in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 288: 101-151. Rohl H.J., Schmid-Rohl A., Oschmann W., et al. The Posidonia Shale (Lower Toarcian) of SW-Ger- many: An oxygen-depleted ecosystem controlled by sea level and palaeoclimate. Palaeogeography, Palaeoclimatology, Palaeoecology, 2001, 165: 358 27-52. Tanabe K. Early Jurassic macrofauna of the oxygen- depleted epicontinental marine basin in the Toyora area, West Japan. Proceedings of Shallow Tethys, Saito Ho-on Kai Special Publication, 1991, 3: 147-157.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Simultaneous Formation of Trench-slope Basin Deposits in the Southern Chichibu Terrane During Oxfordian, Outer Zone of Southwest Japan N. Ishida Faculty of Science, Niigata University, Niigata 950-2181, Japan (E-mail: nao.ishida21@mbn.nifty.com) In the Outer zone of Southwest Japan, distribution of the Upper Mesozoic strata including terrigenus shallow-marine carbonates (Torinosu-type limestone) is well known. This study focuses on the sedimentary and tectonic evolution of the Upper Jurassic Torinosu- type limestone bearing strata (TLBS) in the Southern Chichibu Terrane, a Jurassic to Early Cretaceous accretionary complex. Although TLBS are common in this terrane, the presence of shallow-marine carbonates in accretion-related deep-marine siliciclastics with accreted ocean-floor and seamount fragments remains unexplained. This study addresses the Miyazono Formation, an Upper Jurassic TLBS in the Southern Chichibu Terrane, western Kyushu. The formation, established by Ishida (2009), is subdivided into three members: the Kure- kogawa Mudstone Member, the Nakamichidani Sand- stone Member and the Kamiarachi Mudstone Member, in ascending order. Total thickness of the formation is approximately 2,600 m. Conglomerates in the Nakami- chidani Sandstone Member contain clasts of terrigenus limestone and marl. The formation overlies the Momigi Complex, which is considered to be an ancient accretionary prism. In the Miyazono Formation, radiolarian zones from the upper part of Kilinora spiralis Zone to the Pseudodictyomitra carpatica Zone (lower Upper Jurassic to upper Upper Jurassic; Oxfordian to upper Tithonian) are recognized (Ishida, 2009). The underlying Momigi Complex also includes the Kilinora spiralis Zone, no significant depositional hiatus is present between the Momigi Complex and the Miya- zono Formation. The Miyazono Formation conformably overlies the ancient accretionary prism, the Nakamichidani Sandstone Member is dominated by coarse-grained sediment, and the thickness of the formation is unusual for a trench-slope apron sediment. These characteristics indicate that the main part of the Miyazono Formation was deposited in a trench-slope basin rather than a trench-slope apron. These features of the Miya- zono Formation are strikingly similar to those of the Nias beds in the forearc of Sunda Trench, which are considered to be the ‘typical example’ of trench-slope basin deposits (Underwood and Moore, 1995). Based on studies of modern and ancient examples (e.g. Moore et al., 1980; Underwood et al., 2003), trench-slope basin deposits are divided into two litho- facies that reflect basin evolution; lower immature basin facies and upper mature basin facies (Under- wood and Moore, 1995). In the case of the Miyazono Formation, immature facies correspond to the Kureko- gawa Mudstone Member and mature facies correspond to the Nakamichidani Sandstone Member. The upper- most Kamiarachi Mudstone Member consists of trench-slope apron facies. Upper Jurassic TLBS are present over 900 km of strike-length in the Southern Chichibu Terrane. These TLBS have certain characteristics in common: proxy-mity to basement, simultaneous initiation of sedimentation, and similar sedimentary facies. The basal parts of most of the Upper Jurassic TLBS in the terrane belong to the Kilinora spiralis Zone, indicating that initiation of TLBS deposition in the Oxfordian was probably both simultaneous and widespread. All TLBS exhibit fine-grained immature basin facies in their lower parts, and comparatively thick TLBS contain mature coarse-grained sediments in their upper parts. The Upper Jurassic TLBS contain Middle Jurassic Torinosu-type limestone bodies as exotic clasts. The Torinosu-type limestone bodies are presumed to have been originally deposited in shallow-marine environ- ments on accretionary prisms, such as trench-slope breaks, during the Middle Jurassic. They were later broken and/or eroded, and their fragments were displaced into deeper trench-slope basins in the Late Jurassic. The tectonic and sedimentary evolution of TLBS of the Southern Chichibu Terrane started in the early Late Jurassic (Oxfordian). On the accretionary prisms, which were formed by the frontal delamination during the Middle Jurassic (Matsuoka, 1992), several trench- slope basins were formed in the forearc regions along the eastern margin of Asia. Initially, fine-grained sediment and fragments of accreted elements were transported into the basins from surrounding unstable slopes. During the Late Jurassic, these trench-slope basins were filled by terrigenous siliciclastics, which were supplied through conduits that cross-cut older sedimentary bodies including shallow-marine carbonates on the trench-slope breaks. Filling of the basins was completed in the late Late Jurassic (Tithonian), after which large volumes of seamount- type limestone (Sanbosan limestone) accreted from the latest Jurassic to the Early Cretaceous. One plausible cause for the simultaneous 359
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