|Zusammenfassung||The development of the Andes reflects the interaction of several processes, which in turn were triggered by the subduction of the oceanic Nazca plate along the South American plate margin. The principal processes include magmatism on the South American continent, thickening of the crust by shortening, and climate change. Shortening along the eastern flank of the Central Andes led to the development of a fold-thrust belt and a related retroarc foreland basin. In southern Bolivia, the fold-thrust belt consists of an internal zone (the Eastern Cordillera), an external zone (the Subandean Belt), and a transition zone (the Interandean). Eastward of this fold-thrust belt adjoins the flat plain of the Chaco, which forms the recent retroarc foreland basin of the Central Andes.
The Chaco Basin forms an ideal natural laboratory to study the effects of orogenic processes relating to foreland basin development. The study area encompasses the Subandean Belt and the Chaco Basin in southern Bolivia between 18°-22° S.
The Cenozoic sediments of the Subandean fold-thrust belt and the Chaco foreland basin reach up to 7500 m in thickness and are composed of five distinct formations:
1. The basal Petaca Formation is composed (from the base to the top) of calcretes, reworked pedogenic clasts, and fluvial sandstones interbedded with mudstones. The sandstones of the Petaca Fm are quartz-rich and show east-to-west-directed paleocurrents, suggesting extensive recycling or highly abrasive processes acting on rocks of continental provenance. The upper part of the Petaca Formation was deposited along the western flank of the forebulge whereas its lower part represents depozones located on the forebulge.
2. The Yecua Formation overlies the Petaca Formation. In the northern part of the Chaco Basin, the Yecua Formation consists of vary-colored mudstones, thin-bedded coarse-grained sandstones, marl with shell hash and ooids, and thinly interbedded mudstone-sandstone couplets. The lithologies denote a coastal environment with humid to semi-arid floodplains, shorelines, tidal areas and restricted shallow-marine environments. Sandstones are very quartz-rich and reflect north-to-south paleocurrents, indicating cratonic source rocks. In the southern part of the foreland basin, the Yecua Formation consists of mudstones interbedded with thin sandstone beds, suggesting a floodplain and overbank depositional environment. The lithologies of the Yecua Formation represent a wetland environment, probably located in a very distal foredeep.
3. The Tariquia Formation overlies the Yecua Formation and contains sandstones and mudstones, in which the thickness of the sandstone increases stratigraphically upwards at the expense of the mudstone. Sandstones show decreasing quartz content with respect to the underlying strata, indicating a provenance change from cratonic and continental provenance to a quartzose recycled orogen provenance, with paleocurrents directed west-to-east. The depozone of the Tariquia Formation corresponds to a central foredeep.
4. The Guandacay Formation overlies the Tariquia Formation and consists of thick sandstone and subordinate mudstone. The sandstone includes a significant amount of granules and cobbles. The sandstones of the Guandacay Formation carry a considerable amount of lithic fragments and feldspar, reflecting a quartzose recycled-orogen provenance. West-to-east directed paleocurrents confirm an orogen provenance. The Guandacay Formation corresponds to a proximal foredeep depozone.
5. The youngest unit of the foreland basin fill, the Emborozú Formation is composed of thick, well-rounded cobble- to boulder conglomerates and thick-bedded sandstone. Sandstone petrography shows a high content of lithic fragments and feldspar, representing a quartzose to lithic recycled orogen provenance. West-to-east directed paleocurrents confirm a very proximal orogen provenance. The Emborozú Formation was deposited in a wedge-top depozone.
A biostratigraphic study of ostracodes and foraminifera, which occur in the Yecua Formation in the northern Chaco Basin, yield a shallow-marine depositional environment. However, isotope ratios (strontium, oxygen, and carbon) of these microfossils suggest a water system that mainly consisted of cratonic water runoff and subordinate Andean water runoff. The isotopic compositions within the shells of marine species indicate a very short-lived shallow-marine incursion into a lacustrine environment or wetland located between the backarc of the central Andes and the Brazilian Shield.
Radiometric age dates, biostratigraphy, and lithostratigraphy of the Cenozoic foreland basin formations indicate a strongly diachronous age of the aforementioned, largely lithostratigraphically defined formations. The age of deposition for each formation decreases to the east and south. The eastward-decreasing age of the Cenozoic formations is a function of the eastward propagation of the deformation front since the Oligocene. The southward-decreasing age of each formation depends on the distinct and oblique eastward propagation of the deformation front between 17° and 23° S. The deformation front propagated in the north earlier into the Interandean and Subandean region than in the south, which resulted in significantly older depositional ages for identical lithofacies in the north.