ICE-PROXIMAL STRATIGRAPHY AND ACTIVE TECTONICS: AN EXAMPLE FROM SOUTHERN BOLIVIA

Late Carboniferous ice-proximal stratigraphy of the Peru-Bolivia master basin records a carbonate setting in the northwest yielding to tectonically- and glacially-influenced siliciclastics in the southeast. These sandstones and diamictites are the Macharetí and Mandiyutí groups of the Bolivian Subandes and southern Tarija Basin. Previous authors have suggested that these strata are the lateral facies equivalents of deltaic and marine Mississippian strata below Early Pennsylvanian to Permian carbonate ramps in central and northern Bolivia. Palynological analysis of diamictites and shales confirms a Pennsylvanian age for the glacial units. In the study area, the Macharetí and Mandiyutí groups consist of repeating successions of fluviodeltaic/marine? sands and glacial diamictites bound by significant unconformities. Each group can vary in thickness by orders of magnitude (0 to 1000m) confirming deposition of these units within paleovalleys. The more massive facies stacking patterns are difficult to trace laterally and are dissimilar to well dated cyclothemic carbonate sequences to the north. New stratigraphic sections show significant lateral changes and show problems with earlier mapping outside of type sections. Extreme lateral variability of both facies and thickness of the Macharetí and Mandiyutí groups (and underlying units) reflects primarily changes in tectonics and the growth and decline of glaciers; active loading, resedimentation and slumping on all scales additionally overprint these complex rocks. Only an active tectonically divided southern Peru-Bolivia Basin with a complex series of subbasins and paleovalleys might explain the observed stratal geometry, facies changes, and biostratigraphy. Questions of regional time-transgression, opening and closing of subbasins, and balance with laterally correlative and overlying fusulinid, conodont and ash-dated carbonates are under further study.