Clay mineral diagenesis and red bed colouration: A SEM study of the Gercus Formation (Middle Eocene), northern Iraq

Abstract

Detailed high magnification scanning electron microscopic (SEM) study supported by petrographic, mineralogical, and geochemical (major and trace element) analysis of the Gercus Formation clastics (sandstones and mudstones) from northern Iraq shows that the main authigenic components are carbonate (dolomite and calcite), clay and haematite minerals with rare quartz, feldspar, and gypsum. The carbonates and sulphate minerals were the main early diagenetic phase in the Gercus Formation and were produced from the post‐depositional breakdown of ferromagnesian minerals. These latter are unstable in the oxidizing conditions of the interstitial waters below the desert surface. The main authigenic clay minerals are illite, illite‐smectite, and palygorskite. The presence of tangential illite coating and illite‐smectite in the sandstones can be related to their desert origin. Palygorskite is associated with dolomite, calcite, and haematite. These mineral associations may indicate early diagenesis in evaporitic and saline conditions. It also confirms the syndepositional and diagenetic origin for the clayey‐haematitic matrix and pigment of the Gercus Formation. Haematite occurs as extremely fine‐grained crystals mixed with authigenic clays and exhibits a range of shapes (e.g., fibrous to lath‐shaped aggregates, small spherical grains, specularite crystals, blade‐shaped, rosette‐like clusters, and a lump of bright spheroid‐shaped haematite/goethite). The arid to semi‐arid conditions together with the oxidizing and alkaline interstitial water favoured the noted mineral assemblage within the Gercus Formation.