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Abstract
CHAPTER VI
SUMMARY AND CONCLUSIONS
The present work deals with the study of stratigraphy, systematic paleontology, microfacies analysis and depositional environments of some Miocene sediments, Cairo-Suez District, Egypt.
VI.1 Stratigraphy:
The marine Miocene sequence exposed at gabals Geneife (97.1 m thick), Homeira (110.9 m thick) and Gharra (141.7 m thick) in Cairo-Suez District is a transgressive-regressive sequence lying unconformably between continental sediments of Oligocene and post Middle Miocene.
The Lower Miocene deposits in the studied sections consist of sandstones and mudstones with minor intercalations of sandy limestone. They differ from the Lower Miocene algal limestone of the Sadat Formation (Abdallah and Abdel-Hady 1966). Thus the name Gharra Formation of Said (1971) is feasible to these Lower Miocene deposits.
On the other hand, the Middle Miocene rocks consists mainly of algal, reefal, and chalky limestones with oyster banks, and minor mudstone and sandstone intercalations. These facies differ from the clastic facies of the Hommath Formation (Abdallah and Abdel-Hady 1966). Thus, the auhor follows the rock unit Genefe Formation (Said, 1990) for the Middle Miocene rocks exposed in the studied areas.
Based on the abundance of macro-invertebrate fauna especially bivalves, gastropods and echinoids with some foraminifera, six biohorizones have been recorded, they are from younger to older: Chlamys (Argopecten) submalvinae zone, Chlamys (Macrochlamis) sardoa zone, Pecten (Pecten) cristato-costatus zone, Pecten (Flabellipecten) flabelliformis zone, Pecten (Pecten) ziziniae zone and Chlamys gentoni zone.
VI.2 Systematic paleontology and biohorizons:
Sixty two macrofossil species (40 bivaves, 16 gastropods, 6 echinoids) have been identified, described and illustrated. They belong to 34 genus and 26 families. Their Paleogeographic and biogeographic distribution indicated Atlantic-Mediterranean affinity as well as the Indo-Pacific one in few.
The fossiliferous sandy limestones (beds 5 in Gabal Geneifa , 11 in Gabal Homeira and 6 in Gabal Gharra) nearly contain a common content of fauna (oysters, pectinids and many other bivalves, gastropods, echinods, algae and bryozoans) which can be considered a horizon for correlation of the middle part of Gharra Formation in the study area. In addition to the abundance of Alectryonella plicatula (Gmelin) subsp. virleti Deshayes and Crassostrea frondosa (De Serres) subsp. rohlfsi Fuchs in these beds in the three sections can be considered a biohorizon for correlation.
The fossiliferous sandy limestones (bed no. 13 in Gabal Homeira and bedno. 10 in Gabal Gharra) nearly contain a common content of fauna (oysters, pectinids and many other bivalves, gastropod molds, barnacles and algae) can be considered a horizon for correlation of the upper part of Gharra Formation. In addition to the abundance of Pecten (Pecten) ziziniae Blankenhorn, Pecten beudanti Basterot, Pecten (Oppenheimpecten) benedictus (Lamarck) in the two sections can be considered a biohorizon for correlation.
The fossiliferous sandy marl (bed 11 in Gabal Geneifa and the lowermost part of bed 13 in Gabal Gharra) with their high abundance of pectens are correletable. The abundance of Chlamys (Argopecten) macrotis (Sowerby), Pecten (Pecten) cristato-costatus Sacco, Pecten beudanti Basterot, Pecten (Oppenheimpecten) benedictus (Lamarck) and Chlamys gentoni (Fontannes) in the two sections can be considered a biohorizon for correlation for the lower part of Genefe Formation.
The fossiliferous sandy limestone (beds 16 in Gabal Homeira and 13 in Gabal Gharra) nearly contain a common content of fauna (oysters and pectinids) can be considered a horizon for correlation of for the lower part of Genefe Formation in the study area. In addition to the abundance of Alectryonella plicatula (Gmelin) subsp. Virleti Deshayes and Crassostrea frondosa (De Serres) subsp. Rohlfsi Fuchs in these beds considered a biohorizon for correlation.
VI.3 Micofacies and depositional environments:
49 thin sections from the total collected samples were subjected to microfacies investigation. The examined thin sections have been categorized into two main groups:
1- Limestone group, which represents 55 % of the studied thin sections. They differentiated into four main microfacies types: wackestone (sandy dolomitic and algal), packstone (sandy bioclastic, sandy foraminiferal bryozoan and sandy echinoidal), grainstone (sandy fossiliferous,sandy echinoidal foraminiferal, sandy algal and sandy molluscan) and framestone (coralline).
2- Clastic group, which represents 45 % and includes the following microfacies types: quartz arenite (calcareous, dolomitic calcareous, calcareous fossiliferous and calcareous ferrugineus) and sandy-silty shale.
The marine Miocene sequence in the study areas could be divided into two sedimentary facies: the lower one is a clastic-dominated facies, which represents the Gharra Formation, while the upper facies is caarbonate-dominated and represents the Genefe Formation.
VI.3.1 Clastic-dominated facies:
The lower part of this facies is an erosional base with extra-and intraformational clasts, followed upward by cross-bedded sandstones and topped by thinly laminated mudstone, while its upper is mainly intercalated with sandy limestone, oyster banks, mudstone and sandstone.
This sedimentary facies is characterized by the following microfacies associations: calcareous quartz arenite, dolomitic calcareous quartz arenite, calcareous fossiliferous quartz arenite, calcareous ferrugineous quartz arenite, sandy echinoidal foraminiferal grainstone, sandy dolomitic wackestone, sandy fossiliferous grainstone, sandy echinoidal packstone, sandy algal grainstone, sandy bioclasts packstone, and sandy foraminiferal bryozoan packstone.
Based on microfacies associations, sedimentary structures and fossil content, the Gharra Formation is deposited in an environment ranging from tidal flat for the lower part to typical reef flank for coral and algal reefs and high energy on shoals for the grain-supported fabrics (grainstone and packstone).
VI.3.2 Carbonate-dominated facies:
This sedimentary facies consists essentially of limestones and mudstones. The limestones are thick-bedded, compact, massive, chalky, jointed, fractured, bioturbated and highly fossiliferous with coralline red algae, Amphistigina sp., Miogypsina sp. and oysters. Mudstone is thinly laminated, moderately compact, nonfossiliferous, with lenticular beds of fine sandstone.
In the study area the carbonate-dominated facies is characterized by the following microfacies associations: Sandy bioclasts packstone, sandy foraminiferal bryozoan packstone, sandy molluscan grainstone, foraminiferal algal wackestone, sandy bioclasts packstone, sandy echinoidal foraminiferal grainstone, sandy algal grainstone, algal coralline framestone and coralline framestone.
The microfacies associations, fossil content and sedimentary structures stated that the environment of deposition was reefal environment for the lower part, due to the presence of abundance of coralline red algae, oysters, Amphistigena sp, Miogypsina sp. to shelf lagoons with open circulation and an intertidal environment with low energy conditions for the deposition of mudstones in the top most part of Genefe Formation.