青藏高原古岩溶的性质、发育时代和环境特征

The Nature, Formation Age and Genetic Environment of the Palaeokarst on the Qinghai-Xizang Plateau

青藏高原目前多处所见岩溶地貌主要属第三纪古岩溶之地下部分经后期剥蚀而出露于地表的。风化壳红土和洞穴次生化学沉积等古岩溶相关沉积也多以残留形态出露在已经发生解体的高原主夷平面的南和东南缘。风化壳红土中所含粘粒部分的主要化学成分为SiO2、AI2O3 和 Fe2O3;粘土矿物多属“伊利石－高岭石”型组合,少数样品属“高岭石－伊利石”型组合。据硅酸系数和粘土矿物组合判断,古岩溶风化壳红土的发育阶段处在化学风化的初期,但由于目前所见红土仅反映当时风化壳剖面根部的化学风化状况,故其较弱的风化指数仍能间接指示古岩溶发育时期湿热的地表环境。扫描电镜观测结果亦表明,风化壳红土中石英砂的表面结构特征以化学溶蚀形成的为主,机械侵蚀形成的为辅,反映了高原风化壳红土的长期残留特征,对应风化壳发育时期的湿热环境。

The karst landforms scattering on the Qinghai-Xizang (Tibet) Plateau can begenetically classified as the Tertiary underground karst, which gradually emerged to theground surface following later period's denudation with the uplift of the plateau during theQuaternary Period. The relative deposits of the Tertiary palaeokarst processes, such as theresiduum and speleothem, were discovered recently on the plantation surface of the plateau,where both eastern and southeastern fringe has geologically been disintegraed. The results offission track dating using the speleothem calcite revealed that the formation age of thepalaeokarst and hence the planation surface is between l9.0 and 7.0 Ma B.P. The residuumhas a fine size distribution. The principal chemical components of the clay portion of theresiduum consist mainly of SiO2, AI2O3, and Fe2,O3. The clay minerals composition of the clayportion belongs to illite-kaolinite-type for most of the residuum samples, andkaolinite-illite-type for a few samples. It can be judged from the silicic acid index and theclay minerals composition that the formation of the residuum of the Qinghai-Xizang Plateauwas in its initial phase. However, such a weak chemical weathering index only reflected theweathering degree in the bottom or lower parts of the weathering crust.