晚泥盆世古土壤的辨识及其发生学特征

The Identification of Late Devonian Paleosol and Pedogenetic Character

运用野外形态观察、元素地球化学和磁学相结合的方法对新近发现的甘肃东部地区晚泥盆世古土壤剖面进行了辨识和分析,从而得出:1)该套地层中残留有原土壤发生过程的痕迹,如根迹、浅褐色晕圈状根迹,原生石膏层,红色古土壤层内富含成壤钙结核及结核层。同时,可见圆顶柱状结构、次棱角块状结构等古土壤特有的结构类型,这些特征表明该套地层为古土壤。2)古土壤发生层次明显。可识别出红色淀积层(B层)和灰绿色母质层(C层)。B层磁化率向上增强,常量及微量元素Al_2O_3、Fe_2O_3、K_2O、Sr含量及Al_2O_3/SiO_2比值向下减少,Zr、Ti向下富集,以及风化指数CIA向下增加。磁化率及元素含量和比值特征反映了B层古土壤的风化成壤程度向下增强。3)依据发生学特征初步判断古土壤的类型主要为始成土、旱成土和变性土,反映了当时较干旱的气候条件。晚泥盆世沉积地层中古土壤层的发现对指示重大地质事件和研究环境变化方面有着重要的地学意义。

In this paper, we use methods of field observing, elemental geochemistry and magnetism to identify Late Devonian paleosol and pedogenic characters, the conclusions are as follows：1） residual traces of soil during pedogenic processes, such as root traces, drab-haloed root traces, the original gypsum which have consistent bedding with red soil layers, calcium nodules and calcium layers oc-curred in red paleosol. It is also found that domed columnar peds and blocky subangular peds, which are the special peds of paleosol. These morphological characteristics show that they are buried paleosol sequences. 2） red B layer and gray-green C layer can be identified from the profile, the magnetic sus-ceptibility increases from bottom to top in B layer. The contents of macroelement and trace element,＆nbsp;such as Al2 O3、 Fe2 O3、 K2 O、 Sr, and the ratio of Al2 O3/SiO2 decrease from top to bottom. On the contrary, the contents of Zr, Ti are enrichment, as well as increased Weathering index CIA. On the whole, magnetic susceptibility and element contents indicate that the paleosol weathering degree of B layer increase from top to bottom. 3 ） The paleosol types are mainly calcareous Vertisol, Aridisolpa-leosol and Natrargidpaleosol, which indicates a drier climate in Late Devonian. Therefore, the identi-fication of paleosol from the sedimentary strata provides an important significance to recognize the Late Devonian stratigraphy in study area, and to obtain more understandings about Devonian environmental and climate changes.