NEW PALEONTOLOGICAL AND GEOLOGICAL DATA ON THE ORDOVICIAN AND SILURIAN OF BOLIVIA

The oldest vertebrates of South America are from the thick Anzaldo (=C uchupunata) Formation in central Bolivia. At the scale of the basin, t he Anzaldo is overlain by the San Benito, Tokochi, Cancaniri, Llallagu a and Kirusillas/Uncia formations. The Anzaldo was classically dated C aradoc (early Late Ordovician), but recent paleontological data have s uggested a Llanvirn age (early Middle Ordovician). The only significan t fossil invertebrates recently collected in the Anzaldo, viz., lingul id brachiopods, give an age not older than Late Ordovician. Fossils fr om the Tokochi suggest a Caradoc age. The microfossils (acritarchs and foraminifers mainly) collected in the Cancaniri and Kirusillas/Uncia formations indicate an Ashgill to Wenlock age (late Late Ordovician to late Early Silurian) for these formations. A Caradoc (or perhaps olde r) age thus seems more correct for the Anzaldo Formation. These new pa leontological data have major implications on our knowledge of the Ord ovician-Silurian basins of Bolivia: 1) transition from a Middle Ordovi cian marine foreland basin to a Late Ordovician-Llandovery glacial-mar ine to turbidite trough in the Altiplano occurred in the (late?) Carad oc; 2) a major sea-level rise developed around the Llandovery-Wenlock boundary; 3) a fossiliferous limestone member of shallow origin and ea rly Wenlock age is present approximately between Cochabamba and Santa Cruz. Copyright (C) 1996 Elsevier Science Ltd & Earth Sciences & Resou rces Institute