闽西晚白垩世红层的古环境探究

中国东南地区白垩纪红层通常被认为是陆相红色碎屑沉积物,是河流湖泊相沉积,但其古环境存在争论.本文以闽西晚白垩世红层为研究对象,利用环境磁学、粒度、地球化学、古土壤分析等方法,选取连城(LC)和冠豸山(GZS)两个剖面,分析探讨红层的古环境.结果显示:(1)闽西红层主要以细的粉砂颗粒为主,黏土和砂含量较少,表现为粉砂和砂互层,夹有薄层细粒砂砾层;样品磁化率偏低,主要载磁矿物为硬磁性矿物赤铁矿.(2)闽西红层有较高的风化程度,是暖湿气候下的中等风化程度;轻重稀土元素分异明显,Ce元素富集,Eu元素相对亏损,与上地壳(UCC)的分配模式相似,表明沉积物经历了充分混合,物质为混合沉积产物.(3)闽西红层虽然发育了一定的古土壤,但是成壤程度不强,没有明显的古土壤粘化层(Bt)和钙积层(Bk).由此推断,闽西红层沉积物在沉积前已在源区经过相当程度的风化过程,之后经过混合动力搬运沉积在盆地,沉积后风化成壤弱.可见,红层本身并不能直接反映沉积区环境,需结合古土壤发育特征判断沉积环境特征和变化.(4)闽西晚白垩世红层表现为相对干旱半干旱的古环境,红层中主要的着色矿物赤铁矿主要形成于源区,反映了地表透水性良好的干燥氧化条件,而不是"水成"环境.本文可为白垩纪红层古环境研究提供新的思路.

Study on the Late Cretaceous paleoenvironment documented by red beds in the western Fujian province

The Cretaceous red beds in southeastern China are regarded as terrestrial clastic sediments, whose paleoenvironmental implication is still in debate. They are conventionally regarded as lacustrine-fluvial sediments in basin. In present study, the late Cretaceous paleoenvironment documented by red beds (Liancheng and Guanzhaishan section) in the western Fujian province were investigated based on environmental magnetism, grain size, geochemistry and paleosols. The results show that:(1) The red beds in the western Fujian, mostly in form of interbedded siltstone and sandstone with gravels, are dominated by fine silt particles with less clay and sand. The dominant magnetic carrier in red beds is hematite, which attributes to low values of magnetic susceptibility in the two sections. (2) The weathering degree, reflected by chemical index of alteration (CIA), is moderate as those formed in warm and humid climate. In addition, the rare earth elements (REE) of red beds show enriched light rare earth element (LREE), depleted heavy rare earth elements (HREE),enrichment of Ce and negative Eu anomaly, which resembled that of average upper continental crust (UCC), thus indicating thoroughly mixing sediments. (3) Although paleosols were recognized in the red beds, in particular GZS section, these paleosols were weakly developed with no evident of B horizon (e.g. Bk and Bt) in paleosol profile. The conflict between moderate chemical weathering degree and weak pedogensis infers that the red bed sediments had experienced to an extent chemical weathering in source area before deposition. After deposition, the red sediments were hardly chemically weathered. It should be noted that the red beds solely cannot indicate paleoenvironment, which must take paleosols into account. (4) The late cretaceous paleoenvironment in western Fujian was suspected to be arid and semiarid. The hematite in red beds were mainly derived from source area, indicating well-drained dry oxidizing environment, rather than aqueous condition. This work provides an alternative prospect in studying Cretaceous red beds.