Analysis of Conglomerates and Associated Sedimentary Rocks of the Daito Ridge, Deep Sea Drilling Project Site 445

The geological history of the Daito Ridge is revealed in the clasts of proximal channel conglomerates recovered at DSDP Site 445. The clasts are the product of a middle Eocene transgression of the Daito Ridge. The major clasts, in order of abundance, are volcanic rocks, middle-Eocene bioclasts, and Cretaceous reef limestones. Pebbles of basalt within the Cretaceous reef limestones help confirm the late-Mesozoic origin of the Daito Ridge. From the lack of Paleocene shallow-water fossils, it is believed that the Daito Ridge remained in deep water after the Late Cretaceous global sea-level rise until the middle Eocene. During the middle Eocene, the bioclasts were derived from contemporaneous carbonate deposition upon the ridge and reworked along with the volcanic rocks and some Cretaceous limestones to make up the conglomerates that were recovered at Site 445. Post-depositional zeolitization of the conglomerates indicates thermal activity on the ridge at least through the middle Eocene. Evidence supporting the island-arc origin of the ridge consists in detritial minerals derived from blueschist and greenschist metamorphic facies. These minerals are found in the matrix of the conglomerate and associated sedimentary rocks.