The Paleogene Hyuga Group in the Shimanto Supergroup, southern Kyushu, Japan, is lithologically divided into four formations: the Aradani, Tashiro, Unama, and Ouchibaru. These formations are characterized by imbricate structure, and each formation is repeatedly sliced and distributed by thrusts dipping to the north (landward). The mudstone unit of the Tashiro Formation is composed mainly of massive mudstones with thin sandstone interbeds. It occurs in seven belts, of which the Shiitani, Nagano, and Unama belts have larger widths than others. Based on a biostratigraphic study of abundant planktonic foraminiferal fossils from the Tashiro Formation, five planktonic foraminiferal zones were recognized in the strata of these three belts. They are Morozovella spinulosa Zone (I), Globigerinatheka index Zone (II), Globorotalia cerroazulensis cocoaensis Zone (III), Pseudohastigerina barbadoensis Zone (IV), and Chiloguembelina cubensis Zone (V) in ascending order. They are firmly correlated with the well‐calibrated zones known from low‐latitude regions and are assigned to latest Middle Eocene, earliest Late Eocene, Latest Eocene, Earliest Oligocene, and Early Oligocene age, respectively. On the basis of the distribution of the fossil zones in the three belts the stratigraphy and geologic structure of mudstone unit of the Tashiro Formation are summarized as follows: 1. The strata of each belt are cut into many sheets mainly by thrusts dipping to the north (landward), forming an imbricate structure, and the same strata appear repeatedly many times with a disordered array within a single belt: 2. Many of the sheets show a normal sequence, but a few sheets are overturned: 3. The thickness of each sheet is less than 200 m: 4. In each belt, the older planktonic foraminiferal zones occur widely in the southern part and the younger ones in the northern part. Therefore, the strata of each belt show a tendency of northward (landward or inward) younging: 5. If the distribution of the foraminiferal zones of the three sections is viewed collectively, the upper zones (IV–V) are more widely distributed in the Shiitani belt in the southernmost part of the investigated area than the Nagano belt that is in the central part: 6. To date, the youngest zone (V) has been found only in the Shiitani belt. Therefore it is concluded that each belt shows a roughly northward (landward) younging internally, but the arrangement of the overall foraminiferal zones among the three belts analyzed shows a southward (oceanward) younging. The stratigraphic arrangement and structure of the three mudstone belts of the Tashiro Formation may be characteristic of accretionary prisms.
ASJC Scopus subject areas
- Geochemistry and Petrology