广丰盆地晚白垩世周田组红层沉积特征及古气候意义

Sedimentary Characteristics and Paleoclimatic Significance of the Late Cretaceous Zhoutian Formation Red Beds in the Guangfeng Basin

中国东南地区白垩系陆相红层分布广泛,是了解白垩纪温室气候陆地沉积响应的重要载体。但对这些红层的沉积环境尚有“水下”和“水上”之争议,古气候意义还需要进一步挖掘。以广丰盆地毛村剖面晚白垩世周田组为研究对象,通过野外剖面测制及古土壤样品色度、磁化率、碳酸钙含量等实验测试,分析沉积环境与古气候。周田组红层岩石类型可划分为古土壤、砂岩和砾岩。古土壤几乎分布于整个剖面,其中钙质结核、钙板层发育,可见遗迹化石及土壤滑擦面。砂岩与砾岩仅分布于部分层位,砂岩以中—粗砂岩为主,砾岩以中砾岩为主,发育平行层理和粒序层理构造。研究区古土壤样品的碳酸钙含量与低频质量磁化率联系密切,可作为气候变化的替代指标,而色度指标具有区域性,对该研究区的气候指示不明显。碳酸钙含量与磁化率曲线变化明显,记录了红层沉积阶段气候干湿交替的变化趋势。周田组红层沉积于强氧化环境并伴有季节性降水,古气候具有由相对湿热逐渐变化为干热的趋势。

Cretaceous terrestrial red beds are widespread in southeastern China,and they are archives for under-standing the continental sedimentological response to the Cretaceous greenhouse climate.It remains controversial as to whether the red beds were deposited in“subaqueous”or“superaqueous”environments,and the paleoclimate needs to be further explored as well.Based on measurements of the Maocun section in the Guangfeng Basin,the depo-sitional environments and paleoclimate of the Late Cretaceous Zhoutian Formation red beds were studied by chroma,magnetic susceptibility and calcium carbonate content analysis of paleosol samples.Three types of rocks,namely pa-leosol,sandstone and conglomerate,are distinguished in the Zhoutian Formation.The paleosol is distributed almost throughout the measured section,being characterized by abundant carbonate nodules,carbonate deposition layers,trace fossils,and slickensides.The sandstone observed in some horizons is mainly medium-to coarse-grained.The pebbly conglomerate occurs in parallel bedding and graded bedding structures.The calcium carbonate content of the paleosol samples is closely related to low-frequency mass magnetic susceptibility,which can be used as a climate proxy.In contrast,the chroma index reflects only regional changes,thus is less indicative of climate.Both the calci-um carbonate contents and magnetic susceptibility have clear trends,which probably reflect cyclical dry and wet con-ditions.Therefore,the red beds of the Zhoutian Formation were interpreted to have been deposited in a intense oxidiz-ing environment with seasonal precipitation,and the paleoclimate may have changed between wet and dry conditions.