Ocean Environment Changes During Cretaceous/Paleogene Boundary At Gamba Basin, In Southern Tibet

The Gamba basin of southern Tibet, is located in the east of Tethys tectonic region, whereexposes complete strata from late Cretaceous to Paleogene. The strata yield abundantfossils, and accurate biostratigraphic works have been done by previous studies. TheCretaceous/Paleogene boundary was recognized by the extinction of larger benthicforaminifora Orbitoides-Omphalocyclus fauna and the first appearance ofRotalia-Smoutina-Lockhartia fauna. The boundary is between the Zongshan and Jibulaformations. Both formations are disconformity. There is a short time exposing facebetween them. At the contact, the Paleocene clastic rock of Zongshan Formation overliesdirectly on the latest Cretaceous shelf carbonate rock. It indicates a regional tectonicprocess. The boundary extinction is mainly influenced by sea level changes in southernTibet. The regression occurred from late Campanian of Cretaceous to early Danian ofPaleogene. Sr isotopic curve is well correlated with global temperature and sea levelchanges. An apparent negative excursion at bed 22 is correlated with the globaltemperature decline. The ⁸⁷Sr/⁸⁶Sr ratio of 0.707605 can be related to the tectonic processand sea level rising resulting from the rapid Indian Ocean ridge expansion in Campanian.Decrease of detrital weathering materials is resulted from ingression. Other horizonsshow apparent positive excursions, which are correlated with the global temperature shift,we relate to the sea level regression resulting from the Indian Ocean ridge expansion inlate Masstrichtian and early Danian stages. Continental crust and ocean crust at northmargin of Indian Plate subducted and caused continental uplift. Increase of detritalweathering materials is resulted from orogeny activity. Magnetic susceptibility variationis controlled by ancient sea level changes. It is particular if the supply of detrital materialwas low, when the magnetic susceptibility was mainly controlled by the content ofauthigenic minerals. If the supply of detrital materials was high, the average susceptibilityvalue would be higher, but there is still a correlation between susceptibility and sea levelcurves. A sea level depress would lead to an increase in the content of the detritalmaterials, and therefore to an increase in the susceptibility values. Beds with highermagnetic susceptibility are darker in color in the Gamba basin of southern Tibet. Most ofthem are ferruginous cement, more quarts and rock fragments, fine grain sandstone. It isconcluded that they occurred in oxidation environment, more detrital materials, longdistance transport and deposited in high-energy environment. There are carbonate rocksbelow the Cretaceous/Paleogene boundary and sandstone above the boundary. It isconcluded that the sea level was dropping down. The Gamba basin uplifted partly andseawater withdraw were the result of continental collision when Indian plate moved tonorth. According to the fossil assemblages and lithology, it is suggested that, during theCretaceous/Paleogene boundary time, Gamba basin was in a warm tropical, subtropicalshallow shelf-circulating environment. It can be recognized as carbonate gentle slope andcarbonate shallow bank facies.