Duricrust
and karst products in and around Farafra Oasis, Western Desert, Egypt.
Mohamed I. El Sayed
Dept. Geology, Fac. Sciences, Beni Suef, Egypt |
Farafra Oasis and the surrounding areas
present extensive development of duricrusts and karst products. Silcretic duricrust caps
scattered isolated hills, covering the floor of the low-areas and as in-place deposits in
the lower horizon of the Tarawan Formation (late Paleocene). Two main types of silcretes
are present: porcellaneous developed by precipitation of microcrystalline quartz in the
intergranular pores of chalk and mudstone, and massive-vitreous type formed by replacement
of limestone and dolostone. The silcretes are interpreted as groundwater precipitates. The
main source of silica is from clay mineral diagenesis in which ascending translocation is
considered.
Gyperete is restricted to low-areas. Its gypsum crystals are granoblastic with cloudy
appearance and minute inclusions. The main source of gypsum came from the gypsum veinlets
characterizing the Dakhla Shale (middle Paleocene).
A proposed sequential model for the studied sites, especially in Wadi Hennis, involves
dissolution by the end of Eocene, followed by formation of silcrete and development of
cavities in the Khoman Chalk (Maastrichtian). Later on, coarse calcite crystals were
precipitated in the cavities. Another phase of dissolution took place in the Khoman Chalk
resulting in collapsing of the overlying rocks and development of a drainage system as in
Wadi Hennis. During Quaternary, arid environment prevailed with intermittent wetter
episodes witnessing deposition of gypcrete and paleosols in addition to partial
silcretization in the low-areas.
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