Facies stacking patterns in high-frequency sequences influenced by long-term sea-level change on a Permian Panthalassan oceanic atoll: An example from the Akiyoshi Limestone, SW Japan

Detailed core examination of Middle Permian (Guadalupian) lagoonal successions of mid-Panthalassan atoll carbonates (Akiyoshi Limestone) revealed distinct stratigraphic variation of facies and sequence stacking patterns. The long-term sea-level change reconstructed from the atoll stratigraphy of the Akiyoshi includes a Bolorian to Midian gentle sea-level fall, punctuated by a short-lived Murgabian sea-level rise. These sea levels can be regarded as eustasy, judging from the monotonous subsidence of the Akiyoshi atoll and its good correspondence with the sea-level curves of other areas. Two sequence types are defined. Sequence type A consisting of shoal to lagoon grainstone occurred in low accommodation during the sea-level highstand-to-falling stages of the late Murgabian to Midian. This sequence type is bounded by two successive subaerial exposure surfaces and exhibits the uneven but generally low δ13C values throughout the section. In contrast, sequence type B occurred in increasing accommodation during the sea-level rise of the early Murgabian. This sequence type represents a composite sequence comprising parasequences and consists of various types of facies such as peritidal lime-mudstone, lagoonal packstone and boundstone, and shoal grainstone. Sequence type B is characterized by consistently high δ13C values except for the negative shift immediately below the sequence-top surface. These lines of evidence demonstrated that long-term eustatic sea-level change obviously controls the variations of sequence types in atoll carbonates.