Strontium isotopic ages of the Torinosu-type limestones (latest Jurassic to earliest Cretaceous, Japan): Implication for biocalcification event in northwestern Palaeo-Pacific

The ages of the Late Jurassic to the Early Cretaceous shallow marine limestones in Japan (the Torinosu-type limestones) were evaluated by Sr isotopic stratigraphy. The ⁸⁷Sr/ ⁸⁶Sr ratios of 17 brachiopods collected from three sections revealed ages that are consistent with the previous assumption based on biostratigraphy. The age of the upper horizon of the Koike Limestone Member (the Soma-Nakamura Group, NE Japan) ranges from 150.6Ma to 149.2Ma (the latest Kimmeridgian to the early Tithonian). The limestones from two sections (the Torinosu and Imaidani Group) on Shikoku Island of SW Japan yield the younger ages, which evaluate the depositional rate of the two limestones 6.3-14.5m/m.y. By applying these rates, the depositional period was estimated for the whole limestone of each section; from 151.1Ma (the latest Kimmeridgian) to 140.3Ma (the latest Berriasian) for the Torinosu Group, from 151.5Ma (the late Kimmeridgian) to 143.0Ma (the early Berriasian) for the Imaidani Group. These results confirm active carbonate deposition in the shallow fore-arc platform in SW Japan from the Tithonian to the Berriasian. The biocalcification event recognized in relevant age in the Tethyan region was likely expanded to the Palaeo-Pacific. The carbonate deposition in the platform was terminated by sea level fall during the early Valanginian.