Study on paleoclimatic, paleoceanic, and paleoenvironmental evolution of the Triassic basin in the northern belt of the South Qinling
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摘要:
在野外地质调查工作基础上，开展露头剖面沉积相研究，结合粒度分析结果，划分沉积相类型，通过砂岩样品主量元素、微量元素分析，碳酸盐岩C-O同位素分析，探讨三叠系盆地古气候、古海洋、古环境演化过程。结果表明：（1）研究区三叠系自下而上发育下统扎里山组、马拉松多组浅海环境开阔台地相碳酸盐岩（盆地南部）和隆务河组半深海环境斜坡相深水砂岩，中统郭家山组浅海陆棚相和光盖山组半深海环境陆棚斜坡相沉积，以及上统大河坝组浅海陆棚环境沉积；（2）砂岩样品化学蚀变指数（CIA）值、成分变化指数（ICV）值，以及Rb/Sr–K2O/Al2O3关系图解表明，研究区三叠系经历了两次“温暖湿润—半干旱、干旱”的古气候演化过程，并造成古水深相应的两期由深变浅的过程，以及砂岩沉积水体盐度的变化，温暖湿润气候带来的降水使得海水盐度大幅度降低，而炎热的半干旱、干旱气候条件使得海水盐度大幅度升高；（3）研究区三叠系砂岩形成于氧化—弱氧化的古水体环境，这种环境虽有利于水生生物的发展繁盛，能够为沉积有机质奠定物质基础，却不利于沉积有机质的保存。
Abstract:On the basis of field geological investigations, the sedimentary facies of outcrops were studied, and sedimentary facies types were classified based on the results of particle size analysis. Additionally, through major and trace element analyses of sandstone samples, and C-O isotope analysis of carbonate rocks, the process of paleoclimatic, paleoceanic, and paleoenvironmental evolution in the Triassic basin was discussed. The results show that: (1) From bottom to top, the Triassic strata in the study area consist of open platform carbonate rocks of the Lower Triassic Zhalishan Formation and Malasongduo Formation in the southern part of the basin, semi-deep sea slope facies of the Longwuhe Formation, shallow marine shelf facies of the Middle Triassic Guojiashan Formation, semi-deep sea shelf slope facies of the Guanggaishan Formation, and shallow marine shelf facies of the Upper Triassic Daheba Formation; (2) Values of the chemical index of alteration (CIA), index of compositional variability (ICV) , and the relationship between Rb/Sr and K2O/Al2O3 ratios in sandstone samples indicate that the Triassic period in the study area experienced two major paleoclimatic shifts: from warm and humid to semi-arid and arid conditions, which caused changes in the paleobasin's water depth, fluctuating from deeper to shallower stages, and influenced the salinity of the water. The precipitation brought by the warm and humid climate greatly reduced the salinity of seawater, while the hot semi-arid and arid climate greatly increased the salinity of seawater. (3) The paleowater body developed in the Triassic sandstone in the study area presents a weakly oxidizing environment, which is conducive to the development and flourishing of aquatic organisms. This environment provided a basis for the formation of sedimentary organic matter, though it was not ideal for the preservation of sedimentary organic matter due to the oxidizing conditions.
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图 13 南秦岭北带三叠系盆地砂岩V/Cr、Ni/Co、V/(V+Ni)和U/Th比值分布特征（据Jones and Manning, 1994）
Figure 13.
