular fosilífero intercalado entre la unidad mixta pelítica/carbonática (transfacies) correlacionable con la base de la Formación Los Azules. Éste se caracteriza por un límite erosional de espesor milimétrico y por una distribución mecánica de bioclastos e intraclastos, por encima del mudstone oscuro. La capa bioclástica se interpreta como el depósito de un evento tempestítico acaecido en la plataforma carbonática. Se determinan las microfacies carbonáticas y las asociaciones de macro y microfauna. Se registran y describen los trilobites Mendolaspis salagastensis y especies indeterminadas de Agnostidae, Illaenidae, Leiostegiidae, Encrinuridae, Telephinidae y Trinucleidae. Se reconocieron braquiópodos indeterminados de la familia Obolidae. Los graptolitos son comparables con Levisograptus cf. austrodentatus y en base a la fauna de conodontos se reconoce la Zona de Yangtzeplacognathuscrassus de edad darriwiliense temprana (Ordovícico Medio). Se discuten las implicancias bioestratigráficas de estos registros, las que permiten la correlaciónde estos niveles de la Precordillera de Mendoza con otras regiones de la Precordillera de San Juan, del Noroeste argentino y del mundo. Se brindan además algunas consideraciones paleoambientales surgidas del análisis de estos registros faunísticos y de las microfacies carbonáticas.

A tectonically affected Middle Ordovician succession crops out at the Los Guanacos quarry in the sierra de Las Higueras, in the Precordillera of Mendoza, western Argentina. This is represented by the San Juan Formation which is transitionally covered by a mixed carbonate-shaly unit of dark colours, 1.5 metres thick (transfacies) which can be interpreted as the base of the Los Azules Formation. A 5-cm thick intra-bioclastic pack/grainstone bed interbedded in the lower part of this unit has been interpreted as a tempestite. It is characterized both by a millimetre-thick erosional boundary, just above the dark mudstone layer and strong mechanical distribution of bioclasts and intraclasts, suggesting sedimentary structures associated with a storm event of high energy possibly occurring in the middle part of the platform. Towards the bottom, a hummocky structure thin interval, under the dark mudstone, is interpreted as the record of another tempestite event. The mudstone represents the post-storm event reflecting the low energy of the marine water environment. The lenticular layer contains predominantly lingulid shells, trilobites and graptolites, all highly fragmented, which suggests they have been reworked and subsequently mixed and quickly redeposited during a storm event that caused the mortality of these faunas. The trilobites Mendolaspis salagastensis and indeterminate species of Matagnostidae, Illaenidae, Leiostegiidae, Encrinuridae, Telephinidae and Trinucleidae have been recorded for the first time at this locality. The conodont fauna allows the identification of the Yangtzeplacognathus crassus Zone, which indicates the lower Darriwilian (Middle Ordovician). Graptolites assignable to Levisograptus cf. Levisograptus austrodentatus and Levisograptus sp. are described for the first time in the studied area and biostratigraphic implications validate the early Darriwilian age for the bearing levels. Finally, based on the studied records, correlation with different regions at the Precordillera, northwestern Argentina and worldwide, as well as some paleoenvironmental considerations, are additionally discussed.