13:45 〜 15:15
[SGL22-P02] Biostratigraphy and assemblage of middle Cretaceous planktic foraminifera in the Vocontian Basin, southeastern France
キーワード：白亜紀、浮遊性有孔虫、生層序
Planktic foraminifera is useful tool for international biostratigraphc correlation, and numerous planktic foraminiferal biostratigraphy have long been studied for the Cretaceous deep-sea cores and sections. However, these deep-sea sediments contain few macrofossils, the relationship between macrofossil and microfossil biostgraphies have not been well constrained. Vocontian Basin sequences exposed in southeastern France were accumulated in the continental slope in northern Tethys Sea, and contain abundant calcareous marine macro and microfossils. The Vocontian Basin sequence is regarded as one of the representative sequence in Tethyan realm because GSSPs of the Bathonian (Jurassic), Hauterivian, Albian and Cenomanian (Cretaceous) are placed in the sequences. Recent studies elucidate detailed stratigraphy of the calcareous nannofossils, macrofossils, carbon- and osmium-isotope stratigraphies of the Albian-Turonian interval in the Vocontian Basin (e.g., Giraud et al., 2013, Du Vivier et al., 2014, Gyawali et al., 2017). On the other hand, planktic foraminiferal biostratigraphy has not studied for the Albian-Turonian sequence except for boundaries of Albian/Cenomanian and Cenomanian/Turonian (Kennedy et al., 2000; Grosheny et al., 2006; Takashima et al., 2009). In this study, we developed Cretaceous planktonic foraminiferal biostratigraphy and assemblage of the Vocontian basin and integrated it with other stratigraphic records such as carbon isotope, calcareous nannofossil and ammonites.
The studied 1030-m-thick sequence of the Vocontian Basin ranges from Middle Albian to Turonian and consists mainly of hemipelagic marl and limestone with intercalations of black shales in OAE1d and OAE2 horizons. Following twelve planktic foraminiferal zones were identified from the studied sequence; Ticinella primula zone, Ticinella praeticinensis zone, Pseudothalmanninella ticinensis zone, Thalmanninella appenninica zone, Thalmanninella globotruncanoides zone, Thalmanninella reicheli zone, Rotalipora cushmani zone, Thalmanninella greenhornensis zone, Praeglobotruncana algeriana zone, Whiteinella archaeocretacea zone, Helvetoglobotruncana helvetica zone, and Marginotruncana schneegansi zone. Most of our result are consistent with the result of Geologic Time Scale 2020, there is a significant discrepancy between integrated stratigraphy of the Vocontian Basin and GST 2020 as for the stratigraphic position of Thalmaninella reicheli Zone. Our integrated stratigraphy demonstrates that Th. reicheli zone occurs in the Mid Cenomanian Event I, while Geologic Time Scale 2020 implies that Th. reicheli zone is placed much lower level than MCE I.
We also reconstructed the changes in marine environment during middle Cretaceous. Based upon the planktic foraminiferal assemblage, studied period divided into six intervals. From the OAE1d to Albian/Cenomanian boundary, it would be oligotrophic or mesotrophic condition. It turned eutrophic from the Albian/Cenomanian boundary to Lower Cenomanian Event II, and from Lower Cenomanian Event III to just before OAE2, it could be oligotrophic. At OAE2, the drastic change in planktic foraminiferal assemblages suggests marked eutrophic conditions. Eutrophic or mesotrophic conditions would have remained in the Turonian interval after OAE2.
Reference
Du Vivier, D. C. A. et al., 2014. Earth and Planetary Science Letters, 389, 23-33.
Giraud, F. et al., 2013. Cretaceous Research, 46, 43-58.
Grosheny, D. et al., 2006. Cretaceous Research, 27, 629-640.
Gyawali, R. B. et al., 2017. Newsletters on Stratigraphy, 50, 111-139.
Kennedy, J. W. et al., 2000. Cretaceous Research, 21, 591-720.
Takashima, R. et al., 2009. Palaeogeography, Palaeoclimatology, Palaeoecology, 273, 61-74.
The studied 1030-m-thick sequence of the Vocontian Basin ranges from Middle Albian to Turonian and consists mainly of hemipelagic marl and limestone with intercalations of black shales in OAE1d and OAE2 horizons. Following twelve planktic foraminiferal zones were identified from the studied sequence; Ticinella primula zone, Ticinella praeticinensis zone, Pseudothalmanninella ticinensis zone, Thalmanninella appenninica zone, Thalmanninella globotruncanoides zone, Thalmanninella reicheli zone, Rotalipora cushmani zone, Thalmanninella greenhornensis zone, Praeglobotruncana algeriana zone, Whiteinella archaeocretacea zone, Helvetoglobotruncana helvetica zone, and Marginotruncana schneegansi zone. Most of our result are consistent with the result of Geologic Time Scale 2020, there is a significant discrepancy between integrated stratigraphy of the Vocontian Basin and GST 2020 as for the stratigraphic position of Thalmaninella reicheli Zone. Our integrated stratigraphy demonstrates that Th. reicheli zone occurs in the Mid Cenomanian Event I, while Geologic Time Scale 2020 implies that Th. reicheli zone is placed much lower level than MCE I.
We also reconstructed the changes in marine environment during middle Cretaceous. Based upon the planktic foraminiferal assemblage, studied period divided into six intervals. From the OAE1d to Albian/Cenomanian boundary, it would be oligotrophic or mesotrophic condition. It turned eutrophic from the Albian/Cenomanian boundary to Lower Cenomanian Event II, and from Lower Cenomanian Event III to just before OAE2, it could be oligotrophic. At OAE2, the drastic change in planktic foraminiferal assemblages suggests marked eutrophic conditions. Eutrophic or mesotrophic conditions would have remained in the Turonian interval after OAE2.
Reference
Du Vivier, D. C. A. et al., 2014. Earth and Planetary Science Letters, 389, 23-33.
Giraud, F. et al., 2013. Cretaceous Research, 46, 43-58.
Grosheny, D. et al., 2006. Cretaceous Research, 27, 629-640.
Gyawali, R. B. et al., 2017. Newsletters on Stratigraphy, 50, 111-139.
Kennedy, J. W. et al., 2000. Cretaceous Research, 21, 591-720.
Takashima, R. et al., 2009. Palaeogeography, Palaeoclimatology, Palaeoecology, 273, 61-74.