Birth and death of seamounts in the Panthalassa Ocean: Late Triassic to Early Jurassic sedimentary record at Mount Sambosan, Shikoku, Southwest Japan

Mid-oceanic shallow water carbonate platforms are usually characterized by long-term deposition due to the continuous subsidence of the underlying volcanic edifices and thus provide good records of biotic evolution and environmental changes. However, since most of the pre-Jurassic oceanic crust has been subducted, very few remains of Triassic atoll-like platforms are preserved in the Circum-Pacific region. Therefore, it is crucial to precisely document these Triassic carbonate occurrences to better understand the global evolution of climates and ecosystems during the Early Mesozoic in the huge Panthalassa Ocean. In the Sambosan Accretionary Complex (Southwest Japan), several Upper Triassic shallow water limestone blocks are especially well exposed at Mount Sambosan in Shikoku Island. Microfacies analyses associated with biostratigraphic data (reef assemblages, foraminifer associations and conodonts) allow us to reconstruct the geological history of these seamount fragments from the first settlement of reef biota in the Ladinian-Early Carnian to platform growth during the Late Carnian to Rhaetian. Finally, the demise of the carbonate platform occurred during the Latest Triassic to Early Jurassic, followed by platform collapse and accretion to the East Asia margin in the Late Jurassic-Early Cretaceous. Furthermore, cathodoluminescence microscopy reveals that the Sambosan limestone experienced several diagenetic episodes including early cementation, dissolution and dolomitization. At the global scale, the comprehensive sedimentary record at Mount Sambosan shows that evolution of shallow water ecosystems in the Panthalassa Ocean is consistent with biotic and environmental changes documented in coeval carbonate platforms from the Tethys Ocean.