Ishida N. (2007) Sedimentary and tectonic evolution of the Upper Jurassic trench-slope basin deposits in the Southern Chichibu Terrane, Outer zone of Southwest Japan // Graduate School of Science and Technology, Kumamoto University. PhD Thesis. 78 pp.
In Outer zone of Southwest Japan, the distribution of the Upper Mesozoic strata including 'Torinosu-type limestone' are well known. The Torinosu-type limestone is a generic name for the Mesozoic shelf-type limestone in Outer zone, which sometimes contains reef components like as corals and stromatopoloids with terrigenous clasts. In Outer zone, the Torinosu-type limestone is specific element in contrast with the accreted seamount-type limestone. This study reveals the sedimentary and tectonic evolution of the Upper Jurassic Torinosu-type limestone bearing strata (TLBS) distributed in the Southern Chichibu Terrane. Although distribution of the TLBS is also well known in the Southern Chichibu Terrane, their comprehensive formation model has not been presented. This study describes lithostratigraphy, biostratigraphy and lithofacies of the Miyazono Formation, which is one of the Upper Jurassic TLBS in the Southern Chichibu Terrane, in the Itsuki-Gokanosho area, western Kyushu. The Miyazono Formation subdivided into three members: Kurekogawa Mudstone Member, Nakamichidani Sandstone Member and Kamiarachi Mudstone Member in ascending order. Conglomerates in the Nakamichidani Sandstone Member contain pebbles of the Torinosu-type limestone and marl. Total thickness of the Miyazono Formation reaches ca. 2600m. The Miyazono Formation covers the Momigi Complex, which is a Middle Jurassic chert-clastic complex regarded as an ancient accretionary prism. The Miyazono Formation yields the radiolarians in the Kilinora spiralis Zone to the Pseudodictyomitra carpatica Zone, and is correlated with lower Upper Jurassic to upper Upper Jurassic (Oxfordian Stage to upper Tithonian Stage). Because the Momigi Complex also includes the Kilinora spiralis Zone, remarkable timegap is not estimated between the Momigi Complex and the Miyazono Formation. The Miyazono Formation certainly covers the ancient accretionary prism conformably. The Nakamichidani Sandstone Member is dominated by coarse-grained sediments. Further, the Miyazono Formation is comparatively thick for the trench-slope apron sediments. These features indicate that the main part of the Miyazono Formation was deposited in the trench-slope basin. Actually, the features of the Miyazono Formation are extremely similar to those of the Nias beds in the forearc of Sunda Trench, which is the 'type example' of trench-slope deposit. Based on the investigations about the modern and ancient examples, the trench-slope basin deposits are divided into two lithofacies, immature basin facies and mature basin facies. The former dominates the lower part of the trench-slope basin deposits and the latter dominates the upper part. This lithofacies transition reflects the evolutionary stages of the basin. In the case of the Miyazono Formation, the immature facies correspond to the Kurekogawa Mudstone Member and the mature basin facies correspond to the Nakamichidani Sandstone Member. The lithofacies of the Kamiarachi Mudstone Member indicate that this member is considered to be a trench-slope apron deposit. The Upper Jurassic TLBS are distributed intermittently over 900km long in the Southern Chichibu Terrane. Among these TLBS, some common features are found: basement, initiation time of sedimentation, sedimentary facies and so on. Because most of the TLBS in the Southern Chichibu Terrane include the Kilinora spiralis Zone in the basal parts, the sedimentation of the TLBS was certainly initiated contemporaneously and extensively in Oxfordian age. All TLBS include mudstones with exotic blocks corresponding to the immature basin facies. In addition, comparatively thick TLBS includes coarse-grained sediments corresponding to the mature basin facies in the upper halves. According to the age data of the twelve Torinosu-type limestone bodies in Outer zone, the Upper Jurassic TLBS contain the Middle Jurassic Torinosu-type limestone bodies as exotic blocks. The Torinosu-type limestone bodies were formed under the shallow-marine environment on the accretionary prisms like as trench-slope breaks. After that, they were broken or eroded and were transported into the trench-slope basins during Late Jurassic time. The tectonic and sedimentary evolution of the TLBS in the Southern Chichibu Terrane is summarized in chronological order as below. In the forearc regions of eastern margin of Asia, accretionary prisms, which were formed by frontal offscraping, were evolving during Middle Jurassic age (Togano Stage). On these accretionary prisms, several trench-slope basins were formed in early Late Jurassic (Oxfordian) time. Initially, fine-grained sediments and allochthonous blocks derived from surrounding unstable slopes were transported into the basins. During Late Jurassic time, these trench-slope basins were aggraded by terrigenous sediments, which were supplied through conduits cutting the sedimentary bodies including the Torinosu-type limestones on the trench-slope breaks. Aggradation of basins was almost finished in late Late Jurassic (late Tithonian) time. After that, collisions of the Sanbosan seamounts occurred during Early Cretaceous age (Sanbosan Stage). One plausible cause of this basin formation event is the motion change of the subducting Izanagi Plate. This deformation event in early Late Jurassic time may be one of the representation of the global tectonic events in the circum Pacific regions.