Sedimentary history with biotic reaction in the Middle Permian shelly sequence of the Southern Kitakami Massif, Japan

Abstract

AbstractThe transition of sedimentary environments and their organism‐related responses is exemplified in the active margin setting of the Middle Permian in the Southern Kitakami Massif. The transition in the sedimentary environment began with a shallowing‐upward sequence at an upper slope to an outer shelf depositional setting that was associated with a delta system which transported abundant botanic remains in the Hoso‐o Formation. By contrast, the sediments of the overlying Kamiyasse Formation, the base of which is roughly equivalent to the lowermost Capitanian, were deposited at a lower shoreface to the outer shelf setting, which originated from complex depositional sources, including beach, near shore reef mounds and a nearby independent area of shallow and hard substratum with a considerable difference of depth. This unique depositional setting resulted in the seemingly ‘mixed’ fauna associated with this formation. The depositional environment of the overlying Kurosawa Formation is similar to that of the Hoso‐o Formation, but with far fewer monotonous biotic remains. A comparison of lithological characteristics and fossil remains with those of neighboring regions suggests that the independent mounds were generated as if the talus formed elsewhere so as to provide a large amount of skeletal elements to a deeper basin in the earliest Capitanian. Accordingly, the complex sedimentary setting observed in the Kamiyasse Formation occurs widely throughout the South Kitakami Massif, making possible the ‘mixed’ and seemingly diverse fauna from a mixture of multiple allochthonous origins.