Litho- and Biofacies of Mudstone in the Pliocene-Pleistocene Kazusa Group, Boso Peninsula, Central Japan

Abstract

The Pliocene-Pleistocene Kazusa Group, deep- to shallow-marine clastic deposits attaining 3, 000 m thick, exposed in the Boso Peninsula, central Japan, is predominated by apparently monotonous mudstone. Based on the sedimentary structure, nature and intensity of bioturbation, and containing mega-fossils, the mudstone can be divided into following five different facies; (1) Greenish gray massive fine-grained mudstone facies (facies A). Moderately bioturbated, fine-grained mudstone, in which small, horizontal tube burrows are quite common. Benthic assemblages indicate the bathyal environments. (2) Bluish gray mottled coarse-grained mudstone facies (facies B). Intensely bioturbated, coarse-grained mudstone containing large vertical tube burrows. Lower-neritic environments are suggested from benthic assemblages. (3) Light gray massive fine-grained mudstone facies (facies C). Intensely bioturbated, fine-grained mudstone, in which large vertical tube burrows are abundant. Benthic assemblages indicate environments from outer shelf to uppermost bathyal. (4) Facies composed of alternating beds of greenish gray fine-grained mudstone and bluish gray fine-grained mudstone (facies D). This is the low-density turbidite facies; the bluish bed is a layer formed by muddy turbidity current and intercalated in the greenish gray massive fine-grained mudstone similar to the facies A. (5) Greenish gray laminated fine-grained mudstone facies (facies E). Parallel and narrowly spaced lamination is distinct in this fine-grained mudstone. Degree of bioturbation is low, and benthic megafossils are quite rare. The distribution of facies types changes stratigraphically, the facies A is seen in the lower part of the group, and is replaced upward by the facies B, and C, successively. In general, grain size and degree of bioturbation increase upward. The succession probably reflect the shallowing upward environmental change of the group. The facies D, the muddy turbidite, is found locally within the distribution of the facies A. Thin parallel laminated mudstone of the facies E suggests an oxygen-deficient environment. The in situ bed of the facies E is distributed in a restricted western part of the Yoro area, while they occur commonly as allochthonous blocks of the slump deposits in the facies A of the eastern area. The facies E might originally be deposited in the shallower western area, and most part of the facies might be slumped eastward, and preserved as allochthonous blocks.