表 1 南秦岭北带盆地三叠系碎屑岩样品微量元素地球化学分析结果
Table 1. Geochemical analysis results of trace elements of Triassic clastic rock samples in the northern belt of the South Qinling
样品编号 PM105
-26YQPM104
-83YQHD0013
-YQHD0017
-YQPM104
-119YQHD0015
-YQHD0012
-YQPM103
-54YQPM103
-38YQLP303
-YQ含量/% SiO2 62.01 67.09 60.87 64.59 63.21 78.37 55.94 51.99 59.21 71.56 Al2O3 13.29 12.47 13.74 11.37 12.88 5.70 7.26 11.72 11.53 10.01 TFe2O3 6.38 5.82 6.60 5.50 6.04 3.80 2.90 5.87 5.17 5.55 MgO 2.65 1.61 3.05 1.82 2.00 0.75 1.28 2.39 1.53 1.78 CaO 4.25 3.21 3.90 4.93 4.66 4.54 15.48 10.71 8.46 2.98 Na2O 2.97 2.52 1.86 2.77 2.30 0.07 0.86 0.08 0.91 0.18 K2O 1.66 2.14 2.39 1.40 2.56 1.29 1.13 1.83 2.04 1.90 TiO2 0.56 0.61 0.66 0.49 0.66 0.47 0.51 0.61 0.56 0.60 MnO 0.08 0.05 0.05 0.09 0.07 0.05 0.06 0.11 0.07 0.05 P2O5 0.13 0.15 0.14 0.13 0.17 0.11 0.14 0.16 0.14 0.14 含量/10－6 Rb 63.30 82.30 98.20 57.20 116.00 59.60 51.30 87.50 106.00 86.30 Ba 220.00 260.00 259.00 211.00 406.00 307.00 204.00 228.00 588.00 255.00 Th 8.12 11.00 11.60 8.61 13.90 9.33 11.00 12.20 10.60 11.20 U 1.75 1.79 2.32 1.81 2.34 1.99 2.31 2.60 2.07 2.35 K 13780.47 17723.67 19815.65 11597.18 21260.11 10684.01 9388.98 15191.72 16951.63 15739.62 Ta 0.86 0.96 1.23 0.78 1.24 0.80 0.83 1.05 1.15 0.98 Nb 12.60 12.90 17.00 11.20 16.50 11.10 11.50 15.30 15.30 13.70 Sr 159.00 145.00 204.00 178.00 229.00 139.00 837.00 390.00 234.00 155.00 P 584.34 671.55 606.14 575.62 732.60 466.60 623.59 697.72 623.59 601.78 Zr 150.00 252.00 164.00 165.00 248.00 371.00 226.00 157.00 208.00 239.00 Hf 4.33 7.62 4.80 4.59 7.79 10.10 6.58 4.26 5.82 6.62 Ti 3374.62 3668.32 3932.06 2943.05 3968.02 2799.19 3068.92 3668.32 3380.61 3608.38 Sc 9.21 8.72 13.30 8.82 10.30 5.28 5.55 10.60 10.30 9.43 Co 17.50 9.64 18.00 12.10 12.20 6.35 7.57 11.70 10.80 14.10 Cr 49.80 52.70 63.00 45.80 46.70 53.10 38.20 45.20 45.20 68.00 Li 53.30 26.80 55.50 44.20 20.30 22.90 25.80 41.20 35.50 35.20 Ni 28.80 20.40 33.80 21.90 20.70 18.90 16.70 23.00 18.70 33.00 Pb 18.70 9.53 11.80 8.69 8.20 12.30 8.77 22.50 19.90 17.10 V 59.30 61.30 90.30 58.70 66.50 43.50 45.80 63.60 63.10 71.00 相关指标 CIA 53.45 55.45 51.68 61.17 54.00 77.65 64.08 83.86 68.82 79.09 ICV 1.95 1.93 2.09 1.86 1.68 2.58 5.05 2.75 2.35 1.66 V/Cr 1.19 1.16 1.43 1.28 1.42 0.82 1.20 1.41 1.40 1.04 Ni/Co 1.65 2.12 1.88 1.81 1.70 2.98 2.21 1.97 1.73 2.34 V/(V+Ni) 0.67 0.75 0.73 0.73 0.76 0.70 0.73 0.73 0.77 0.68 U/Th 0.22 0.16 0.20 0.21 0.17 0.21 0.21 0.21 0.20 0.21 
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