新近纪深海大洋红层的分布分类及成因

Distribution,classification,and origin of Neogene deep oceanic red beds

新近纪大洋红层是一种在氧化条件下形成的红色-粉色-棕色海洋沉积物。白垩纪大洋红层研究成果众多,然而新近纪大洋红层研究缺乏系统的分析和总结。本文以新近纪大洋红层为研究对象,通过对全球大洋钻探井位资料的分析,发现39个钻井含有新近纪大洋红层。通过对不同大洋区域、不同钻井内93个红层离散样品的主量元素CaO,Al2O_(3)和SiO_(2)含量进行三元图投点分析,将新近纪大洋红层分为铝质和钙质大洋红层,其中绝大多数为铝质大洋红层。进一步分析了部分钻井中离散样品的磁化率变化、反射光谱一阶导数曲线、热退磁曲线和交变磁场退磁曲线变化,结果表明新近纪大洋红层的致色矿物以赤铁矿为主。12个钻井中新近纪大洋红层离散样品的低总有机碳含量和低CaCO_(3)含量指示较低的生物生产力,此外19个钻井中红层的沉积速率平均为4 mm/ka,而非红层的沉积速率平均为85.3 mm/ka,指示红层的沉积速率相对较低。总结和分析新近纪大洋红层的以上特征,本文认为较低的有机质堆积速率为铁氧化物(赤铁矿)的形成提供了有利的氧化条件,使得沉积物中的铁氧化物在成岩阶段得以保存,最终形成全球分布的新近纪大洋红层。

Oceanic red beds(ORBs)are red to pink to brown marine sediment formed in oxic conditions.Previous studies mainly focused on the Cretaceous ORBs,but the systematic analysis and summary of Neogene ORBs are not comprehensive.This paper selects the ORBs in Neogene as the research object.Through the analysis of global ocean drilling sites data,we found that 39 sites contained Neogene ORBs.Based on the ternary diagram of the contents of the main elements CaO,Al2O_(3),and SiO_(2) of 93 discrete samples of ORBs at different oceans,the Neo-gene ORBs were divided into Al-ORBs and Ca-ORBs,whereby most of them were Al-ORBs.Further analysis of the magnetic susceptibility changes,the first-derivative curve of the reflectance spectroscopy,thermal demagnetization curve,and alternating field demagnetization curve of the discrete samples in sites indicate that hematite is the main mineral imparting the red color of Neogene ORBs.The low total organic carbon and low CaCO_(3) content of ORBs in 12 sites indicate lower biological productivity.Moreover,the average sedimentation rate of ORBs in 19 sites was 4 mm/ka,while the average deposition rate of non-red beds was 85.3 mm/ka;this feature also showed that the depo-sition rate of the red layer was relatively low.Based on the summary and analysis of the above characteristics of Neogene ORBs,the low organic materials accumulation rate provides favorable oxic conditions for iron oxide(hematite)formation;the iron oxides can be preserved,which causes coloration during the diagenesis,leading to the formation of global Neogene ORBs.