Palaeo-environmental and provenance reconstruction of the Lower Permian mudstones (lower Ecca Group) in the main Karoo Basin, South Africa

Geel, C.; Bordy, E. M. (in review) Palaeo-environmental and provenance reconstruction of the Lower Permian mudstones (lower Ecca Group) in the main Karoo Basin, South Africa Assessing the composition and origin of the organic matter rich mudstones, which dominate the early basin fill of the main Karoo Basin of South Africa, are essential for reconstructing the palaeo-environment, palaeo-climate and potential provenance areas in southern Gondwana during the Early Permian. This study addresses these topics by investigating the sedimentary geochemistry and petrography (e.g., major oxides, trace elements, mineral compositions, sedimentary structures, textures) of the lower Ecca Group, which was sampled in newly drilled boreholes. Our results show that the rate of weathering decreased over time as the environment changed from an ice-house to a more green-house setting (i.e., the Chemical Index Alteration is 72–78 in the Prince Albert Formation and 64–66 in the Collingham Formation). Comparison of the V/Cr, V(V+Ni) and Ni/Co ratios and Fe-S-TOC ternary plot show fluctuations between anoxic and oxic conditions during deposition. Furthermore, there is evidence, albeit somewhat equivocal, for changing palaeo-salinity levels, low sedimentation-rates and high bio-productivity. Trace element Principle Component Analysis, the Index of Compositional Variability and Discriminant Function Analysis of the older mudstones (Prince Albert and Whitehill Formations) imply different provenance regions with felsic to intermediate vs mafic signatures in the western and eastern MKB, respectively. The youngest unit, the Collingham Formation, has a more uniform composition across the region, and was mostly sourced from the quartz-arenites and granites most likely associated with the Cape Fold Belt.吉尔（Geel, C.）、博迪（Bordy, E. M.）（待刊）：南非主卡鲁盆地下二叠统泥岩（下埃卡群）的古环境与物源重建。占南非主卡鲁盆地早期沉积充填主体的富有机质泥岩，其组分与成因解析，对重建早二叠世冈瓦纳南部的古环境、古气候及潜在物源区至关重要。本研究通过对新钻探钻孔采集的下埃卡群样品开展沉积地球化学与岩石学分析（涵盖主量氧化物、微量元素、矿物组成、沉积构造与结构），对上述科学问题展开研究。研究结果显示，随着沉积环境从冰室期向温室期转变，化学风化速率随时间逐渐降低：阿尔伯特组（Prince Albert Formation）的化学蚀变指数（Chemical Index Alteration）为72~78，科林厄姆组（Collingham Formation）为64~66。通过对比钒铬比（V/Cr）、钒/(钒+镍)比（V/(V+Ni)）、镍钴比（Ni/Co）以及铁-硫-总有机碳三元图，发现沉积时期水体环境在缺氧与富氧之间波动。此外，尽管存在一定不确定性，但已有证据表明古盐度、沉积速率与生物生产力均发生了显著变化。对较老的阿尔伯特组（Prince Albert Formation）与怀特希尔组（Whitehill Formation）泥岩开展的微量元素主成分分析（Principle Component Analysis）、成分变异指数（Index of Compositional Variability）及判别函数分析（Discriminant Function Analysis）结果显示，主卡鲁盆地（MKB）东西部分别具有长英质-中性与铁镁质的物源端元特征。研究层位中最年轻的科林厄姆组（Collingham Formation）区域组分均一性更强，其物源主要来自与开普褶皱带（Cape Fold Belt）相关的石英砂岩与花岗岩。