SEDIMENTOLOGY OF THE MACHARETí AND MANDIYUTí GROUPS, SOUTHERN BOLIVIA: NEW INSIGHTS INTO A COMPLEX SET OF SILICICLASTICS

Numerous depositional environments, ranging from continental to deep marine, have been suggested for the Macharetí and Mandiyutí groups in Bolivia including fluvial and deltaic to submarine fans and glaciomarine deposits. Both the Macharetí and Mandiyutí groups vary greatly in thickness (0-1000m each) and lithologies (sandstone, shale, and diamictite) due to deposition in paleovalleys and syndepositional tectonics within the Tarija-Chaco Basin (southern Bolivia, northern Argentina, and western Paraguay). Recent work detailing the sedimentology and stratigraphy of the Macharetí and Mandiyutí groups across southern Bolivia sheds new light on these complex rocks. Faceted and striated clasts within the diamictites confirm a glacial origin to the diamictites. While there is little evidence in northern sections of Bolivia to suggest that the diamictites can be classified as true tills, glacial pavements have been described within the Macharetí Group in northern Argentina. Previously unrecognized paleosols have been described throughout the sandstones and even within the diamictites of both the Macharetí and Mandiyutí groups confirming that these units were deposited within a largely continental basin. However, brachiopods and gastropods have been described within the upper formations of both groups suggesting minor marine incursions occurred during deposition. Common deformation features, including synsedimentary slumps, folds, and faults and thick beds of climbing ripples throughout both the Macharetí and Mandiyutí groups indicate high sedimentation rates throughout deposition. Further work on the sedimentology of these complex units will elucidate the climate and depositional history of the Tarija-Chaco Basin in southern Bolivia.