[M-IS28] Global Boundary Stratotype Section and Points

[MIS28-P03] The current state of the Mesozoic GSSPs and contributions of the Japanese Mesozoic to the Geologic Time Scale

★Invited Papers

*Reishi Takashima¹ (1.Tohoku University Museum, Tohoku University)

Keywords:GSSP, Mesozoic, Japan

Most of the Mesozoic stages have been proposed in the 1800s for the neritic-continental shelf strata exposed in Europe. Each stage was originally proposed as a lithostratigraphic unit, and biostratigraphic correlations among the European strata were conduct based on macrofossils derived from the type strata. Later, with the progress of stratigraphic studies, it was revealed that most of the stage boundaries in the strata of the type areas exhibits unconformity or hiatus, which led to the redefinition and revision of the stage boundaries. At present, most of the Global Boudanry Stratotype Section and Points (GSSPs) are placed in the strata of hemipelagic facies, which is less affected by sea-level change. The primary markers that define the stage boundaries are not only the datum of macrofossils, but also microfossils such as conodonts, planktonic foraminifera, and Carpionella, as well as paleomagnetic reversals. Recently stratigraphic correlation among the GSSPs and the sections that differ biogeographic realm from the GSSP has greatly improved based on integration of of bio-, magneto-, carbon-, strontium- and osmium isotope stratigraphies. On the other hand, in Europe, where the most of the Mesozoic GSSPs are located, the strata intercalate few tuffs due to the passive continental margin depositional setting, which often makes it difficult to determine the absolute age of the stage boundaries. In such strata, the ages of the stage boundaries have been estimated based on cyclostratigraphy, but for strata older than the Eocene, calibration by radiometric ages are necessary, and the ages of the Mesozoic stage boundaries has been changed many times in the Geologic Time Scale.
Since Japanese Mesozoic strata were accumulated in a different biogeographic realm from Europe throughout the Mesozoic period, they do not yield enough age-diagnostic fossils common with GSSPs, and therefore stratigraphic correlations using fossils alone is not sufficiently accurate. On the other hand, the Japanese Mesozoic formations, especially the Cretaceous strata, intercalate a lot of volcaniclastic sediments because they were deposited in an active continental margin setting. In recent years, integrated stratigraphy combining various isotope stratigraphy and paleomagnetic stratigraphy, in addition to biostratigraphies of various taxa fossils, has been developed in the Japanese Mesozoic formations. These integrated stratigraphies successfully identified the precisive stratigraphic levels of stage boundaries and paleoenvironmental events in the Japanese Mesozoic strata by correlation with GSSPs. By measuring the radiometric ages of volcaniclastic sediments intercalated at or near these levels, Japanese Mesozoic strata have the potential to make a significant contribution to the international geologic time scale.