Palaeoenvironmental implication of resedimented limestones shed from Mississippian–Permian mid-oceanic atoll-type buildup into slope-to-basin facies, Akiyoshi, Japan

摘要

For better understanding of long-term environmental changes in a mid-oceanic atoll-type buildup (Akiyoshi Limestone Group), we examined the atoll-derived carbonate sediments that were contemporaneously shed into the spicular chert of slope-to-basin facies (Ota Group) in the Akiyoshi area, southwest Japan. Both of these two units are dated as upper Viséan to Capitanian, and interpreted as having laterally passed into each other on and around a mid-oceanic seamount. The resedimented limestone occurs as slab-like bodies, and isolated and laterally discrete smaller pods, and thin beds and laminae in the spicular chert of the Ota Group. The composite succession (ca. 105 m thick) measured in slab-like bodies is divided into the lower and upper units, each of which consists of the sharp-based calcarenite, siliceous lime–mud/siltstone, and spicular chert with blocks, beds, and laminae of calcarenite and siliceous limestone in ascending order. The calcarenite forming slabs and smaller blocks are massive and dominantly comprises densely packed, coarse crinoid debris and subordinate bryozoan fragments with no or sparse matrix. The siliceous lime–mud/siltstone consists of siliceous sponges spicules, minute calcitic skeletal debris, and peloidal particles, with the matrix of a mixture of lime–mud and microcrystalline quartz. The calcarenite and siliceous lime–mud/siltstone beds and laminae in the spicular chert occasionally contain graded and parallel laminae. The massive calcarenite, and graded calcarenite and siliceous limestone were formed by redeposition mainly of skeletal sands presumably by grain flows with minor debris flows, and by mixing of sponge spicule-rich siliceous sediments and redeposited lime–mud and skeletal debris by muddy turbidites of distal facies, respectively. These redeposited carbonate sediments were exported from the Akiyoshi Limestone Group onto the spicular sediments of slope-to-basin facies. Conodonts, fusulinids, and smaller foraminifers from the resedimented limestone indicate that the redeposition of the carbonate sediments took place dominantly in late Viséan to late Bashkirian, and subordinately in Sakmarian to Capitanian time. The absence of the Moscovian to Asselian resedimented limestone implies much-reduced carbonate shedding during this period. The correlation with the interpreted palaeoenvironmental episodes in the Akiyoshi buildup shows that late Viséan to late Bashkirian and Sakmarian to Capitanian carbonate-shedding events correspond to the times of generally warm climate and high sea-levels in the Akiyoshi buildup. Moscovian to Asselian time, which lacks records of carbonate shedding, nearly corresponds to the Moscovian to Gzhelian cooling and sea-level lowering in the Akiyoshi buildup. We consider that the shedding of carbonate sediments from the Akiyoshi atoll is its depositional response to the warm climate and elevated sea-levels and is compared with a sedimentary system of the highstand shedding of carbonates. Atoll-type carbonates equivalent to the Akiyoshi Limestone Group in the Akiyoshi terrane also show comparable responses of carbonate shedding into slope and slope-to-basin facies.