Middle Ordovician conodonts and graptolites at Los Cauquenes range, central Precordillera of San Juan, Argentina

Abstract

The Ordovician System is superbly represented in the Precordillera of western Argentina, at the Andean foothills. During the Middle Ordovician, an important paleogeographical rearrangement of depocenters and source areas took place associated with the demise of the Eopaleozoic carbonate platform of the Precordillera. This critical interval is recorded in the basin through widespread deposition of black shales over the fossiliferous limestones of the San Juan Formation, punctuated by local deposition of olistostromes, debris flows, conglomerates, and turbidites (e.g., Astini et al., 1995; Keller, 1999). Fossil dating of the lowest black shales and equivalent units indicates a diachronous deposition from north to south in the Precordillera basin (Hünicken, 1985; Albanesi et al., 1998).