Marine Paleobathymetry and Paleotopography of the Hokuroku District during the Time of the Kuroko Deposition, Based on Foraminiferal Assemblages

The marine paleobathymetry and paleotopography of the Kuroko deposits are reconstructed, based on foraminiferal assemblages occurring in bore holes of the Hokuroku district, Akita Prefecture, Northeast Japan. Judging from the fossil planktonic foraminiferal assemblages, Hokuroku district was predominated by a subtropical water mass during the time of. the Kuroko deposition. Correlating the benthonic foraminiferal assemblage of the Hokuroku district with those of present-day subtropical Kuroshio area, the paleobathymetry of the Kuroko deposits is inferred to range from 500 to 2,500 meters. Reconstructed submarine topography indicates the existence of a ditch-like depression at that time. Arenaceous foraminiferal assemblages distributed in the Hokuroku district may have inhabited in an extraordinary acid water condition which was caused by the dissolution of volcanic gas supplied from the sea bottom. Introduction In general, distribution of an organism is controlled by many environmental factors. However, it is difficult to determine the kinds of environmental factors which mainly control the distribution of the organism. A specific assemblage occurs in particular natural environments. Therefore, the nature of an environment can be reconstructed by means of surveying animal assemblages. In the case of foraminifera, present-day assemblages are well distinguished in relation to both the geographic distribution and the water depth. Since HORIKOSHI (1960) first stressed the hydrothermal sedimentary origin of the Kuroko ore, discussions arose as to what kind of sedimentary environment existed at the time of the Kuroko deposition. It has been considered by Japanese mining geologists that Kuroko was formed in shallow seas, about 200 to 500 meters deep (TATSUMI and WATANABE, 1971; SATO, 1974; LAMBERT and SATO, 1974) without basing any clear paleontological evidence. Recently, GUBER and OHMOTO (1978) questioned this idea and proposed that the Kuroko deposition took place in a deep-sea environment. However, it is still not clear in what kind of submarine topography the Hokuroku district was situated during the time of the Kuroko deposition because their discussions were based on data obtained from only one or two bore holes. The purpose of this study is to estimate the paleobathymetry and to reconstruct the submarine topography for Kuroko deposits of the Hokuroku district, Akita Prefecture, Northeast Japan. All the data on benthonic foraminiferal occurrences at the Hokuroku district are based on reports by the Metal Mining Agency of Japan (1965, 67, 68, 69). For detailed informations on foraminiferal species, readers should refer to those reports. In this article, the distribution of the present-day benthonic foraminifera off the coast of Japan are compiled. And these data are used for analyzing areal distributions of fossil benthonic foraminiferal assemblages associated with the Kuroko of the Hokuroku district. Received October 27, 1978, in revised form May 24, 1979. * Institute of Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422.