Stratigraphy, Crinoid Columnals, and Radiometric Age of Carboniferous Kanayama Limestone Embedded in the Cambrian Hitachi Volcano-plutonic Complex

The stratigraphic relationship of Kanayama limestone which is approximately 20 m thick and is narrowly distributed 700 m long in the strike direction within the Akazawa Formation of the Cambrian Hitachi volcano-plutonic complex, is examined. The complex including the Akazawa Formation was metamorphosed under conditions with greenschist transitioning to amphibolite facies. Five lithologic units are recognized based on the lithology, grain size, and sedimentary structure of the Kanayama limestone exposed along the first ridge trail. Because all units are mainly composed of very fine- to fine-grained calcite detrital grains with well-developed parallel- and wave-ripple lamination, Kanayama limestone appears to have been deposited as calcarenite under a shallow-water environment dominated by waves and water currents. It unconformably overlies the Akazawa Formation and seems to be in fault contact with the Akazawa Formation above the uppermost unit. Upper Devonian to Carboniferous crinoid columnal fossils (Cyclocion sp.) are scattered from the uppermost unit. U–Pb radiometric ages of detrital zircons ranging from 545 to 338 Ma, with the youngest at 338 Ma from a zircon grain margin, suggest that the Kanayama limestone is probably early Carboniferous and later in age. The two age assignments support that the proposition that Kanayama limestone was deposited during the Carboniferous. On the other hand, zircon ages of 1480-395 Ma obtained from sandy schist intercalated in limestone of the Carboniferous Daioin Formation indicate that the formation has a different provenance from Kanayama limestone. This difference suggests that each limestone, in spite of its current proximity, was initially deposited as a different formation.