Notes on the Permian Copacabana Group in Bolivia

Abstract

The Permian Copacabana Group consisting of limestone, grey shale, and sandstone, is exposed, being scattered over the districts of Escoma, Carabuco, Ancoraimes, Isla del Sol, Peninsula de Copacabana, Tiquina, Peninsula de Cumana, Peñas, Yaurichambi, Colquencha, Santiago Llallagua, Apillapampa, Torotoro and elsewhere, in the Altiplano, Cordillera Oriental and Subandes regions. General trend of the distribution is from NW to SE through Titicaca Lake. In the Altiplano region, the Copacabana Group covers conformably the alternation of sandstone and shale of the continental Carboniferous System, and is unconformably overlaid by the clastic reddish rocks of the Lower Cretaceous Puca Formation.
The Copacabana Group is represented predominantly by limestone and indicates the uniform sedimentary environment in the region. The limestone, light grey or dark grey in colour, contains such fossils as fusulinids, brachiopods, corals and others. The Group shows that the region was under the shallow sea. Judging from the fusulinid species, the Group is presumed to be Wolfcampian to Leonardian in age.
The Group was deposited during the southward transgression from Peru into Bolivia in early Permian Period. The sea might be a southern extension of the Tethys, and it transgressed from northwest to southeast, cutting obliquely the axis of Cordilleras Oriental to the north of Poopo Lake. It is thought that the Potosi and the Tarija regions, the southern part of Cordillera Oriental, were elevated before the Group was deposited.
In the Titicaca Lake region, the Group, together with the Lower Cretaceous Puca Formation, forms a large syncline, which is the Tiquina syncline extending from NW to SE, the axis of which passes through Tiquina channel. The axes of some other anticlines and synclines in this region run parallel to that of the Tiquina syncline. In the Cordillera Oriental region, the Group was also folded with the axes running from NW to SE. These geologic structures were cut off by the thrusts trending NW to SE. The thrusts were formed before the Tertiary System was deposited.
In Bolivia, there were two remarkable crustal deformations, one was after the deposition of the Devonian System and the other was between the latest Cretaceous Period and the earliest Tertiary Period. The Bolivia region was emerged and suffered the denudation from the late Permian to the end of the Jurassic, after the Copacabana Group was deposited.