2020 Volume 71 Issue 3 Pages 85-199
Stratigraphy of the Neogene Taga and Hitachi groups in the Kitaibaraki-Takahagi area (southern Joban area, Ibaraki Prefecture, Japan) has been established based on field survey and diatom biostratigraphic analysis. The both groups are the part of sedimentary sequences deposited in the Early Cretaceous to Recent Off-Joban forearc sedimentary basin.
The Miocene Taga group in this area, ranging in age from 16.6 Ma to 7.5 Ma, is a sedimentary complex made of three different kinds of deposits: shelf to slope deposits, submarine channel fills and submarine slide scar fills. The shelf to slope deposits include the Takaku, Kidoba, Kohama, Juogawa and Kushigata formations. In this study, the Takaku Formation, a unit previously treated as a distinct group (Takaku Group), is placed at the lowest part of the Taga Group on the basis of diatom biostratigraphy and field evidence on the stratigraphic relations with below and above. The formation unconformably overlies the Lower Miocene Yunagaya Group, and consists of basal lag conglomerate, poorly stratified bioturbated sandstone in the lower part, and massive sandy mudstone in the upper part. The Kidoba, Kohama, Juogawa and Kushigata formations are composed of diatomaceous massive mudstone or sandy mudstone deposited in the slope environment. The submarine channel fills of the Taga group consist of mudstone and sandstone that buried 18 small submarine channels (0.3 to 1.2 km in width) incised into the submarine slope environment. They form a nested sedimentary complex, preserving discrete 14 units (Units T1–T14) separated by erosional surfaces with hiatuses from each other, although Unit T13 is not distributed in this area. The stacking patterns of submarine channel fills reflects alternate periods of erosion and deposition and the long-lived nature of the channels ranging several million years. Although some units of the channel fills are composed of sandstone deposited from turbidity current, much of the units are characterized by hemipelagic massive diatomaceous mudstone with thin sandstone layers. Each of the submarine channel fills shows a synclinal structure that probably formed by post-burial compaction of mudstone. As submarine slides scar fills of the Taga Group, two units (the Takado and Kokaigahama Units) are recognized in the uppermost part of the group. The both units consist of sandy mudstone and mudstone overlying shelf to slope deposits of the Kohama and Kushigata formations, respectively with sharp and flat erosional surfaces.
The Hitachi Group distributed in the souther part of the studied area and the further southwars ranges in age from latest Miocene (ca. 7 Ma) to Pliocene (ca. 3 Ma). The group consists of 11 units (Units H1–H11) of submarine channels fills produced by repeated burial and re-incision within channels. Among 11 units, Units H6–H11 are distributed in this area, most of which are composed mainly of sandy mudstone, and to the lesser amount, interbedded mudstone and sandstone, and amalgamated thick sandstone. Five small submarine channels of 0.3–0.9 km wide are recognized in the Hitachi Group of this area, and their spatial and temporal migration is broadly reconstructed.