Post-Depositional
Changes of Middle Eocene Limestones of North West Minia Area, Egypt
Ali Kh. ElGindy, Abd ElAziz A. Abd ElAziz,
Fathy H.A. Soliman and Ramadan Sayed
Geology Department, Fac. of Sciences, Minia Univ., Egypt |
Post-depositional processes on the Middle
Eocene limestones, NW Minia area, Egypt, include micritization, cementation,
silicification, ferrugination, glauconitization, compaction, dolomitization and
dedolomitization. Micritization is more advanced in the Minia (Lower Lutetian), Samalut
and Sath ElHadid Fms than the Midawara Fm (Middle - Lutetian). Four cement types are
recognized: Dark micrite, fibrous calcite, sparry calcite and syntaxial overgrowth.
Moreover, the meteoric water has caused calcitization and aggrading recrystallization of
both the matrix and skeletal particles. The sediments of Minia and Samalut Fms were
subjected to significant compaction (mechanical or chemical) with a minor effect on Sath
ElHadid Fm.
Three forms of silica are recognized: Chert nodules, interconnected chert bands and
mottled cherts in the carbonate rocks of Midawara and Sath ElHadid Fms. Dissolved silica
could have been produced epigenetically through leaching processes on arenaceous rich
sequences. Glauconite was formed authigenically as an indirect effect of organism on
the Midawara sediments in pre-burial stages. The oxidation of Fe bearing detrital minerals
probably have resulted, among other sources, in the formation of Fe oxides in the Midawara
facies. Two types of dolomite are observed in the Minia Fm. The first is the
fine-crystalline dolomite that was probably formed at an early diagenetic stage. The
second is the coarse crystalline, formed by mixing meteoric-marine water and br
neomorphism of the precursour dolomite in later stages of diagenesis. The non- pervasive
dolomite of the Midawara and Sath ElHadid Fms might have resulted from the depletion of Mg
ions from former high Mg-calcite and clay minerals.
Dedolomitized crystals are recorded in the Minia facies, probably formed by meteoric
water.
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