Geology of the Copacabana Group in the Colquencha Area, Central Bolivia

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  • ボリビア中部Ｃｏｌｑｕｅｎｃｈａ地区のＣｏｐａｃａｂａｎａ層群について
  • ボリビア チュウブ colquencha チク ノ Copacabanaソウグ

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The Copacabana group distributed between the two ranges of the Andes Oriental and the Andes Occidental in the central part of Bolivia represents one of the standard stratigraphic sequences of the upper Palaeozoic sediments in Central Andes. In 1973 the writers carried out the geological reconnaissance at the Colquencha area, Altiplano, paying special attention to that group. The Palaeozoic formations in this area consist of Silurian, Devonian, Carboniferous and Permian systems, the last of which is called as the Copacabana group. The group estimated about 300 m in thickness is well exposed at the northeastern foot of mountainland near Colquencha. It lies conformably on the Carboniferous system consisting of the alternation of sandstone and shale. The group is mainly composed of fine-to coarsegrained reddish sandstone, black or dark grey shale and dark or reddish limestone. Its lower part is dominated by massive sandstone, and the upper part by the alternation of sandstone, shale and limestone. The limestone contains such fossils as Brachiopods, Gastropods, Fusulinids, Corals, Crinoids, Bryozoans and so forth with calcareous or siliceous angulated nodules.<BR>The writers found Pseudoschwarina, Schwagerina, “Rugosofusulina” and other fusulinaceans from the upper-most horizon of the group in this area. Judging from these fossil evidences, it is thought that the upper-most horizon of the group assigns to the Lower Permian. The geological structure of the Palaeozoic strata is characterized by a syncline with axis running NNE-SSW and by the faults of NW-SE. The deformation of the Palaeozoic strata might had been completed before the Tertiary sediments were deposited.

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