A multidisciplinary study of upper Neogene continental deposits from Corque basin, central Altiplano, Bolivia: sedimentary evolution and faunal changes
Résumé
The Altiplano formed a broad Cenozoic inverted sedimentary basin (> 200 km wide) with up to 8 km
of fluvio-lacustrine filling in various sub-basins. This work is focused on the Corque basin which recorded
evolution of the central Altiplano and Neotropical terrestrial mammalian assemblages during the late
Neogene. Indeed, this area is inferred to have experienced a rise from ∼1,500 to 4,000 m, between ∼10
and 6 Ma. Paleoclimates rapidly changed from warm climate at 10 Ma, with a mean annual temperature
(MAT) ˜20◦C, to the current highland climate (MAT ˜8-9◦C). These series yielded both rich land mammal
assemblages essential for late Neogene biochronology and interbedded volcanic tuffs ranging a ∼10–2.8
Ma interval, marked by drastic faunal changes by the Miocene-Pliocene transition. A multidisciplinary study
of late Neogene deposits from the Corque basin near the village of Pomata allows for describing the
sedimentary evolution, refining chronostratigraphy through Ar/Ar datings of interbedded volcanic tuffs,
and implementing the corresponding fossil record. The studied section encompasses the Totora, Pomata,
and Mauri Formations, with 15 interbedded volcanic tuffs suitable for dating. The Totora Fm is mainly
composed of continental fine-grained sediments yielding desiccation features indicating temporary
emersions and some palynomorphs in finest sediments. Paleoenvironments correspond to ephemeral
braided streams and lakes. In the channels, eastward paleocurrent directions indicate that sediments came
from the Cordillera Occidental. The Pomata Fm begins few meters above an index volcanic tuff that crops
out over the whole basin and consists of mud or sand-supported mass flow deposits yielding numerous
terrestrial mammal fossils. The most conspicuous taxa are the mesotheriid notoungulate Plesiotypotherium
achirense and new echimyid and caviid rodents. The upper surface of the Totora Fm, eroded, corresponds
to an angular unconformity underlying the Mauri Fm. The latter starts with an index volcanic tuff (Toba 76;
5.34 +/- 0.003 Ma) and corresponds to four thinning upward fluvial sequences, each one starting with
cross-bedded conglomerates/coarse-grained sandstones, passing upwards into fine-grained argillaceous
sandstones. In these southward-flowing deposits, we recently found a new nothrotheriid ground sloth,
Aymaratherium jeani. The erosion of previous units and the change in transport direction may be related to
a major phase of relief growth.