The Late Miocene to Pliocene paleoenvironmental evolution process in Zhaotong Basin on the southeastern margin of the Qinghai-Tibet Plateau

doi:10.13745/j.esf.sf.2023.11.33

Abstract

Abstract:

Situated at the southeast margin of the Tibetan Plateau, the Yunnan region is pivotal for investigating Late Cenozoic climatic changes. While considerable research has focused on the paleoclimate and paleoenvironmental evolution of Yunnan, the understanding of climate change from the Late Miocene to Pliocene primarily relies on carbon isotope and pollen records. Consequently, there is a dearth of high-resolution, continuous paleoclimatic records documenting humidity changes during this period. This study utilizes sediment cores from the Late Miocene to Pliocene in the Zhaotong Basin, northeastern Yunnan Province. Through sediment grain size analysis, the sedimentary sequence, lithological characteristics, and sedimentary structures indicate that the Zhaotong Basin was predominantly characterized by swamp-subfacies sedimentary environments during 8.8-6.2 Ma, transitioning to shallow lake subfacies during 6.2-2.8 Ma, and lakeside subfacies during 2.8-2.6 Ma. Grain size parameters of sediments in the Zhaotong Basin exhibit a drying trend of the South Asian monsoon during the Late Miocene to Pliocene. Combined with clay mineral and chemical weathering results from borehole data in the early period, it is inferred that the South Asian monsoon gradually weakened from the Late Miocene to Pliocene, primarily influenced by global cooling and decreasing global CO₂ concentrations.

Key words: Zhaotong Basin, Late Miocene, Pliocene, paleoenvironment

CLC Number:

P532
P534.62

LI Pei, ZHANG Chunxia, LUO Hao, LIU Zhicheng, GAO Zhanwu. The Late Miocene to Pliocene paleoenvironmental evolution process in Zhaotong Basin on the southeastern margin of the Qinghai-Tibet Plateau[J]. Earth Science Frontiers, 2024, 31(4): 326-339.

Figures/Tables 9

Fig.1 Schematic diagram showing the location of Zhaotong Basin on the southeastern margin of the Tibetan Plateau (a), schematic geological map of the area surrounding borehole ZK1 in the Zhaotong Basin (b). Modified after [6].

Table 1 Lithologic description and stratigraphic sequence of borehole ZK1

深度/m	岩性特征	层位
0.0~5.8	灰色、灰白色砂质粉砂,灰色泥质粉砂夹褐黄色赤铁矿团块。团块呈不规则形状,一般小于4 cm,从上往下团块数量减少	I
5.8~22.4	深绿灰色黏土。夹两层螺化石层,其中18.86~19.06 m的化石含量相对较少,而且螺体都小于1 cm;19.5~19.7 m含大量螺化石,螺体为3~5 cm	II
22.4~63.0	灰绿色粉砂质黏土。共夹一层劣质褐煤(32.1~32.25 m),一层泥质粉砂(32.5~32.6 m)和7层螺化石层。化石层分别为:①25.50~25.55 m;②24.30~24.35 m;③31.28~31.30 m;④31.95~32.05 m;⑤32.5~32.6 m;⑥42.5~43.3 m	III
63.0~64.7	深褐色褐煤。含炭化木和水草化石	IV
64.7~72.1	黑褐色泥炭。中间夹有3层黑褐色碳质黏土层	V
72.1~104.5	深褐色褐煤。含炭化木和水草化石	VI
104.5~111.6	深灰-灰色黏土	VII
111.6~120	浅灰色灰岩(二叠纪灰岩)	VIII

Fig.2 Magnetostratigraphic age framework of borehole ZK1 in the Zhaotong Basin (modified after [2]). (a), (b) and (c) represent lithology, inclination and magnetic polarity zonation; (d) geomagnetic polarity timescale (GPTS) (adapted from [19]); (e) thirteen age control points of sediment from borehole ZK1 in the Zhaotong Basin based on the standard geomagnetic polarity timescale.

Table 2 Variation range of clay, silt, sand content, median particle size, and average particle size in afferent zones of borehole ZK1

分带		黏土(<2 μm)含量/%	粉砂(2~63 μm)含量/%	砂(>63 μm)含量/%	中值粒径/μm	平均粒径/μm
Unit I	范围	7.0~37.8	60.3~93.1	0~34.1	4.4~52.4	8.9~47.4
Unit I	平均值	17.4	75.6	6.9	10.1	19.4
Unit II	范围	4.1~32.8	45.7~93.3	0.01~21.6	4.0~29.2	9.9~49.7
Unit II	平均值	13.7	81.1	5.22	10.0	17.85
Unit III	范围	6.5~36.5	28.6~68.8	2.6~65	3.2~104.8	9.3~140.0
Unit III	平均值	17.5	54.7	28.6	29.8	49.4

Fig.3 Variation of sediment grain size with depth and lithology in borehole ZK1, Zhaotong Basin

Fig.4 Frequency distribution curve and probability accumulation curve of each sedimentary zone of borehole ZK1 in the Zhaotong Basin

Fig.5 Comprehensive C-M map of sediment grain size of borehole ZK1 in Zhaotong Basin

Fig.6 Sedimentary facies division and field stratigraphic sedimentary structural characteristics in Zhaotong Basin during the Late Miocene to Pliocene

Fig.7 The various contents of grain size (a-c)、median grain size (d), the ratio of clay mineral content (Kao/(Sm+Ver(HIV)))[17] (e), the chemical weathering index of CIA and the ratio value of K/Al[18], global ice volume as indicated by marine oxygen isotope (h)[37], δ18O record (PDB) of palaeosol carbonate nodules from the Siwalik Group in northern Pakistan (i)[38]. Record: the gray dots in the figure represent the original data of sediment particle size parameters of ZK1 borehole, the curve is smoothed using a 5-point moving average, and the line segments are the trend lines of different stages obtained by linear regression.

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