The Achavil Formation: A new low-grade metamorphic unit in the Upper Cambrian evolution of Famatina

Abstract

A new low-grade metamorphic unit, different from the well-known Negro Peinado Formation, is suggested from sedimentological, stratigraphical, mineralogical, structural and geochronologic criteria developed after detailed field mapping in the region of Famatina. For this new unit the name Achavil Formation (nom. nov.) is proposed and its type locality and regional extension are established. Four main lithofacies allow interpreting shallower water depositional environments than those traditionally accepted for the Negro Peinado Formation. The depositional age for the Achavil Formation ranges between Middle-Upper and Upper Cambrian, considering that the immediately overlying Volcancito Formation ranges between uppermost Cambrian and Tremadocian. This interval would correlate with the Mesón Group in northwest Argentina and not, as it has often been interpreted, with the Puncoviscana Formation. Two superposed deformational episodes inferred to be Cambrian are recorded within the Achavil Formation and clearly predate the deposition of the Volcancito Formation. From our results potential correlations between the Achavil Formation and other lower Paleozoic units in the surrounding regions allow establishing a paleogeographic reconstruction for the western Gondwana margin during this period.