Abstract
The Naradani Formation is located to the south of the Torinosu Group in the Sakawa district of Kochi, southwest Japan, and consists of a linear, narrowly distributed series of marine clastic deposits that contain allochthonous limestone blocks. This formation was previously correlated to the Bajocian–Bathonian Middle Jurassic using brachiopods extracted from the limestone blocks, with later radiolarian correlations suggesting an Oxfordian–Kimmeridgian Upper Jurassic age. The formation has also been interpreted to underlie the Upper Jurassic–Lower Cretaceous Torinosu Group. However, the discovery of two stage-diagnostic aspidoceratid ammonoids, Aspidoceras sp. and Hybonoticeras sp., from muddy sandstones in the stratotype area of the Naradani Formation casts doubt on these previous interpretations. Aspidoceras was widespread during the latest Jurassic, and the presence of Hybonoticeras limits this short stratigraphic interval to the Kimmeridgian–Tithonian boundary in various Tethyan sections. This clearly suggests a latest Jurassic age for the Naradani Formation and generally supports the radiolarian biostratigraphy age-assignment. However, the ranges of ammonoids from the Naradani Formation overlap those from the Torinosu Group, meaning that it is not possible to determine an age-difference between these two lithostratigraphic units. Further research is needed to test other hypotheses, for example to determine whether heterotopic facies relationships exist between the Naradani Formation and the Torinosu Group.
