Abstract
Siliceous micrite crops out in close association with bedded chert and nodular chert in the Middle Jurassic melange of the Mino terrane in the Mt. Funabuseyama area, central Japan. The siliceous micrite and chert form a characteristic lithologic association of a siliceous micrite-chert facies (ca. 60 m thick) in a bedded chert succession. The siliceous micrite is light gray and occurs as thin, lenticular beds in thin-bedded chert and as irregular-shaped blocks in thick-bedded to massive chert. The siliceous micrite is described as radiolarian lime-mudstone that includes subordinate thin-shelled bivalves within a lime-mud matrix. Conodont and radiolarian fossils indicate a Carnian to lower Norian age for the siliceous micrite and associated bedded chert.
We interpret the Upper Triassic siliceous micrite as deep-water sediment deposited in a pelagic setting within a mid-oceanic realm. This interpretation implies that short-lived fluctuations of the carbonate compensation depth occurred in Carnian to early Norian time, probably due to the intermittent accumulation of lime mud derived from calcareous nannoplankton.
The Upper Triassic siliceous micrite-chert facies differs in containing siliceous micrite from the coeval bedded chert of the oceanic plate stratigraphy of the Mino terrane. We postulate that compared with the Upper Triassic chert, the siliceous micrite-chert facies accumulated on a topographic high at relatively shallow water-depths, probably near the carbonate compensation depth. The Upper Triassic siliceous micrite-chert facies is inferred to have accumulated on the lower slope of a mid-oceanic seamount. We suggest an alternative age of Early Permian or Middle-Late Triassic for the formation of the mid-oceanic seamount.
