Jérôme Chablais

Aulotortus friedli from the Upper Triassic gravitational flow deposits of the Kumagawa River (Kyushu, Southwest Japan)

Abstract. The benthic foraminifer Aulotortus friedli is reported from the Upper Triassic carbonates (capped seamount) of the Sambosan Accretionary Complex in Kyushu, Southwest Japan. Specimens were collected from shallow-water limestone clasts in a debris flow sequence near the Ose locality along the Kumagawa River. The limestone clasts mainly represent shoal facies where the benthic foraminifer is extremely abundant. Because of the remarkable preservation of the Japanese specimens, they allow further description of the involutinid A. friedli. In addition, a detailed sedimentary and environmental analysis of the gravitational deposits where A. friedli occurs is given. Using microfacies analysis together with the biostratigraphic range of A. friedli and associated foraminifers, a new interpretation is proposed of a seamount collapse process that occurred during the Norian-Rhaetian and preceded the major collapse event of the Sambosan seamount, which has been placed in the Middle to Late Jurassic.

New Crustacean Microcoprolites of the upper Triassic Limestones of the Sambosan Accretionary Complex, Japan

Abstract We report here the first crustacean microcoprolites from the Sambosan Accretionary Complex (AC) in Japan. Three species of crustacean (order Decapoda) microcoprolites - Payandea japanica n. sp., Favreina tosaensis n. sp., and a species belonging to the genus Parafavreina? sp. are described from the Late Carnian to Rhaetian shallow-water limestones of the Sambosan AC, which were originated on extinct and subsiding volcanoes forming a mid-oceanic atoll-type buildup in the Panthalassan Ocean. Coprolites, which occur within a lagoonal to back-reef facies, provide new insight into the micropaleontology and sedimentology of the Sambosan limestones. Their presence in this tropical shallow-water depositional setting heightens their status of sedimentary facies indicators of such environments. Furthermore the occurrence of these Tethyan coprolites genera in Japan improves our knowledge of the distribution of Triassic crustacean decapods across the Panthalassan Ocean.

Upper Triassic (Ladinian?-Carnian) reef biota from the Sambosan Accretionary Complex, Shikoku, Japan

The Middle and Late Triassic was a time of important reef development. This evolution, which is primarily documented in the Tethys realm, comprised several phases from the Anisian to the Rhaetian. To help elucidate the less constrained reef evolution in the Panthalassa domain, samples of reef limestone were collected from several localities along the Sambosan Accretionary Complex in Shikoku Island, southwest Japan. In this paper, we report a well-preserved and comprehensive reef biota, including several taxonomic groups, such as scleractinian corals, calcified sponges, calcareous algae, foraminifers, and microproblematica. Seventeen species are described for the first time in Japan among the 33 that are identified in this study. The assemblage-based biostratigraphy and index taxa indicate a Ladinian?-Carnian age. This new finding corresponds to an older reef limestone than has been previously identified in the Sambosan Accretionary Complex and may represent the initiation of shallow-water carbonate deposition on western Panthalassa seamounts. This work also provides valuable insights on reef ecosystem biodiversity in the Panthalassa domain during the Middle? to Late Triassic.

Mid-oceanic shallow-water carbonates of the Panthalassa domain: new microfacies data from the Sambosan Accretionary Complex, Shikoku Island, Japan

During the Late Triassic, carbonate platforms expanded on continental shelves and island arcs. They have been studied in detail in the Tethys realm but coeval mid-oceanic shallow-water environments of the Panthalassa domain have received less attention. To fill this gap, we investigated the Upper Triassic limestone of the Sambosan Accretionary Complex (SAC), southwest Japan. A comprehensive sampling of the Upper Triassic limestone has been performed in 16 localities at Shikoku Island. Eight microfacies, ranging from pelagic to lagoonal settings, were identified, including two microfacies that are described in the SAC for the first time. Quantitative microfacies analysis assesses the composition of the SAC limestone and its biostratigraphy is refined. Finally, a speculative depositional model of the SAC carbonate platforms is proposed based on modern and ancient analogues. Instead of a true atoll-type platform with well-defined facies belts as suggested by previous studies, this new model suggests that the typical Sambosan platform was more probably a carbonate bank with submerged margins and a mosaic of microfacies in the platform interior. Special attention is given to the factors that have probably controlled the carbonate sedimentation at the top of the seamount(s).