Abstract The recently constructed Lokoja-Abuja highway and a quarry which cut through the Patti Formation, provided opportunity for the field study and sampling of the exposed ironstone interbeds within the carbonaceous shales and clays in Ahoko-Idu axis of the southern Bida Basin, Nigeria. The origin and paleoenvironmental significance of the ferruginous intervals were examined in this paper. Investigation reveals that the ferruginous bands occur as three lithofacies comprising of laminated, bioturbated and spheroidal concretionary facies. The primary components and depositional features of the host shales (detrital minerals, wood and herbaceous fragments, laminations) are largely preserved in the ironstones indicating compositional similarity and therefore a diagenetic origin for the ironstones. Results of XRD reveal substantial amount of siderite and illite but minor amounts of kaolinite, haematite, calcite and magnetite. Major elements such as Fe2O3, SiO2, Al2O3, CaO, MgO and P2O5 range from 41.50 to 54.60%, 6.10–23.30%, 3.64–7.00%, 1.40–13.60%, 0.30–1.20% and 0.10–8.80% respectively. The current data is suggestive that kaolinite was diagenetically replaced by siderite at early stage of burial in a reducing environment. Migrating pore water, probably enriched with alkali metals like Ca and Mg, sourced from sea water in the depositional basin and abundance of organic matter in the primary sediments are interpreted to be part of the factors responsible for the siderite precipitation. Both the host shale and the ferruginous beds contain significant amount of MgO, CaO, relics of dissolved bioclasts and body fossils. The geochemical proxies combined with the presence of magnesian siderite in the ironstones indicate that the host shales were deposited in marine environment with occasional freshwater influence. The study concludes that the ironstones are marine to non-marine, sideritic and diagenetic. The laminated and fossiliferous ironstone facies was deposited at the peak of marine transgression during the Upper Cretaceous time and represents maximum flooding surface. Shallowing up and regression into fresh water swamp condition was marked by the occurrence of freshwater sideritic ironstone.

中文翻译：

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尼日利亚比达盆地Maastrichtian Patti组铁岩相成因及古环境分析

摘要 最近修建的 Lokoja-Abuja 高速公路和一个贯穿 Patti 组的采石场，为尼日利亚比达盆地南部 Ahoko-Idu 轴线碳质页岩和粘土内暴露的铁岩夹层的实地研究和取样提供了机会。本文研究了含铁层段的起源和古环境意义。调查表明，铁质带以层状、生物扰动和球状胶结相三种岩相形式出现。寄主页岩的主要成分和沉积特征（碎屑矿物、木材和草本碎片、叠层）大部分保存在铁矿石中，表明成分相似，因此是铁矿石的成岩成因。XRD 结果显示大量菱铁矿和伊利石，但少量高岭石、赤铁矿、方解石和磁铁矿。Fe2O3、SiO2、Al2O3、CaO、MgO、P2O5等主要元素的含量分别为41.50～54.60%、6.10～23.30%、3.64～7.00%、1.40～13.60%、0.30～1.20%和8.8。目前的资料表明，高岭石在埋藏早期在还原环境中被菱铁矿成岩取代。迁移的孔隙水，可能富含钙和镁等碱金属，来源于沉积盆地的海水和原生沉积物中丰富的有机物，被解释为菱铁矿沉淀的部分因素。宿主页岩和铁质层都含有大量的 MgO、CaO、溶解的生物碎屑和体化石的遗迹。地球化学指标与铁矿石中镁菱铁矿的存在相结合，表明寄主页岩沉积在海洋环境中，偶尔会受到淡水的影响。该研究得出的结论是，铁矿石是海相到非海相、菱铁矿和成岩岩。层状和含化石铁岩相沉积在上白垩纪海侵高峰期，代表了最大洪水面。浅化和回归淡水沼泽条件的标志是淡水菱铁矿的出现。层状和含化石铁岩相沉积在上白垩纪海侵高峰期，代表了最大洪水面。浅化和回归淡水沼泽条件的标志是淡水菱铁矿的出现。层状和含化石铁岩相沉积在上白垩纪海侵高峰期，代表了最大洪水面。浅化和回归淡水沼泽条件的标志是淡水菱铁矿的出现。