Campanian-Maastrichtian (Latest Cretaceous) Palynofloral Changes In East Asia

The Cretaceous was a time of long-term climate stability with warm equable climates resulting from higher atmospheric greenhouse gas content by volcanic activity. However, it is insisted the global cooling had occurred during the Campanian-Maastrichtian by recent works. Terrestrial palynomorph and palynofloristic provinces are expected to explain the relationship between biota and the cooling event.The Songliao Basin in Northeast China preserves the Late Jurassic-Paleogene succession. This basin yields abundant palynomorph and has a potential to provide new insight into temporal variation of the Late Cretaceous biota under the cooling climate. In this basin, the Cretaceous Continental Scientific Drilling Project was carried out to recover nearly complete Cretaceous terrestrial sedimentary records named the Songke Core-1 (North) (SKI (N)) and the Songke Core-1 (South) (SKI (S)). The lithostratigraphic succession of the SKI formed during the Late Cretaceous-Paleogene, as determined by biostratigraphy, radiometric dating, magnetostratigraphy, stable isotope, and cyclostratigraphy. However, the detailed stratigraphic positions of stage boundaries have been controversial. To consider palynofloral changes during the Campanian-Maastrichtian, the position of the Campanian/Maastrichtian (C/M) boundary within the SKI is one of the most important data.The Izumi Group is the upper Cretaceous marine deposit distributed in southwest Japan. This group yields mollusk, radiolarian, and terrestrial-derived palynomorphs. Therefore, the Izumi Group is favorable to establish an integrated mega-and micro-biostratigraphy that is applicable to the Campanian-Maastrichtian terrestrial deposits.This study focuses on fossils obtained from the SKI (N) and the Izumi Group. The late Campanian-early Maastrichtian palynomorph biostratigraphies are established in both study areas. The stratigraphic position of the C/M boundary is determined by chronostratigraphic correlation between palynomorph biostratigraphy of the SKI (N) and integrated mega-and micro-biostratigraphy of the Izumi Group. In addition, relationships between lithology and palynofloral composition are considered with palynological data obtained from the SKI (N). On the basis of them, this study discusses temporal variation of palynofloristic provinces during the late Campanian-early Maastrichtian.In the SKI (N), lithostratigraphy and stratigraphic distribution of palynomorph are confirmed from the uppermost Nenjiang-lower Mingshui formations of the SKI (N). These formations are composed of meandering river and lake facies. The study section yielded 102 genera and 282 species. The abundances of palynomorph specimens were generally low in uppermost Nenjiang-Middle Sifangtai formations. Meanwhile, those of the upper Sifangtai-lower Mingshui formations were relatively higher than lower part of the study section. The upper part of study section yielded 95 genera and 253 species of spores and pollen. Most of the palynomorph assemblages were dominated by gymnosperm pollen, followed by spore of pteridophyte, and angiosperm pollen. On the basis of the first occurrences of marker genera within the succession, this study established four palynomorph interval zones (in ascending order):the Jianghanpollis, Chenopodipollis, Toroisporis, and Betulaceoipollenites interval zones. The first two zones are found from flood plain and/or levee deposits. The third one is recognized from lake basin plain-lake basin slope deposits. The last one is confirmed in delta deposits within lakeshore.The Campanian-Maastrichtian biostratigraphic framework is established in the Izumi Group. On the basis of the first occurrence of marker genera and species composition of the genus Aquilapollenites, this study defined four interval zones (in ascending order):the Tricolporopollenites, Betulaepollenites, Toroisporis, and Hymenophyllumsporites interval zones. At present, palynomorph-bearing horizons are insufficient to define zonal boundary precisely. However, the Betulaepollenites/Toroisporis zonal boundary is regarded as the same stratigraphic position of the Pachydiscus awajiensislNostoceras hetonaiense boundary.In other words, the BetulaepolleniteslToroisporis zonal boundary approximates the C/M boundary within the Izumi Group.On the basis of co-occurring genera and species composition of Aquilapollenites, I attempt to correlate palynomorph biostratigraphy between the Izumi Group and non-marine deposits within the Songliao Basin (SKI (N)). As a result, the ChenopodipollislToroisporis zonal boundary within the SKI (N) could coincide with the BetulaepolleniteslToroisporis zonal boundary within the Izumi Group. In this case, it is implied that the former boundary corresponds with the P. awajiensis/N. hetonaiense and/or C/M boundaries. On the basis of the results, the C/M boundary within the SK1 (N) could be at 748.5 m depth according to palynomorph biostratigraphic correlation with the Izumi Group.Judging from lithofacies and palynomorph assemblage, this study considers that environmental changes are the major factor that influenced the formation of component of Campanian-Maastrichtian assemblage in the Songliao Basin. Comparison biostratigraphic result of this study with previous palynomorph biostratigraphy shows similarity that triprojectate pollen such as Aquilapollenites is regarded as marker taxa in all areas during the Campanian-Maastrichtian. However, Wodehouseia spp. have yet to be confirmed in the study section within the Songliao Basin, although this genus appeared in other areas during the same time. This is one of the important clues to understand regional difference of local flora.The Songliao Basin was considered to be assigned to the Aquilapollenites Provinces by previous works. Judging from compositions, palynomorph assemblages obtained from the Jianghanpollis Interval Zone (late Campanian) is similar to palynofloral component of the Schizaeoisporites Province. Meanwhile, components of the both provinces are mixed in upper three zones. It is suggested that components of the Aquilapollenites Province came into the Songliao Basin within the Schizaeoisporites Province during the latest Campanian-early Maastrichtian. As a result, mixed palynofloristic province would have been formed around the Songliao Basin. Thus, it is expected that the palynofloristic province changed frequently during the Late Cretaceous. To explain the transition of provinces, future works need to investigate stratigraphic distribution of each genus and species.