New records of Ordovician graptolites and conodonts from Estancia San Isidro and Empozada formations, San Isidro Creek, Mendoza Precordillera, Argentina

摘要

The San Isidro creek is a classical locality for the study of the Ordovician System in the Precordillera of Mendoza Province. The Estancia San Isidro and Empozada formations bear allochthonous rocks, Cambrian-Ordovician in age, within an Ordovician shaly matrix. Graptolites, conodonts, and trilobites are abundant in these units. The present contribution deals with a late Darriwilian graptolite association, which was collected from the green-greyish shaly facies of the Estancia San Isidro Formation at the northern flank of San Isidro creek. This fauna refers to the Pterograptus elegans Zone or probably correlates with the Hustedogrdptus teretiusculus Zone, which was documented for other areas of the Precordillera. The graptolite assemblage is composed of diplograptids dominated by Archiclimacograptus Mitchell, and a low proportion of glossograptids and dichograptids. It allows us to assign a late Darriwilian age for the time interval including the deposition of the olistostromes of the Estancia San Isidro Formation. At the southern margin of the San Isidro creek, a conglomerate bed close to the top of the Estancia San Isidro Formation yielded an early Darriwilian conodont fauna, referred to the lower Lenodus variabilis Zone or slightly older. This fauna reveals the age of clasts that make up the conglomerate. Early Floian graptolites from allochthonous shales in the basal Empozada Formation are described. The conodont Amorphognathus cf. ordovicicus Branson and Mehl, recorded from the lower member of the Empozada Formation together with graptolites of the Dicellograptus complanatus Zone, indicates a late Katian age for the autochthonous strata.