Response of Carboniferous - Permian mid-oceanic seamount-capping buildup to global cooling and sea-level change: Akiyoshi, Japan

We discuss the response of a Carboniferous - Permian, mid-oceanic atoll-type carbonate buildup to global cooling and sea-level fluctuation during Gondwana glaciation. Our examination focused upon the stratigraphic distribution of emergence-indicative diagenetic features as a proxy for sea-level falls, and we correlated their timing with hiatuses indicated by fusuline biostratigraphic discontinuities and the demise of warm-adapted metazoan reef builders. We examined the Pennsylvanian (Moscovian) to Lower Permian (Asselian) shallow-marine limestone succession (ca. 500 m thick) of the Akiyoshi Limestone Group, southwest Japan, reconstructed as a mid-oceanic seamount-capping buildup in the Panthalassan Ocean. The examined succession is dominated by atoll - lagoon facies with subordinate coral - Chaetetes patch reefs and calci-microbial mounds. We have recognized several stratigraphic intervals, less than 60 m thick, in the examined succession in which emergence-indicative diagenetic facies are concentrated. These facies include the microcodium structure, pendant cements, recrystallization, and algal casts. Each of the intervals occurs immediately below a fusuline zone boundary and represents a sea-level fall event, which lasted presumably 1 Ma or less, possibly reflecting the sea-level fluctuation of third-order cycles. The emergence event in late Kasimovian is the most prominent in the series of Moscovian through Asselian emergence events and resulted in a dissolution unconformity indicated by a fusuline biostratigraphic discontinuity. We infer that many intercalations of less freshwater-altered limestone in these stratigraphic intervals resulted from short-lived emergence - resubmergence episodes possibly due to the high-frequency glacio-eustyaic sea-level fluctuation. We also showed that the demise of warm-adapted metazoan reef builders of corals and Chaetetes happened nearly synchronously with the culmination of Gondwana glaciation. We hypothesize that the demise of these warm-adapted reef builders is a biotic response of the Akiyoshi buildup to the severe and prolonged global cooling during the Pennsylvanian to Early Permian icehouse period. The Akiyoshi metazoan reef builders were replaced by calci-microbes, phylloid algae, and calci-sponges that exhibit an increasing significance during the Lower to Middle Permian. Our results reveal the distinct sedimentary and biotic responses of a Panthalassan atoll to global cooling and sea-level fluctuation during Gondwana glaciation in the Southern Hemisphere. We conclude that Carboniferous - Permian mid-oceanic atoll stratigraphy has significant archival potential for recognizing the global palaeoclimatic change.