ELGUNNA CAMBRIAN - LOWER CRETACEOUS
SANDSTONES, SOUTHERN SINAI, EGYPT:
COMPOSITION AND PALEOCLIMATIC INFERENCES
ATEF M. IBRAHIM
Geology Dept., Fac. Sciences, ElAzhar Univ., Cairo, Egypt
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Systematic variations in compositional maturity of ElGunna sandstones Araba Fm, Cambrian; Naqus Fm, Ordovician and Malha Fm, L. Cretaceous reflect the change in climatic conditions during deposition. Source rocks and tectono-environmental setting however remained relatively unchanged throughout deposition of these sandstones. To present these features some textural and mineral maturity indices have been tested.
ElGunna sandstones succession at G. ElRoum is of 3 sandstone types characterized by the following sequence of change (oldest to youngest) in compositional maturity expressed in Q/ F/R %s 81.14:0:6.43
>> 79.4:0:8.2 >> 79:0:8.21. The differences are a function of the changing paleoclimate from (warm humid
>> gradual change toward temperate humid >> temperate humid). At G. Morawha, the succession consists of 2 sandstone types, and the sequence of maturity change from (oldest to youngest) is 85.3:0:6
>> 81.6:0:12.3. i.e, warm humid >> gradual change toward temperate. A log/log plot of the ratio of the total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to total feldspar plus rock fragments is also a sensitive discriminator of the studied sandstones with differing climatic heritage.
ZTR index and bulk chemical composition data support interpretation of climate from framework minerals, but themselves are not sensitive enough to be unequivocal indicators.
The optimum conditions for the production and preservation of a distinctive climatic signature conditions on sand composition are met in recycled orogen provenance. Such settings are characterized by recycled orogenic and low rank metamorphic source rocks, long transport distance, deposition in non marine environments and shallow burial diagenesis.
ElGunna sandstones (G. ElRoum and G. Morawha) have similar diagenetic history characterized by stages of authigenesis: Stage I typified by neomorphic development of kaolinite and quartz, and Stage II dominated by replacement reactions involving production of illite, montmorillonite, Fe oxide and carbonate minerals. Various arguments suggest that early formed cement in sandstone was a product of meteoric water whose chemistry was controlled by climate. The nature and distribution of early silicate cement in ElGunna sandstone support this supposition.
The nature of the silicate cement was a function of groundwater chemistry, as controlled by climate. During times of high precipitation (relative humidity and low, ionic concentration of groundwater), pore water was dilute promoting authigenic formation of kaolinite and quartz. Illite and montmorillonite in subordinate amounts in the area are pseudomorphic replacement of kaolinite.
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