PETROGRAPHY, GEOCHEMISTRY AND GENESIS OF TUFA
DEPOSITS OF BIR DUNGUL AREA, SOUTH WESTERN
DESERT, EGYPT.
ESMAT A. ABOU EL ANWAR
Geologica/ and Geophysical Sciences Dept. National Research Center, Cairo, Egypt.
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During the Lower Eocene, Southern Egypt, including Bir Dungul area was elevated and remained until the present time. The geomorphic features of the Bir Dungul area, southern Western Desert are a re suit of intensive weathering processes, which helped in the formation of many paleoerosion surfaces of different elevations. The oldest one is that of the Eocene Dungul Formation (400 m a.s.l.), followed by the Paleocene Garra Formation (300 m a.s.l.) and finally the younger surface of the Paleocene Kurkur Formation (200 m a.s.l.). These surfaces are covered by fresh water deposits; tufa and travertine.
The tufa deposits can be differentiated petrographically into four facies: (1) porous bioclastic tufa. (2) cellular algal tufa, (3) stromatolite algal tufa, and (4) hard massive tufa. Diagenatically, tufa deposits were subjected to two paragenetic stages.
Impression fragments of dicotyledon (Moraceae and Saliceceae) and monocotyledon (Typhacea) leaves as angiospermous indicate that they lived in normal to warm temperature and tropical climates.
Geochemically, the low content of Na indicates its loss during diagenesis which occurred in an environment less saline than seawater; brackish and/or fresh water. The Sr content indicates that the tufa formed from dissolved bicarbonate with high concentration of MgO,
Fe2O3 and P2O5 and under control of bacterial activity.
The petrographical and geochemical observations, besides the study of the fossil plants reveal that the deposition of the tufa occurred in shallow lakes during wet and warm conditions accompanied by high water table, alkaline spring recharge and considerable vegetation cover. |
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