ボリビアの白亜系について

In Bolivia the Cretaceous System is distribution widely in the Altiplano, Andes Oriental, Subandes, and eastern hill regions. It is underlain unconfermably by the Palaeozoic formations, lacking most of the Triassic and Jurassic sequence. The System is characterized by the predominance of reddish or brownish red rock facies, and is named by STEINMANN (1904) as the Puca Group. The name was derived from “puca” which means red in Quechus. The whole section of the System is distributed in the Altiplano and Andes Oriental regions, and the upper part of the System is in the Subandes and eastern hill regions.<BR>1) In the Andes Oriental region the System forms a large syncline named the Huarachani-Mocomoco syncline. The System is divided into the Huayrapata, Ococoya, Suches, Moho and Huancané Formations in decending order. The Huancané and Moho Formations are well exposed along the northeast coast of Titicaca Lake, but the Suches, Ococoya, and Huayrapata Formations are typically seen in the area upper course of the Suches River which flows from northeast into Titicaca Lake. The Moho Formation is a marine deposits, consisting of fine-to medium-grained sandstone, brownish red or black shale and dark grey massive limestone. There are thin fossil beds in several horizons. These fossils are mostly of echinoids and brachiopods which consist poor number of species. The Huayrapata Formation yields Viviparus, Melanoides and others. Judging from these fossils the formation is considered to be a non-marine deposit.<BR>2) The system in the Titicaca basin, consisting of the Muñane, Vilquechico, Cotacucho, Moho, and Huancané Formations, forms a large synclinorium with axes trending from NW to SE.<BR>3) In the Miraflores district, southern Bolivia, the System can be classified stratigraphically into six formations, as follows : El Molino, Chaunaca, Aroifilla, Miraflores, Tarapay, and La Puerta Formations in descending order. The Miraflores Formation consist chiefly of black or dark grey massive limestone, and contains pelecypods, brachiopods and ammonites. Based on these fossils the formation corresponds to the Moho Formation in Altiplano and Andes Oriental regions. It has been considered as Cenomanian in age. The formation is stratigraphically important in central Bolivia, because it is the most pronounced marine beds in the Cretaceous and can be used as the horizon marker. The El Molino Formation is another shallow marine deposits, yielding brachiopods, pelecypods and vertebrates. The formation is late Cretaceous in age.<BR>4) In the Altiplano region near Titicaca Lake, Subandes and eastern hill regions, the uppermost Cretaceous sediments are continous to the lower Tertiary formations which are composed of the remarkable reddish facies quite similar to the Cretaceous red beds. It is very difficult to define the boundary between the uppermost Cretaceous and the lower Tertiary sediments.