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A >15-20 km-thick succession in southern Bolivia forms the most-complete stratigraphic record in western Gondwana. Upper Neoproterozoic–Carboniferous clastic rocks record ∼300 Myr of marine, nonmarine, and glacially influenced sedimentation in diverse basin systems generated by variable tectonic regimes along the western edge of Gondwana during active and passive-margin conditions. New provenance results help resolve key uncertainties regarding source regions and sediment dispersal patterns. The findings are integrated with spatial variations in stratigraphic thicknesses to evaluate regional patterns of basin subsidence, magmatism, and deformation during long-term evolution of the western Gondwanan margin in the central Andes. Detrital zircon U–Pb geochronological data for 17 sandstone samples reveal sedimentary input from Precambrian cratonic basement provinces and pre-Andean basement, magmatic arc, and fold-thrust belt source regions. The basement age signatures indicate derivation from the flanking Brasiliano (900-560 Ma) and Pampean (650-500 Ma) provinces to the south and east, and the distal Rio de la Plata craton (2400-2000 Ma) along the eastern South American margin ∼1000–1500 km to the southeast. Although the greater Amazonian craton was not a major contributor, subordinate Amazonian signatures from the Sunsás (1300-950 Ma) province to the east and northeast selectively fed the northern basin regions of the central Andes. Despite the lack of Paleozoic igneous rocks in Bolivia, detrital zircons of Ordovician age attest to the pre-Andean influence of the subduction-related Famatinian magmatic arc. Limited Devonian-Carboniferous igneous material was contributed locally from western pre-Andean highlands or regionally by axial northward transport from selected igneous sources in Argentina and Chile. Episodic recycling of Neoproterozoic–Paleozoic basin fill, including a sharp reappearance of Famatinian-age detritus, can be linked to periods of Paleozoic crustal shortening and foreland sedimentation ascribed to Famatinian, Ocloyic, Chañic, or Gondwanide phases of deformation. The spatial distribution of sediment sources along with temporal shifts in sediment routing highlight several stages in the paleogeographic evolution of the western Gondwanan margin preserved in the central Andes. Initial regional subsidence spanned a multiphase Neoproterozoic to early Paleozoic history of Rodinia breakup, Brasiliano-Pampean orogenesis, and post-orogenic back-arc extension prior to final late Paleozoic amalgamation of Gondwana. The early Paleozoic onset of subduction and Famatinian arc magmatism led to high-magnitude subsidence (>10–15 km) likely driven by Ordovician slab rollback in an extensional back-arc basin. Thereafter, intermittent Paleozoic contraction in a poorly understood pre-Andean system (best expressed in the Eastern Cordillera of Bolivia and neighboring segments of northern Argentina and southern Peru) generated transient topographic loads that produced superimposed flexural foreland and successor basin systems. • Southern Bolivia contains the most-complete stratigraphic record of western Gondwana. • Detrital zircon U-Pb ages reveal Neoproterozoic-Paleozoic shifts in sediment sources. • Proximal and distal (>1000 km) cratonic sources fed pre-Andean sedimentary basins. • Slab rollback in a back-arc basin likely caused >10 km of Ordovician subsidence. • Cratonic and Famatinian arc/orogenic sources fed Paleozoic foreland/successor basins. [ABSTRACT FROM AUTHOR]

As the highest part of the central Andean fold-thrust belt, the Eastern Cordillera defines an orographic barrier dividing the Altiplano hinterland from the South American foreland. Although the Eastern Cordillera influences the climatic and geomorphic evolution of the central Andes, the interplay among tectonics, climate and erosion remains unclear. We investigate these relationships through analyses of the depositional systems, sediment provenance and Ar/Ar geochronology of the upper Miocene Cangalli Formation exposed in the Tipuani-Mapiri basin (15-16°S) along the boundary of the Eastern Cordillera and Interandean Zone in Bolivia. Results indicate that coarse-grained nonmarine sediments accumulated in a wedge-top basin upon a palaeotopographic surface deeply incised into deformed Palaeozoic rocks. Seven lithofacies and three lithofacies associations reflect deposition by high-energy braided river systems, with stratigraphic relationships revealing significant (∼500 m) palaeorelief. Palaeocurrents and compositional provenance data link sediment accumulation to pronounced late Miocene erosion of the deepest levels of the Eastern Cordillera. Ar/Ar ages of interbedded tuffs suggest that sedimentation along the Eastern Cordillera-Interandean Zone boundary was ongoing by 9.2 Ma and continued until at least ∼7.4 Ma. Limited deformation of subhorizontal basin fill, in comparison with folded and faulted rocks of the unconformably underlying Palaeozoic section, implies that the thrust front had advanced into the Subandean Zone by the 11-9 Ma onset of basin filling. Documented rapid exhumation of the Eastern Cordillera from ∼11 Ma onward was decoupled from upper-crustal shortening and coeval with sedimentation in the Tipuani-Mapiri basin, suggesting climate change (enhanced precipitation) or lower crustal and mantle processes (stacking of basement thrust sheets or removal of mantle lithosphere) as possible controls on late Cenozoic erosion and wedge-top accumulation. Regardless of the precise trigger, we propose that an abruptly increased supply of wedge-top sediment produced an additional sedimentary load that helped promote late Miocene advance of the central Andean thrust front in the Subandean Zone. [ABSTRACT FROM AUTHOR]

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non-marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow-dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km2) with remarkably few incised channels or stacked-channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non-marine setting (>0·45 mm year−1 for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine-grained to medium-grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine-grained to medium-grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow-dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross-stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross-stratified sandstone encased in mudstone-rich floodplain deposits. The ubiquitous thin-sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow-dominated system are attributed to fluvial fan conditions in an arid to semi-arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long-term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow-dominated fluvial systems. [ABSTRACT FROM AUTHOR]

Examines the development of early to middle Tertiary foreland basin in Bolivia. History of Andean crustal shortening; Presence of interbedded fluvial sandstone and mudstone; Measurement of lacustrine carbonate and clastic rocks; Consistency of the lithofacies and sediment-accumulation history.

Responds to the comments of Thierre Sempere on geologic mapping and structural interpretations in the Eastern Cordillera of southern Bolivia. Stratigraphic seperation diagrams for the major contractional faults in the map area; Evaluation of Semepere's analysis of the El encuentro syncline and Tocloca faults.