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据野外调查和室内鉴定 ,在长安县城南布容期 0 78- 0MaBP形成的黄土剖面中确定了三个时期的风化壳 ,它们由第 1 ,4,5层古土壤和风化破碎黄土构成 .黄土中的风化壳是红褐色古土壤在特定的气候条件下转变而成的 ,厚度在 3～ 6m之间 ,风化壳剖面常由 3～ 4个层次构成 ,发育时间为 5～ 1 0× 1 0 4a不等 .风化壳上部是发育好的红色古土壤 ,中部是含铁质胶膜的风化破碎黄土 ,下部是不含铁质胶膜的风化破碎黄土 .粘土矿物分析表明 ,黄土中的风化壳为伊利石 蒙脱石型和高岭石型之间的过渡类型 .黄土中风化壳发育时的年均降水量在 95 0～ 1 0 0 0mm之间 ,是黄土高原降水丰富的气候事件的显示 .风化壳的发育证明黄土中有些古土壤确属淋溶型森林土壤 ,当时亚热带气候至少迁移到了黄土高原东南部
According to field investigation and indoor appraisal, we have identified three weathering crusts in the loess profiles formed by the Chengnan-Burying Period of 78 ~ 0 Ma BP in the south of Chang’an County, consisting of the first, fourth and fifth layers of paleosol and weathered crushed loess, The weathered crust is a reddish-brown paleosol that has been transformed under certain climatic conditions with a thickness of 3 to 6 m. The weathered crust is often composed of 3 to 4 layers with a developmental period of 5 to 10 × 10 4a range.The weathered crust is well developed red ancient soil, the middle is weathered crushed loess with iron film, the lower is weathering broken loess without iron film.Clay minerals analysis shows that the weathering crust Is the transitional type between illite and montmorillonite type.The average annual precipitation during the development of weathering crusts in the loess is between 95 0 and 100 000 mm and is a manifestation of the precipitation-rich climate events on the Loess Plateau The development of weathering crust shows that some of the paleosol in the loess is indeed leached forest soils that the subtropical climate at least migrated to the southeast of the Loess Plateau