Kimmeridgian? to Paleocene Tectonic Evolution of Bolivia

Abstract

The Puca Group (Kimmeridgian?-Paleocene) of Bolivia recorded the external (distal) tectonic evolution of the Andean back-arc basin in these latitudes. Bolivia had been part of stable cratonic South America until Late Jurassic time, when it was captured by the Andean system during a large-scale extensional “Araucan”-age tectonic event. This episode seems to be related to the onset of large-scale extensional and transtensional conditions in northern Qüle and coastal Peru. In Bolivia, it led to initiation of the tectonically-controlled, highly fragmented Potosí basin filled with unfossiliferous continental siliciclastic deposits (mostly red beds), with relation to the reactivation of the major transversal Khenayani-Turuchipa paleostructural corridor. Extension was locally accompanied by basic volcanism, and created a tüted-block structure, with half-grabens showing topographic downwarps and uplifts upon which younger fine-grained strata onlapped. The oldest and most important extensional episode took place during deposition of the lowermost part of the Puca Group (Condo conglomerates). A younger minor extensional episode developed locally, possibly in Late Neocomian and/or Aptian time. Albian? time saw a large-scale onlap of brown to violet-red mudstones over the previous deposits and, locally, on the Paleozoic basement, indicating a relative change in the tectonic setting, marked by a slow and gentle widening of the sedimentation area without any small-scale extensional manifestations.

Shallow-marine carbonates were deposited during the Cenomanian-Turonian interval, in an area of lesser extension than the Kimmeridgian?-Albian? basin. In this unit, thickness variations are very gentle, sedimentation rates seem very low and no direct indication of synsedimentary tectonics is known. It is thus assumed that this transgression was mainly of global-eustatic origin.

The Senonian-Paleocene sequence consists of a thick pile of mainly mudrocks beds. It was deposited in the external part of the wide, underfilled foreland basin of the paleo-Andes, which at that time were of small size and had produced only a minor flexure of the South American lithosphere. The base of the sequence records a noteworthy extensional episode, accompanied by relatively widespread basaltic volcanism, which indicates coeval thickening of the asthenospheric wedge and thus an increase of the subduction angle, at least between lat 17° and 27°S. This tectonic upheaval marked the real onset of the compressional building of the Andes from the west. In Bolivia, it reactivated Late Jurassic-Early Cretaceous tensional structures and produced new normal faults. Coniacian playa-lakes formed in the low areas of the fragmented topography induced by this episode. Santonian-Campanian deposits (red mudstones and minor sandstones and evaporites, with two thin restricted-marine carbonate intercalations) show a clear and general onlap toward all external areas, which is interpreted as the distal foreland effect of the “Peruvian”-age deformation developing in the coastal regions at that time. Decrease of sedimentation rate and rapid progradation of immature sand from the west in the Middle and Late Campanian suggest relative coeval tectonic quiescence on the Pacific margin. Increase of sedimentation rate, along with arrival of clean sand from the external areas, near the Campanian-Maastrichtian boimdary is interpreted to mark a notable reactivation of thrusting in the coastal regions, accompanied by uplift of a foreland forebulge. This episode is coeval with an important transgression from the north or northwest, which led to deposition of restricted-marine naarl and carbonate facies during Maastrichtian and earliest Paleocene time. Lacustrine and alluvial strata were deposited during most of Paleocene time. No major tectonic event is perceptible during the Maastrichtian-Paleocene interval, but fairly steady subsidence suggests that tectonic loading and hence thrusting was going on in the coastal regions.

A major modification of the regional tectonic system occurred near the Paleocene-Eocene boundary and generated a widespread unconformity in Andean Bolivia.