The Eocene Hiwadatoge Formation, SW Japan: Constraints on the timing of the denudation of the Sambagawa metamorphic rocks

The Hiwadatoge Formation in northwestern Shikoku, Southwest Japan, represents the oldest strata overlying the Sambagawa metamorphic rocks and is, therefore, often used to constrain the timing of the denudation of the Sambagawa metamorphic rocks. Despite its significance, previous studies on the formation are limited, and even the geological relationship between the formation and the Sambagawa metamorphic rocks has yet to be described in detail. To rectify this, we conducted geological mapping and detrital zircon U-Pb dating. The Hiwadatoge Formation is >100 m thick and composed of pebble to granule conglomerate and sandstone, which have local calcareous matrices. The formation is interpreted as southward-prograding shallow marine fan delta deposits. Most of the clasts in the formation were derived from the Sambagawa metamorphic rocks, but igneous rocks of the Ryoke, San-yo, and San-in belts were probably also sources of clastic grains. In addition to foraminifers and dinoflagellate cysts from previous studies, fossil bivalves and detrital zircon U-Pb ages in our study indicate that the formation was deposited during the early Lutetian (middle Eocene). The formation nonconformably overlies the Sambagawa metamorphic rocks and is unconformably overlain by the Kuma Group (and locally by the Ishizuchi Group). Therefore, the Sambagawa metamorphic rocks had been exposed and started eroding by the earliest middle Eocene, at least in the Shikoku region.