Non-pelletal glauconite from the Campanian Qusseir Formation, Egypt: Implication for glauconitization

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• 作者：Hassan Baioumy ; ^{hassanbaioumy@hotmail.com} ; Sabah Boulis
• 关键词：Glauconite ; Egypt ; Mineralogy ; Geochemistry ; Origin ; Glauconitization
• 刊名：Sedimentary Geology
• 出版年：2012
• 期刊代码：81_00370738
• 类别：ear
• 出版时间：1 April, 2012
• 卷：249-250
• 期：Complete
• 页码：1-9
• 文件大小：1703 K

摘要

Glauconitization processes proposed in the literatures include the 鈥渓ayer lattice鈥?theory in which glauconite formed through K uptake by a detrital smectite precursor and the 鈥渘eoformation theory鈥?that does not require a smectite-like precursor.

The non pelletal glauconite bed in the uppermost part of the Campanian Qusseir Formation at the Abu Tartur area and the underlying smectite-rich beds provided a potential example for the glauconitization process. The glauconite occurs as fine (clayey), moderately hard, and homogeneous bed ranges in thickness from 2 to 3 m. Under the optical microscope, glauconite occurs in a unique morphology as very fine green clayey matrix, which differs from the typical pelletal shape of many glauconites. Under the scanning electron microscope, it appears as dense, uniform, and very fine flakes that are composed of Si, Al, Fe, and K. X-ray diffraction analysis of both bulk samples and clay fractions indicates that the glauconite bed is composed entirely of glauconite. Geochemical analysis suggested that the investigated glauconites belong to the 2nd stage of the maturity scale (i.e. moderate maturity); or they can be alternatively ranked as evolved. Chondrite-normalized REE patterns exhibit LREE enrichment relative to HREE ((La/Yb)_N ratios vary from 5 to 8) and slightly negative Eu anomalies (Eu/Eu* from 0.7 to 0.8).

Clay beds that underlie the glauconite bed are composed of two types of smectites; smectite of low K₂O and Fe₂O₃ contents and smectite of high K₂O and Fe₂O₃ contents. Smectite of low K₂O and Fe₂O₃ contents is considered in this study as the initial precursor of glauconite, while smectite of high K₂O and Fe₂O₃ represents the transitional phase between smectite and glauconite. This, in turns, supports the 鈥渓ayer lattice鈥?theory in which glauconite formed by K diffusion to a detrital smectite precursor from the sea-water due to transgression. Absence of mixed-layer minerals suggests that the diffusion process was active and continuous until reaction completion was reached, i.e. formation of glauconite. Maturity and REE patterns suggest the possible formation of the studied glauconite probably at shelf environment at approximately 100 m water depth. The occurrence of glauconite indicates a marine invasion of the upper part of the Qusseir Formation, which was considered previously as non-marine sediments.