Alter Late Middle Pleistocene Combined Stratigraphy of Nannofossil and Tephra in off Senshu, Osaka Bay

Abstract

In the Quaternary stratigraphy, nannofossils serve superior transoceanic datum planes in open sea and fall tephra give the most reliable marker horizons in and around near shore area. Especially, island arcs in the western margin of the Pacific are suitable area to constract the combined stratigraphy which mediates both datum planes.
In the Osaka bay area, a new airport plan was started in 1981. On the course of geological survey, some long cores were taken from the area. In the cores Brs. 56-9, 57-5, 57-9, 57-30 and H-6 are very interesting for the present study. Nakaseko et al. (1984 b) defined the late Pleistocene sequence of off Senshu as the Senshu-oki Group and devided it the lower, Senshu-oki and the upper, Kuhko-shima Formations. Then, they separated the Kuhko-shima Formation to eleven members based on their lithofacies and some kind of microfossil occurrences.
Concerned to nannofossil biostratigraphy, three nannofossil events were detected in the sequence and they were the last occurrence (LO) of Pseudoemiliania lacunosa (Kamptner) Gartner, the first appearance (FA) of Emiliania huxleyi (Lohmann) Hay and Mohler and the start of acme stage (s.a.) of Emiliania huxleyi in ascending order. In the transoceanic correlation, LO of Pseudoemiliania lacunosa is estimated at 440 ka, FA of Emiliania huxleyi 270 ka and s.a. of Emiliania huxleyi 70 ka (Gartner, 1977).
On the other hand six conspicuous marker tephra are defined in the Kuhko-shima Formation based on volcanic glass major chemical composition analyzed with an energy dispersive microspectrometry (EDS). They are Hatchoike, Sakura, Sugawara, V-7, AT and Ah in ascending order. These tephra are found as a volcanic glass-rich layers and spread widely in the core samples in and around the Osaka Bay. Hatchoike is in Ma5 and Sakura is in the lower part of Ma7 clay beds of the group. Both tephra are reliable markers in the Upper Formation of the group which were defined in land sequence of Kinki. The fall ages of Hatcho-ike and Sakura are estimated at ca. 500 ka and 400 ka respectively. Machida and Arai (1992) correlated V-7 to Ata which ejected in 90-110 ka from Ata caldera of southern Kyushu. Aira (AT) and Akahoya (Ah) are reported by many authors and their fall age are estimated at 24 ka and 6.3 ka respectively.
In the present area, following nannofossil events and tephra falls are listed in ascending order : LO of Pseudoemiliania lacunosa, falls of V-15, Sakura, V-14, V-10, Sugawara, V-8, FA of Emiliania huxleyi, s.a. of Emiliania huxleyi, falls of AT and Ah. In general age estimation of these events are correspond with international correlation and established tephrostratigraphy in Japan.