FACIES ANALYSIS, DIAGENENTIC ASPECTS AND DEPOSITIONAL
ENVIRONMENT OF THE CENOMANIAN (RAHA FORMATION) AT
SOUTHWESTERN SINAI, EGYPT.
Alaa Mostafa and Ahmed Kh. Abdel Aal.
Al-Azhar University, Assiut branch, Faculty of Science.
Abstract
On the basis of cyclicity, the outcrops of the Raha Formation at southwestern Sinai could
be classified into three informal units; a lower clastic unit, middle mixed clastic-carbonate unit
and an upper carbonate unit. The lower clastic unit is made up of dolomitic sandstone and
claystone which form the base of each cycle while quartzwacke and dolostone are forming the
tops of these cycles. The cycles of the middle mixed clastic-carbonate unit start at the base with
claystone followed by dolomitic quartzarenite, quartzwacke and ended with limestone and
dolostone. The upper carbonate unit composed entirely of thick limestone beds which
characterize the topmost part of the Raha Formation allover the studied area. The topmost parts
of the mixed clastic-carbonate unit as well as the upper carbonate unit are highly fossiliferous
yielding the most common Cenomanian fauna such as Exogyra flabellata, Exogyra olisiponensis
and Exogyra africana.
Petrographic studies on the Cenomanian rocks revealed that the limestone rocks are
classified into 11 lithofacies. The molluscan shell fragments, foraminiferal tests (mainly
benthonic forams), ostracods and echinoidal plates form the main skeletal framework of the
studied lithofacies arranged in a decreasing order of abundance respectively. Dolostones of the
Cenomanian succession can be differentiated into three types. They are; the ferroan dolomite,
dolomicrite, and glauconitic dolostone lithofacies. Sandstones in the present study could be
classified into the following lithofacies: fossiliferous quartzarenite, glauconitic quartzarenite
and ferruginous quartzwacke.
Cementation, micritization, recrystallization, and dolomitization are the main diagenetic
processes affecting the different carbonate rocks of the Raha Formation. The diagenesis of
sandstone involves the formation of hematite as coating, impregnation and cementing material
for the quartz grains in the studied lithofacies.
The southwestern Sinai hinterland during Cenomanian age can be considered as the
southern most end of the first marine great invasion of the Tethyan Sea coming from the north.
So, the detailed field observations and petrographic investigation of the Cenomanian Raha
Formation in the study area reflect marginal environments for the Cenomanian basin in Sinai.
This marginal site comprises three main environments. These are; intertidal, shelf basin and
carbonate platform environments.
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