New paleontological and geological data on the Ordovician and Silurian of Bolivia
Abstract
The oldest vertebrates of South America are from the thick Anzaldo (=Cuchupunata) Formation in central Bolivia. At the scale of the basin, the Anzaldo is overlain by the San Benito, Tokochi, Cancanri, Llallagua and Kirusillas/Uncia formations. The Anzaldo was classically dated Caradoc (early Late Ordovician), but recent paleontological data have suggested a Llanvim age (early Middle Ordovician). The only significant fossil invertebrates recently collected in the Anzaldo, viz., lingulid brachiopods, give an age not older than Late Ordovician. Fossils from 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 older) age thus seems mote correct
for the Anzaldo Formation. These new paleontological data have major implications on our knowledge of the Ordovician-Silurian basins of Bolivia: I) transition from a Middle Ordovician marine foreland basin to a Late Ordovician-Llandovery glacial-marine to turbidite trough in the Altiplano occurred in the (late?) Caradoc; 2) a major sea-level rise developed around the Llandovery-Wenlock boundary; 3) a fossiliferous limestone member of shallow origin and early Wenlock age is present approximately between Cochabamba and Santa Cruz.