Age of the covering strata in the Kurosegawa Terrane: dismembered continental fragment in southwest Japan

Abstract

Along with the rapid development of radiolarian biostratigraphy, introduction of the new concept of tectonostratigraphic terranes has urged us to reexamine “geosynclinal complexes” which contain various radiolarian-rich rocks such as fine-grained clastic, siliceous, and tuffaceous rocks. Entirely new age data have been discovered from these hitherto poorly known formations. However, until recently, little attention has been paid to the age of geologic units consisting mainly of orderly stratigraphic sequences and coarse-grained clastic rocks such as the “Usuginu-type Conglomerate” in the Kurosegawa terrane, which is characterized by the presence of abundant clasts of granitic rocks. Accurate dating of conglomerate-bearing formations in the Kurosegawa terrane using radiolarian fossils provides a basis for tectonic reconstructions of the Kurosegawa terrane. These formations are fairly well-bedded sequences of mudstone, sandstone and conglomerate, without any exotic clasts of oceanic affinity. They range in age from mid-Permian to Middle Jurassic. Integration of these results indicates that the Kurosegawa terrane originated as a fragment of continental margin characterized by the presence of crystalline basement and coherent covering strata. However, the elements of the Kurosegawa terrane were subsequently severely disrupted into a zone of melange.