Vertical and Lateral Distributions of Benthic Foraminiferal Fauna and the Fluctuation of Warm and Cold Waters in the Middle Pleistocene of the Boso Peninsula, Central Japan
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概要
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Vertical and lateral changes of benthic foraminiferal fauna in southern Kanto, Japan, are described in detail for a relatively narrow stratigraphic interval, the middle Pleistocene U6 horizon. The U6 key tuff bed is important for stratigraphic correlation and is defined as a collective unit which includes nine characteristic tuff layers. These tuff layers outcrop for a distance of approximately 80 kilometers from Yokohama in the west to Kazusa-Ichinomiya in the east. Five stratigraphic horizons within the U6 beds were selected for analysis. Samples were collected from these horizons at seven section localities, which were selected to represent the different lithofacies seen laterally in the clastic part of the U6 beds. Samples used for analysis were taken from just below each of the selected tuff layers. Sample thicknesses at each section locality were adjusted to take into consideration differences in the vertical separation of adjoining tuff layers caused by different sedimentation rates. Thus, all the samples may be regarded as representing nearly the same length of time, calculated to be 86 years. A total of 296 species of benthic foraminifera belonging to 94 genera were identified as well as some selected planktonic taxa. Faunal analyses, including factor analysis, calculations of species diversity and species equitability, were carried out on the benthic foraminifera of this horizon. Sedimentological analyses were made using an automatic grain-size analyser. Four assemblages, whose distributions are controlled by both the movement of surface water masses and the water depth, are distinguished within the stratigraphic interval around the U6 key tuff horizon. The assemblages are : the sublittoral Kuroshio to mixed Kuroshio assemblage composed of the species, Globocassidulina subglobosa (Brady), Bolivinita quadrilatera (Schwager), Pseudononion japonicum Asano, Elphidium crispum (Linnaeus), Cibicides pseudoungerianus (Cushman), Cibicides refulgens Montfort, Bolivina robusta Brady, Hanzawaia nipponica Asano, and Bulimina marginata d'Orbigny; the bathyal Kuroshio assemblage, composed of Cassidulina carinata Silvestri, Bolivina pseudoplicata Heron-Allen and Earland, Bulimina aculeata d'Orbigny, Gyroidina orbicularis d'Orbigny, and Brizalina subspinescens (Cushman); the bathyal mixed Kuroshio assemblage, which consists of Nonionellina labradorica (Dawson), Cassidulinodies kuwanoi Matoba, and Brizalina bradyi (Asano); and the bathyal assemblage which includes both the Kuroshio and the Oyashio water species, Bulimina aculeata, Cassidulina carinata, Bolivina pseudoplicata, Uvigerina akitaensis Asano, and Uvigerina bradyana Fornasini. Using the geographic and vertical distributions of these assemblages within the U6 horizon, both paleoenvironments and faunal successions are reconstructed. The western portion of the studied area is considered to have been a sublittoral environment, in which the Kuroshio and mixed Kuroshio fauna were dominant. In the eastern portion, which is interpreted to have been a bathyal environment, the species changed progressively from the present-day Kuroshio species to those of mixed Kuroshio species, and then to the Kuroshio species again. The vertical change in dominant species is correlative with the movement of the surface water mass as suggested by the planktonic assemblages. In the upper part of the eastern portion, the Oyashio fauna is associated with the Kuroshio fauna.
- 東北大学の論文
- 1977-03-31