Geology of the H ida G aien B elt in the upper K uzuryu‐gawa R iver Area in O no C ity, F ukui P refecture, Central J apan

Geological, paleontological, and geochronological studies of the H ida G aien B elt were carried out in the upper K uzuryu‐gawa R iver area, northern central J apan. The H ida G aien B elt lies between the H ida and M ino belts of S outhwest J apan and is one of the most complex geologic belts in J apan. The geology of the following units in the study area, mostly bounded by longitudinal, high‐angle faults, was particularly reexamined and described: the I se metamorphic rocks, the F ujikuradani, T omedoro, O guradani, M otodo, O otani, and K onogidani F ormations, and the T etori G roup. Among them, the T omedoro and K onogidani F ormations are both composed mainly of greenstone, and were conventionally coupled together as ‘the T omedoro schalstein member’ or ‘the K onogidani F ormation’. However, the conformable relationship between the T omedoro F ormation and overlying M iddle P ermian O guradani F ormation, and the K – Ar and 40 Ar– 39 Ar ages of 75–69  M a ( L ate C retaceous) from the basalt lava of the K onogidani F ormation reveal that they are separate formations with different ages. The O guradani F ormation, consisting of limestone, shale, and sandstone with M iddle P ermian B oreal‐ T ethyan mixed brachiopod fauna, is correlated with the M oribu F ormation in the T akayama area of the H ida G aien B elt, and with the M iddle F ormation of the M aizuru G roup in the M aizuru B elt. The T omedoro F ormation below the O guradani F ormation, in turn, is correlated with the L ower F ormation of the M aizuru B elt. The new L ate C retaceous age data from the K onogidani F ormation and presence of latest C retaceous, post‐tectonic volcanic rocks in the study area finally indicate that the fault‐bound structure of the H ida G aien B elt between the Hida and M ino belts was formed in a very short period in L ate C retaceous age.