GLACIAL CYCLES OF THE LATE PALEOZOIC ICE AGE: TARIJA-CHACO BASIN, SOUTHERN BOLIVIA

The Tarija-Chaco Basin records a set of glacially and tectonically influenced siliciclastics that document the Late Paleozoic Ice Age in southern Bolivia from Late Devonian through Pennsylvanian time. Within the basin are repeating sandstone and diamictite units of largely Pennsylvanian age, the Macharetí and Mandiyutí groups, and restricted Late Devonian and Mississippian deposits. Stratigraphic and sedimentologic data from these units indicate at least six glacial intervals during the Late Paleozoic Ice Age in western Gondwana, recording one in the Late Devonian, one in the Mississippian, and at least four in the Pennsylvanian. Glacial cycles are recorded as thick massive diamictite beds with polished, faceted, and striated clasts, rare glacial pavements, and dropstones that denote advances of the glacial system into the basin. Interbedded sandstone units contain large trough cross-beds, paleosols, and resedimented facies of fluvial and deltaic systems that represent glacial retreat and interglacial periods. Deposition throughout this time is greatly influenced by the Late Devonian-Mississippian Chañic Orogeny which likely contributed to the complex stratigraphy and limited extent of Late Devonian and Mississippian units. Glacial evidence decreases to the north within the Tarija-Chaco Basin and through time following the rotation of Gondwana. Glacial cycles in these units correspond well with recognized cycles across Gondwana, especially those in Argentina, the Parana Basin in Brazil, and in eastern Australia. These cycles, however, do not correspond well with proposed large scale glacial cycles of the Late Paleozoic Ice Age suggested by Isbell et al. (2003), but more closely correspond to earlier cycles proposed by Veevers and Powell (1987).