Sedimentology and lithostratigraphy of the carboniferous Tarija-Chaco Basin, southern Bolivia: Geodynamic and Paleoclimatic evolution

The Late Carboniferous Machareti and Mandiyuti groups, deposited in the Tarija-Chaco Basin of southern Bolivia, record a complex succession of glacially and tectonically influenced siliciclastic rocks providing important information about the geodynamic and climatic history of western Gondwana. Palynological work has redefined the age of the Machareti and Mandiyuti groups to the Pennsylvanian (Bashkirian-Kasimovian-Machareti Group; Kasimovian-Gzhelian-Mandiyuti Group) with few restricted Late Devonian (Famennian) and Mississippian (Visean) units. Extreme lateral variability of both facies and thicknesses of the Machareti and Mandiyuti groups (and underlying Late Devonian and Mississippian units) reflect changes in tectonics and the growth and decline of glaciers. Common soft sediment deformation structures, including synsedimentary slumps, folds, faults, and thick beds of climbing ripples in sandstones and diamictites throughout the Machareti Group indicate high sedimentation rates and significant resedimentation and are caused by active tectonism and glaciation. Ubiquitous trough cross-bedded sandstones of the Escarpment Formation (lower Mandiyuti Group) record an important change in both tectonic activity and climate in the Tarija-Chaco Basin signaling a period of tectonic and glacial quiescence. Renewal of glacial activity in the Kasimovian-Gzhelian San Telmo Formation (upper Mandiyuti Group) is coincident with renewed subsidence of the southern sub-basin of the Tarija-Chaco Basin and demonstrates variable tectonic controls on deposition across the basin throughout this time. Faceted and striated clasts in massive diamictites and rare glacial pavements confirm a glacial origin for several units within the Machareti and Mandiyuti groups.;Facies analysis of glacial units combined with palynological ages of these units indicates six episodes of glaciation in the Tarija-Chaco Basin from the Late Devonian to Pennsylvanian. Glacial, cold-climate indicators decreases to the north and through time in the Tarija-Chaco Basin, reflecting the northward clockwise rotation of Gondwana throughout the late Paleozoic. Paleosols, plant fragments, and terrestrial palynomorphs occur throughout the Machareti and Mandiyuti groups, confirming deposition in largely continental environments. However, brachiopods and gastropods occur in the upper formations of both groups in the southern and central Tarija-Chaco Basin suggesting minor marine incursions occurred during deposition. Faunal connections and shallow marine depositional environments in southern Bolivia suggest that these marine incursions came from the south and not from the north as previously thought. Isopach maps of units and depositional environments across the study area delineate a series of sub-basins within the larger Tarija-Chaco Basin indicating a change in tectonics stresses within the area during that time. Basin reconstructions from the Devonian through the Pennsylvanian show a period of basin inversion in the Late Devonian to Mississippian related to the Chanic Orogeny. Pennsylvanian basin reconstructions and provenance analysis show a change in basin type from a simple foreland basin deepening to the north in the Devonian to a dissected back-arc basin in the Pennsylvanian with at least three depocenters or sub-basins.