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 spheroidshaped 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.

Paleoenvironmental conditions during deposition of Kolosh and Gercus formations in northern Iraq as deduced from clay mineral distributions

“…However, kaolinite is more likely to be inherited from kaolinitic source, since detrital kaolinite is very unlikely to form in seawater [25]. Previous studies confirmed the presence of kaolinite in the river environments (Gercus) [26], which is established in these continental environments due to the presence of acid solutions that have a suitable environment for sedimentation [1]. Detrital origin of kaolinite relates mostly to derivation from igneous rocks that are rich in potash feldspars or from reworking of older sedimentary rocks [27].…”

Paleoclimatic and Redox Condition Changes during Early-Middle Jurassic in the Yili Basin, Northwest China

“…(Figure 6) is notably different from the other formations. Oxidizing depositional conditions are widely used to explain the coloration of red sandstones in the Yili Basin [73,74] and elsewhere [75]. These sedimentary features are supported by the Mo EF values to imply that the sediments in the Sangonghe Fm.…”

Micro-Mechanisms and Implications of Continental Red Beds

“…Most of the hematite is in tiny crystal agglomerates or in individual microcrystals form in the pores of the montmorillonite and exhibit a variety of shapes (Figure 2d) [47]. There are spherical, rod-shaped, leaf-shaped, and well-developed hexagonal flake hematite single crystals, as well as fibrous to lath-shaped, rosette clusters and spherical hematite/goethite aggregates (Figure 2c-f) [47,48]. These morphological features are closely related to the relative abundance of the iron-bearing mineral precursors in the rocks.…”

“…Compared with the hematite in the form of single or polycrystalline aggregates, the latter (mixed with clay minerals) is more widely distributed in time and space, appearing in red beds of all ages. In Mesozoic and Cenozoic red beds, hematite is almost only adsorbed on the surface and/or in the voids of clay minerals [41,48,50]. In older red beds, hematite can be depleted to varying degrees due to later tectonics, groundwater penetration, or the participation of hydrothermal fluids, and then replaced by euhedral to semi-hedral authigenic hematite grains formed in situ and secondary pores or aggregates [42,43].…”