Journal of African Earth Sciences, Vol. 17, No. 1, pp. 125-127, 1993. 089%5362/93 $6.00 + 0.00 Printed in Great Britain Pergamon Press Lui

Short Notes

Pleistocene evolution of the reef terraces of the Red Sea coastal plain between Hurghada and Marsa Alam, Egypt

M. E. E. EL MOURSl

Department of Geology, Faculty of Science, E1Mansoura University, El Mansoura, Egypt

(First received and accepted 21st April, 1993)

GEOLOGIC SETTING OF THE EMERGED REEF TERRACES

The s tudy area represents a strip of about 300 km along the Red Sea coastal plain of Egypt extending from Hurghada in the north to Marsa Alam in the south (Fig. 1). In this sector, seaward from the edge of the Pre-Cambrian shield escarpment, a Creta- ceous to Pliocene sedimentary sequence outcrops unconformably overlain by a staircase of eight emerged reef terraces alternated with a series of clearly defined alluvial gravel terraces (Fig. 2). Their gross lithologic features, geomorphologic setting, stratigraphic relations, fossil contents and altitudes revealed that their carbonate sediments were laid down in three cycles of reef formation by intervals of emergence and weathering or conti- nental deposition of gravel fans (Fig. 2). The oldest cycle is represented by terrace VIII at altitudes varying between +9 and +35 m, the middle cycle is re-presented by terraces VII-V at altitudes varying between +22 and +32 m, while the youngest one is represented by terraces IV-I at altitudes of +9, +6, +3, and +2 m, respectively (Fig. 3).

DEPOSITIONAL AND DIAGENETIC ENVIRONMENTS OF THE EMERGED

REEF TERRACES

The detailed textural, mineralogical and chemi- cal analyses carried out on both the reef-building corals of the emerged terraces and their reef asso- ciated sediments showed that the terraces of the youngest cycle are essentially not equilibrated to the subaerial envi ronment and still preserve me ta s t ab l e minera logy and m a r i n e t ex tu ra l characters. On the other hand, the terraces of both the middle and oldest cycles are completely equili- brated to the subaerial environment and are made

up entirely of the most stable diagenetic low Mg- calcite and exhibit a variety of textural modifica- tions that resulted from the interaction with meteoric water. Moreover, statistical analyses carried out on the mineralogical and chemical data showed that, mineralogical stabilization pro- cesses have essentially occurred via a thin film in a somewhat closed chemical system with no direct communicat ion between the diagenetic solutions at the reaction sites and the pore water, i.e., under meteoric vadose conditions. However, mineralogi- cal stabilization was accompanied by a slight enrichment in Ca 2~ and Mn 2÷ and a depletion in Sr ~÷ and Na ÷ while K ÷, Mg 2÷ and Fe a÷ show no significant changes indicating their association to the detrital component ra ther than to the carbon- ate lattice.

CHRONOLOGY AND TECTONICS OF THE EMERGED REEF TERRACES

The three lowest terraces (I-III) of the youngest cycle give 2~rh/2~O ages between 72.1 + 2.5 and 131.2 + 4.7-131.2 + 4.4 ka BP. These age ranges coincide with the chronology of the last interglacial cycle and could be roughly correlated with the oxygen isotope substages 5a, 5c and 5e, respect- ively. However, the present-day elevations of the dated terraces do not coincide with the palaeo-sea level positions during these times. This suggests that, either the sea level position in the Red Sea during substages 5a, 5c and 5e were different from those usually recognized, or that three dated ter- races, combined with terrace IV, were built during the maximum high sea level s tand of the last interglacial cycle (oxygen isotope substage 5e) while their surfaces are the product of short s tand stills during a cont inuously falling sea level. However, the comparison of the stratigraphic rela-

125

126 M.E.E. EL MougsI

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Fig. 1. Geological map of the study are, compiled and slmplifled after the Geological Survey of Egypt (Qena and Aswan Quadrangles. 1978).

tions of the older terraces, made up of recrystal- lized corals, with both the oxygen isotope recorded and the curve for solar insolation received by the earth at latitude 65°N, allowed the estimation of their min imum ages. The terraces of the middle cycle were considered to have formed during oxy-

gen isotope stage 7 (170-230 ka BP) while that of the oldest cycle was formed dur ing stage 9 (300-330 ka BP). The present-day altitudes of these terraces are the product of eustatic sea level fluctuations combined with differential tectonic uplift.

Pleistocene evolution of the reef terraces of the Red Sea coastal plain between Hurghada and Marsa Alam, Egypt 127

- M . M i o c e n e I . P l i o c e n e J M i d d l e P l e i s t o c e n e J , L a t e P l e i s t o c e n e

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Fig. 2. Generalized diagrammatic cross-section showing the straUgraphic relations, major facies and suggested ages of the recognized fossil coral reef terraces and the post gypsum succession exposed in the Red Sea coast.

• Cofamne rwc~Pst3ne with at:ra~ed sedate

~ ) Co:a~,r,e t~nesffir~ covered by calc~eous bPqch deposl~

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320 - - - e - - . • __ _ e _ Te r race VII J ~ • Co~a,'1;r.e bin•stone cover,Pal by aJluvial depo,~

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® \ ° ,o _ . _ _ ~ _ _ -- "- 1 s . . . - ~ , . w _ ® - - - - - - / \ - - - - - 0 - - - - - ' - - ° - - -~ 0 " " - _ _ T e n a c e II

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/ ," . ¢ 1 # / £" / b l / , ¢ " 4 / / b* r . ~ / / . / i / / / / . / / ' / , * ' / ," / . s

Fig. 3. AIUtudinal correlation of the emerged reef terraces of the Red Sea coastal plain between Hurghada and Marsa Alam, Egypt.

REFERENCES

El Moursl, M. E. E. 1992. Evolution Pl6istoc~ne des terrasses de la Mer Rouge entre Hurghada et Marsa Alam, Egypte. Unpublished Doctorat Thesis, Univer- sity of Aix-Marseille It, France, 245 p.