قشرة الجبسوم في الجزء العلوي من تكوين فتحة في منطقة المحلبية شمال غرب العراق

Gypcrete in the Upper Part of Fat`ha Formation at Mahalabia Area / NW Iraq

Gypcrete with a thickness between (4 - 8) meters occur at the upper part of Fat`ha Formation at Mahalabia area, northwest of Iraq. The gypcrete consists of several zones which are lacking stratification. It has been divided on the basis of textures and characteristic structures into three types. These are from bottom to top: subsurface crust, followed by friable gypcrete and the hard surface gypcrete. This last crust is exposed at the surface and consists of gypsum polygons of different shapes and sizes and are characteristic of this zone. The three zones were formed by different diagenetic processes caused by groundwaters and the alternation of dry and wet periods. Calcrete as well as gypcrete also formed due to these processes. The calcrete exhibits alpha fabrics which often reveal the influence of physico-chemical processes and calcite pseudomorph after gypsum. All these features indicate that the calcrete is associated with gypsum crust, probably during the early stages of diagenesis through pedogenesis. The source of gypsum and carbonates of these crusts were most likely the bed rocks of Fat`ha Formation. The majority of gypsum crystals were formed insitu, mainly from groundwaters by displasive growth within the host sediment.