THE CHARACTERISTICS OF CLAY MINERALS AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM(PETM)IN THE GONJO BASIN ON THE SOUTHEASTERN MARGIN OF THE TIBETAN PLATEAU
The Paleocene Eocene Thermal Maximum(PETM)was a rapid global warming event.To obtain the paleoclimatic response to the rapid warming event in the southeastern margin of the Tibetan Plateau,we studied the fluvial-lacustrine sedimentary strata spanning from the Late Paleocene to the Early Eocene in the Gonjo Basin,which is constrained by a magnetic stratigraphic chronological framework.The RM section(30°51′55″N,98° 18′20″E),located between depths of 1480 m and 1800 m within the larger sedimentary sequence in the Gonjo Basin,has a total thickness of about 320 m.It is mainly composed of fluvial deposits dominated by sandstone and contains multiple gravel layers.High-resolution analyses were conducted on carbonate carbon and oxygen isotopes(264 samples),organic carbon isotopes(204 samples),and clay mineral content(101 samples)from the RM section.Our results show that the fluvial deposits from the upper part of the Lower Ranmugou Formation and the lower part of the Middle Ranmugou Formation in the Gonjo Basin record the PETM event,and there is a globally comparable negative shift in carbon isotopes during the PETM.The starting stratum of PETM is roughly located at 1555 m in the RM section,and the δ13Ccarb shifts from-4.5‰ to-7.2‰,while the δ13Corg drifts from-23.61‰to-28.90‰.Illite(43.41％to 89.03％)was dominant in the clay mineral assemblages of the section,containingI/S(illite/smectite)(12.46％to 26.66％),chlorite(0 to 14.39％)and kaolinite(0.64％to 39.07％).By combining the analysis of carbon isotopes with the variations in clay mineral assemblages,it is believed that the end stratum of the PETM event may be located at 1645 m of the sedimentary sequence in the Gonjo Basin.Therefore,it is estimated that ca.90 m of stratum may have been deposited during the PETM in the Gonjo Basin.Illite was dominant in the clay mineral assemblages of both the upper and lower parts of the section,containing chlorite and a small amount of kaolinite.During PETM,the content of illite and chlorite significantly decreased,and I/S(illite/smectite)emerged.Illite was still dominant,with very high IC(mean:0.8)and ICI(mean:0.44)values,followed by I/S,which contained kaolinite and a small amount of chlorite.The variations of clay mineral assemblages and indexes indicated that before the PETM,the climate in the Gonjo Basin was relatively hot and dry,mainly characterized by physical erosion;during the PETM,with the intensification of global warming,the climate became hot and humid,precipitation significantly increased,and both physical erosion and chemical weathering processes were significantly enhanced.After PETM,chemical weathering significantly weakened,and the climate once again shifted to a hot and arid state.The findings of our research indicated that the PETM warming event may have led to an increase in intense precipitation events,rapid river swelling,and an enhanced sedimentary rate in the southeastern margin of the Tibetan Plateau.
万方数据
维普
