柴达木盆地渐新统上干柴沟组碳酸盐纹层成因及古气候意义
- 收稿日期:
2023-02-14
- 网络出版日期:
2023-09-16
摘要: 纹层是细粒沉积物在湖水分层条件下缓慢沉积的表现，其中碳酸盐纹层对于水体盐度、碱度和生物活动变化敏感，是判断大陆古气候、古水文较为重要的证据之一，但目前针对湖相碳酸盐纹层的研究较少。本文以柴达木盆地西岔沟剖面渐新统上干柴沟组为例，通过常规薄片、荧光薄片和扫描电镜的观察以及X射线粉晶衍射和碳氧同位素分析测试，对咸化湖盆纹层和其中碳酸盐矿物的成因进行了研究，并探讨了渐新世时期柴达木盆地的古气候演变规律。结果表明，西岔沟剖面上干柴沟组纹层中文石、方解石和白云石均有发育，三类碳酸盐矿物均与菌藻体关系密切。其中文石含量较高的样品碳同位素比值较高，白云石富集的样品则氧同位素比值偏高，说明文石的保存与浮游藻类生物勃发有关，白云石主要富集于蒸发作用较强的高盐度阶段。根据西岔沟剖面上干柴沟组碳酸盐矿物含量和碳氧同位素纵向变化规律，得出青藏高原北部在渐新世时期气候经历了由早期湿润到晚期（半）干旱的演化过程。
Formation of carbonate laminae in the Oligocene Shangganchaigou Formation of the Qaidam Basin and paleoclimatic significance
- Received Date:
2023-02-14
- Available Online:
2023-09-16
Abstract: Formation of lacustrine laminae result from water stratification and slow rates of deposition of fine-grained sediments. Carbonate laminae are sensitive to changes in water salinity, alkalinity, and biological activity and can provide important implications for paleoclimate and paleohydrology, which have, however, not received enough attention yet. Here, we take the Oligocene Shangganchaigou Formation of the Xichagou Section, SW Qaidam Basin as an example to study the formation mechanisms of lacustrine laminae, the origins of different carbonate mineral phases in saline lake basins, and their climatic and hydrological significances by observation of conventional thin section, fluorescence thin section and scanning electron microscope, and analysis of X-ray powder diffraction (XRD) and stable carbon and oxygen isotopes. Results show that the Shangganchaigou Formation of the Xichagou Section in the SW Qaidam Basin consist of three kinds of carbonate minerals: aragonite, dolomite, and calcite, all of which show a close association with algal residues. Carbon isotopic values of samples with high contents of aragonite are positive and samples containing rich dolomite show relatively higher δ18O values than those of other samples, indicating that the preservation of aragonite in Oligocene strata was related to algal bloom and dolomite formation resulted from high water salinity. The differed distributions of different carbonate mineral phases reflect a dynamic lake evolving from an early clastic-rich brackish lake to a later saline lake, controlled by early Oligocene humid climate and late Oligocene semi-arid climate.
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