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).