アンデス中部地域の地質・古生物学的研究

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The writer carried out the geological and paleontological studies on the central part of the Andes with the cooperation of Peruvian and Bolivian geologists from late August to December in 1973. The principal purposes of this work are as follows : A) The collection of fossils in various localities based on the stratigraphical point of view. B) The observation of fossils collected from the Andes in order to compare these fossils with Japanese ones. C) To make clear the mode of sedimentation on several strata. D) To define the geological structure of the fossil-bearing strata. E) The palaeogeographical studies on the fossils occurring from the circum-Pacific region including Japan and the central part of the Andes. Although the specimens collected from these area are now under studying, some results of the present work already known are as follows : 1) The Lower Cretaceous system in the Huaraz area is divided into the Goyllarisquizga Group and Pariahuanca, Chulec and Pariatambo formations (in ascending order). The Goyllarisquizga Group is subdivided into Oyon, Chimu, Santa, Carhuaz and Farrat formations, and is characterized by the thick milky coloured quartzite. The Santa Formation cropping out widely in the western side of the Branco Cordillera consists mainly of alternation of fine-to coarse-grained quartzose sandstone and black shale, and yields trigoniids, Ostrea, Ammonites, Brachiopoda and so forth. Judging from the occurrence of trigoniids and Ammonites, this formation may be assigned to Lower Cretaceous. 2) This thick black limestone and thin-bedded black shale exposed at the Yanashallash Pass extend northward from this pass to Pucaraju. These strata are distinctly folded by the crustal movement as seen at the west side of this pass. From the evidences of Ammonites and Inoceramus, the strata of the area may indicate middle Lower Cretaceous rather than Jurassic. 3) The Mitu and Pucara formations distributed in the Lircay area form a large anticline. The Pucara Formation developed in this area is the most important representative of the Mesozoic Group, and is underlain unconformably by the Permian Mitu Formation. The Pucara Formation consists of limestone, sandstone, shale and pyroclastic rocks. The lower part of the formation yields trigoniid fossils as Minetrigoniatype, but the upper one yields Weyla alata, Brachiopoda and other mollusks at several localities. These trigoniids are one of remarkable fossils characterized by the distinct marginal carina and radial ribs on the disk. Judging from the fossil evidences, it is thought that the lower part of the Pucara Formation assigns to Triassic, but the upper one of the formation seems to be Jurassic. 4) The Copacabana Formation is well developed in north- and southward of La Paz, and consists of the alternation of light gray limestone, shale and sandstone. Fusulinids, corals and other fossils are newly found from several horizons at Yaurichambi and Colquencha.
社団法人東京地学協会の論文

アンデス中部地域の地質・古生物学的研究

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