Abstract
Limestone blocks yielding corals, foraminifers and other fossils are embedded in the Lower Cretaceous Ishido Formation of the Sanchu Group. These limestone blocks also contain oncoids consisting of calcibionts. Limestones are lithologically classified into floatstone and mudstone. Occurrence of planktonic foraminiers such as Globuligerina hoterivica (Subbotina), Hedbergella planispira (Tappan), and Globigerinelloides aff. blowi (Bolli) indicates the age of limestone blocks as Early Aptian. Previously, a late Hauterivian to late Barremian age was assigned to the Ishido Formation based on ammonoids. This ammonoid age contradicts the age indicated by planktonic foraminifers. The depositional environment of these limestones is thought to be patch reef based on the oncoid facies and occurrence of microencruster, Bacinnella-Lithocodium association.
