Silurian (Llandovery) monograptids from the Vargas Peña Formation (Paraná Basin, Eastern Paraguay)

Abstract

The Itacurubí Group is a siliciclastic Silurian sequence exposed in eastern Paraguay (Paraná Basin). It includes from bottom to top the Eusebio Ayala, Vargas Peña and Cariy formations. The entire sedimentary group has yielded a rich association of invertebrates, ichnofossils and palynomorphs. The low-diversity graptolite fauna found within the uppermost beds of the Vargas Peña Formation is described herein. The monograptids comprising this fauna are Stimulograptus aff. sedgwickii (PORTLOCK), Monograptus aff. priodon (BRONN) and ?Demirastrites sp. These taxa confirm the presence of late Aeronian-early Telychian deposits. The accompanying fossil assemblage also suggests a Mid-Llandovery age for this unit. The recognized graptolite biozones allow us to correlate the Vargas Pena Formation with other Silurian units in South America. From the palaeoenvironmental point of view this sector of the Parana Basin reflects a shallow shelf with high tolerance to oxygen invertebrates association.