Sequence stratigraphic framework for mixed aeolian,peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

Due to difficulties in correlating aeolian deposits with coeval marine facies, sequence stratigraphic interpretations for arid coastal successions are debated and lack a unifying model. The Pennsylvanian record of northern Wyoming, USA, consisting of mixed siliciclastic–carbonate sequences deposited in arid, subtropical conditions, provides an ideal opportunity to study linkages between such environments. Detailed facies models and sequence stratigraphic frameworks were developed for the Ranchester Limestone Member (Amsden Formation) and Tensleep Formation by integrating data from 16 measured sections across the eastern side of the Bighorn Basin with new conodont biostratigraphic data. The basal Ranchester Limestone Member consists of dolomite interbedded with thin shale layers, interpreted to represent alternating deposition in shallow marine (fossiliferous dolomite) and supratidal (cherty dolomite) settings, interspersed with periods of exposure (pedogenically modified dolomites and shales). The upper Ranchester Limestone Member consists of purple shales, siltstones, dolomicrites and bimodally cross‐bedded sandstones in the northern part of the basin, interpreted as deposits of mixed siliciclastic–carbonate tidal flats. The Tensleep Formation is characterized by thick (3 to 15 m) aeolian sandstones interbedded with peritidal heteroliths and marine dolomites, indicating cycles of erg accumulation, preservation and flooding. Marine carbonates are unconformably overlain by peritidal deposits and/or aeolian sandstones interpreted as lowstand systems tract deposits. Marine transgression was often accompanied by the generation of sharp supersurfaces. Lags and peritidal heteroliths were deposited during early stages of transgression. Late transgressive systems tract fossiliferous carbonates overlie supersurfaces. Highstand systems tract deposits are lacking, either due to non‐deposition or post‐depositional erosion. The magnitude of inferred relative sea‐level fluctuations (>19 m), estimated by comparison with analogous modern settings, is similar to estimates from coeval palaeotropical records. This study demonstrates that sequence stratigraphic terminology can be extended to coastal ergs interacting with marine environments, and offers insights into the dynamics of subtropical environments.     

黑色页岩容矿型Ni-Co矿床: 研究进展与展望

Ni和Co都是重要的战略性矿产资源, 其中黑色页岩容矿型Ni-Co矿床是沉积岩容矿型Ni-Co矿床的重要组成部分, 具有非常重要的工业价值和科学研究意义。本文按照形成时代的先后顺序综述了芬兰Talvivaara组、英国Gwna群、中国华南牛蹄塘组、瑞典Alum Shale组、加拿大Canol组以及中欧Kupferschiefer组等全球范围内几个重要黑色页岩层位赋存的Ni-Co矿床。这些矿床的矿石具有纹层状构造、多金属共生富集、富有机质和硫等共同特征。矿床中多金属的初始来源可能为陆源风化的产物或者海底热液, 而导致溶解于海水中的金属发生沉淀的机制则是极端条件下海洋环境的变化。Ni-Co的共生分异受到同沉积期海水氧化还原环境和后期改造过程的共同控制。全球范围内几个重要的含Ni-Co黑色页岩层位均发育在第一次全球大氧化事件、新元古代大氧化事件和显生宙的地球表生环境突变期, 显示了地球表生环境对该类型矿床的形成具有控制作用。此外, 后期改造过程则有利于金属的进一步富集形成工业矿体。前人针对黑色页岩容矿型Ni-Co矿床的研究极大的推动了人们对这类矿床的认识, 然而, 对这类矿床中Ni-Co共生-分异机制、同沉积过程中多金属的共生关系以及后期改造过程仍需要更深入的探索。以现代分析技术为手段, 开展成矿学、沉积学、地质生物学、地球化学等多学科交叉研究, 将有助于深入理解黑色页岩容矿型Ni-Co矿床的成矿过程以及与地球多圈层协同演化(尤其是表生环境演化)的关系, 从而推动黑色页岩容矿型Ni-Co矿床成矿理论的发展。