Classification and depositional environments of Quaternary pedogenic gypsum crusts (gypcrete) from east of the Fayum Depression, Egypt

Abstract

The eastern boundary of the Fayum Depression (45 m below sea level) is bounded by a 99-m-high hill (inselberg of Girza) that slopes gently eastward towards the Nile Valley. The inselberg and the surrounding plain are encrusted with 50–130-cm-thick gypsum crusts that exhibit well-defined pedogenic horizons. Also, east of the Nile Valley, the Qattamia plateau is encrusted with gypsum crusts that form 50-m-wide ‘pavements’ that are scattered at different levels. The gypsum crusts range in thickness from 20 to 50 cm, and do not form characteristic vertical horizons. The individual gypcrete pavements exhibit massive mottled or powdery structure. Using structural, fabric and textural criteria, three main types of crusts are distinguished: (1) Eluvial horizon—massive to spongy-like powdery gypsum crusts, found at the land surface and consisting of gravels and sands cemented with gypsum; (2) Illuvial horizon A—massive indurated gypsum crusts that consist dominantly of white gypsum with horizontal and sub-vertical black gypsum veinlets; (3) Illuvial horizon B—massive mottled gypsum crusts that consist of carbonate or shale fragments floating in gypsum cement. Petrographically, the gypsum crusts in the Girza and Qattamia areas comprise a varying abundance of microcrystalline, fine lenticular, coarse lenticular, porphyroblastic, prismatic or poikilotopic gypsum. Lenticular and microcrystalline gypsum crystals are usually observed filling rhizotubules in all crusts. The origin of the gypsum crusts is related to pedogenic processes which involved the leaching of gypsum from the underlying, or adjoining, gypsiferous shale or marl layers and the subsequent displacive crystallization of gypsum in the upper soil horizon. Cycles of wetting (rain fall) and drying are believed to account for the dissolution and translocation of gypsum from the upper eluvial horizon and its recrystallization as microcrystalline and fine lenticular gypsum in the illuvial horizon A. The poikilotopic gypsum crystals in the illuvial horizon B reflect the abundance of gypsum-enriched groundwater in the phreatic zone. The dominant processes of gypsum growth are by displacive and/or inclusive crystallization, reflecting formation in pedogenic setting. The existence of gypsum crusts capping the inselberg and the surrounding plain of Girza area resist eastward deepening of the Fayum Depression. On the other hand, the gypsum crusts on the surface of the Qattamia plateau favor the development of relief inversion due to the removal of the surrounding soft materials. Therefore, the gypsum crusts provide useful paleoclimatic and geomorphic indicators to the study areas.