中部アンデス地域の古生代後期古生物群集の解明 平成２年度現地調査報告

Abstract

In succession of our biostratigraphic study of the Paleozoic and Mesozoic Groups in Central Andes, which was started in 1980, more detailed field survey and re-examinination were undertaken during late July to late August, 1990.The present article is a preliminary report on the field survey of the Tarma Group (Middle Carboniferous) developed near Tarma City in Peru and the Copacabana Group (Upper Carboniferous to Lower Permian) developed near and around Titicaca Lake in Bolivia. The journeys in these two countries are shown in Figures 1 and 2.The aims of the present research are mainly as follows : 1) To be completed our biostratigraphic study of the Upper Paleozoic (Carboniferous-Permian) in Central Andes with the conodonts as additional data, and 2) To make clear the boundary between the Carboniferous and Permian Systems in the Copacabana Group.In Peru, the rock samples for conodont analysis were collected from the Tarma Group which consists mainly of dark gray limestone of ca. 70 m thick. The lower half of the limestone is more or less massive but the upper one intercalates slate and shale beds. In the field work, many fusulines, bryozoans, brachiopods, corals, molluscs, etc. were found, especially the genus Fusulinella which indicates the lower Middle Carboniferous were common through the lower to upper horizons. Although no conodont datum has been hitherto known from not only the Tarma Group but also the Paleozoic of South American Continent except for the Carboniferous in Colombia by Stibane (1967), it will become possible the world-wide relationships with the other regions by the conodonts expecting to be found.In Bolivia, the Copacabana Group developed in Yaurichambi, Ancoraimes, Matilde, Cuyavi and Yampupata, all of the routes formerly measured by us in 1982 and 1984 were re-examined and collected some rock samples for conodont analysis, especially around the boundary between the Triticites Zone and Pseudoschwagerina Zone in the Copacabana Group. The Copacabana Group has been divided into three fusuline zones, namely, the Triticites, Pseudoschwagerina and Eoparafusulina Zones in ascending order, and all of which has been recognized to be the Lower Permian (Wolfcampian) in age by previous students since Dunbar and Newell (1946) and so on. Su$eacute;rez-Riglos et al. (1987), however, reported on the conodont biostratigraphy of the Copacabana Group and they divided it into four conodont assemblage zones, namely, Streptognathodus elongatus Ass. Z., Idiognathodus ellisoni Ass. Z., Neogondolella bisselli-Sweetognathus whitei Ass. Z. and Neostreptognathodus pequopensis-Sweetognathus behnkeni Ass. Z. in ascending order. They are of the opinion that, the lower two assemblage zones, which have been recognized as the lowermost Permian Triticites Zone or Triticites nitens Subzone by the previous students including us, should be the uppermost Carboniferous (Virgilian) by the conodont evidence. The first occurrences of Pseudoschwagerina are in the horizons at 25-40 m from the base of the Copacabana Group in Yaurichambi, Ancoraimes and Matilde routes, and at 240-280 m in Cuyavi and Yampupata routes.Accordingly, at present, the correlation of stratigraphic sections is revised and the boundary between Carboniferous and Permian is tentatively placed at the base of Pseudoschwagerina Zone as shown in Figure 4.Furthermore, four lithologic sequences distinguished by the field observation are briefly described.