Origin and paleoenvironmental conditions of iron duricrust in the Paleogene N’kapa Formation, Douala sub-basin, Cameroon

Abstract

• Iron duricrust has brecciated to colloform textures with hematite, siderite, kaolinite, quartz. • Intense weathering of Fe-rich felsic rocks in humid-semiarid, dysoxic-oxic regimes. • Iron duricrust formed as ferricretes via supergene processes by meteoric water. • Global warming, sea-level changes and tectonics controls ferricretes formation. • Isotopic signatures reveal diagenetic altera by meteoric waters, forming hematite. This paper unravels rarely studied paleoenvironmental conditions and origin of iron duricrust in the Paleogene N’kapa Formation, Douala sub-basin, Cameroon. It integrates petrographic and geochemical techniques (mineralogy, trace, major and REEs elements) and isotope of δ 18 O, δD and 87 Sr/ 86 Sr to reconstruct paleoclimate, paleoweathering, paleoredox and paleosalinity, and established mechanisms of formation of iron duricrust. The textures and mineralogy of iron duricrust indicates that they are ferricretes with brecciated to colloform textures composed of hematite, siderite, kaolinite and detrital quartz grains. Paleoclimate proxies revealed humid to semi-arid climate and variation in climatic conditions caused intense-chemical weathering, favouring a change in weathering processes from kaolinitisation to ferricretization. Paleoredox and paleosalinity proxies indicate that the iron duricrust was formed under dysoxic to dominantly oxic conditions in shallow freshwater to somewhat brackish water body with intermediate salinity. The δ 18 O, δD and 87 Sr/ 86 Sr composition of the samples indicate post-depositional alteration due to influx of meteoric waters during the duricrusting process. The equilibration with meteoric water reveals that the iron duricrusts formed primarily through supergene enrichment and diagenetic reworking of detrital sediments. However, some contribution from hydrogenous input occurred in lagoonal settings with fluctuating salinity, which probably facilitated the precipitation of iron directly from water. The remobilization of iron led to the formation of secondary hematite, which could possibly be link to early Paleogene climatic variations and eustatic sea-level changes. These findings provide valuable insights into sedimentary mineral enrichment in response to Paleogene environmental dynamics and offer a reference for comparable iron-rich deposits in African basins and other parts of the world.