CARBONIFEROUS PALEOVALLEYS, SOUTHERN BOLIVIA: COMPLEX STRATIGRAPHY IN GLACIAL WESTERN GONDWANA

Late Carboniferous stratigraphy of Bolivia records a warm carbonate setting in the north, yielding to a complex array of tectonically- and glacially-influenced siliciclastics in the south and east. The Macharetí and Mandiyutí groups of southern Bolivia record glacially influenced deposition in the Chaco-Tarija Basin during the Late Paleozoic ice ages. Palynological analysis of diamictites and shales confirms a Pennsylvanian age for the glacial units. The Macharetí and Mandiyutí groups consist of repeating sequences of fluviodeltaic sands and glacial diamictites. Each group varies greatly in thickness, bound by significant unconformities at their bases. New stratigraphic sections show significant lateral changes in the Macharetí and Mandiyutí groups across the Chaco-Tarija Basin. Deposition of these units within paleovalleys ranges from 0 to 1800 m, with measured sections varying in thickness from 84 m to 1800 m. There is an overall thinning of the sections from south to north, although a few northern sections reach 1000 m. While there are significant changes in section thicknesses across the basin, lithologies (quartz rich lithic to feldspathic arenites) and depositional environments of the sands (fluvial deltaic) and diamictites (glaciomarine) do not vary significantly, suggesting that depositional controls within the individual paleovalleys remain the same across the basin. However, the strongest glacial influence, noted from glacial scours and faceted and striated clasts, occurs in southern sections. Northern sections contain slumps including large blocks of Devonian sands and fewer diamictites suggesting a decrease in climate as a control and an increased tectonic influence. Carboniferous tectonics, glaciations, and paleolatitude changes make correlations and delineation of individual siliciclastic beds’ events difficult.