Sedimentary Environments and Tuff Characteristics of the Ohyamashimo Formation, Lower Cretaceous Sasayama Group, Southwest Japan

Tsuji, M., Sawada, Y., Tanaka, T., Saneyoshi, M.

The 6th International Symposium on Asian Dinosaurs in Japan 2025

Asian Dinoaur Association

Fukui, Japan

n recent years, dinosaur fossil research in Japan has advanced significantly. In particular, the Tetori Group in the Hokuriku region of central Japan and the Sasayama Group in Hyogo Prefecture, western Japan, have yielded a wide array of vertebrate fossils, offering valuable insights into the terrestrial ecosystems of the Japanese archipelago during the Early Cretaceous period. Notably, the vertebrate fossils discovered within the Sasayama Group include numerous dinosaur specimens. Substantial prior research has focused on the age and paleoenvironment of these fossils. According to these studies, the Sasayama Group is subdivided into the lower Ohyamashimo Formation and the upper Sawada Formation, with the majority of vertebrate fossils recovered from the Ohyamashimo Formation. The stratigraphic correlation and depositional age of the formation have been elucidated through analysis of the tuff layer distributed at its base, as well as microfossil biostratigraphy (e.g., ostracods). Previously, the geological age was determined using detrital zircons contained within the basal tuffs and was estimated at 112.1 ± 0.4 Ma. However, the depositional processes of these tuffs have not been examined in detail, and their volcanological and petrological features remains unclear. Elucidating these factors will contribute to a more precise determination of the geological age of the fossil-bearing sediments and provide deeper insights into the paleoenvironmental context of the vertebrate fossils. In this study, we investigated the depositional processes and geochemical properties of the tuffs distributed at the base of the Ohyamashimo Formation. This presentation reports preliminary findings. The tuff distributed at the base of the Ohyamashimo Formation was collected from three distinct localities; however, this presentation focuses on the tuff exposed along the Ohyamashimo Route, where the deposit is thickest and best preserves the stratigraphic succession. The tuff at this route comprises two discrete units interbedded with typical clastic sediments. The stratigraphically lower tuff unit attains an approximate thickness of 8 m, whereas the upper unit measures approximately 3.5 m in thickness. Each tuff unit comprises massive tuff, laminated tuff, conglomerate, tuffaceous sandstone, and sandy tuff. Notably, the laminated tuff and certain tuffaceous sandstones frequently exhibit pervasive graded bedding and crudely stratification, with occasional occurrences of wave ripples in specific lithofacies. Each tuff unit is composed, in ascending order, of massive tuff, laminated tuff, and sandy tuff. In specific horizons, a transition from laminated tuff to massive tuff and subsequently back to laminated tuff is also observable. Furthermore, all units display a sheet-like geometry, with no evidence of basal scouring. These stacking patterns and lithological characteristics suggest that the tuff units were deposited by sediment gravity flows within a stable water environment or through suspension settling and fallout. In previous studies, the Ohyamashimo Formation was interpreted as consisting primarily of fluvial deposits. This study suggests the presence of stable water environments, including gravity-driven currents; therefore, broader regional investigations will be necessary in future research. To clarify the volcanological and petrological features of the tuff, this study conducted macroscopic observations, petrographic analysis of thin-sections, mineralogical analysis using EPMA, and whole-rock geochemical analysis using XRF on massive tuff samples. Massive tuff is composed of accretionary lapilli, pumice fragments, fibrous and bubble-wall volcanic glass, and mineral fragments (feldspar, quartz, biotite, apatite, zircon, and iron oxides), along with altered/weathered minerals, and a fine-grained matrix. Some feldspars show evidence of secondary alteration, and a substantial portion of biotite has chloritized. Other constituent minerals likely preserve their primary compositions. XRF analysis yielded the following whole-rock chemical composition: SiO2= 74.3–78.9 wt%, TiO2= 0.02–0.04 wt%, Al2O3= 12.1–15.2 wt%, Fe2O3= 0.94–1.61 wt%, MnO= 0.02–0.04 wt%, MgO= 0.41–0.94 wt%, CaO= 0.40–2.20 wt%, Na2O= 1.01–2.99 wt%, K2O= 3.43–5.04 wt%, and P2O5= 0.02–0.05 wt%. In future research, we will investigate the characteristics of minerals presumed to preserve their primary compositions, in order to elucidate the fundamental nature of the tuff.