Biostratigraphy and palaeoecological interpretation of the Miocene–Pleistocene sequence at El-Dabaa, northwestern Egypt

Thirteen shallow boreholes were drilled by the Qattara Project Authority (QPA) on the top of the second limestone ridge in the El-Dabaa area, along the Mediterranean coast, northwest Egypt. Five foraminiferal biozones could be recognized in ascending stratigraphic order: the Heterostegina costata, Praeorbulina sicana, Borelis melo melo, Globigerinoides obliquus extremus zones and an un-named zone, embracing the Middle Miocene Marmarica Formation and the Pliocene–Pleistocene Alexandria Formation. The deposition of the fossiliferous carbonate rocks of the Marmarica Formation probably took place in a warm water, inner shelf environment (0–20 m palaeodepth) subjected to some current activity, and with salinity ranging from normal to slightly hypersaline (35–50‰). The sediments of the oncolitic/shelly rocks were deposited in slightly deeper water than the overlying fossiliferous rocks and in less agitated conditions, as indicated from the high percentages of rotaliids and rarity of miliolids. The foraminiferal associations in the Alexandria Formation indicate a clear, relatively agitated, near-shore marine depositional environment in a vegetated inner neritic zone (10–25 m palaeodepth) with 25°C surface water isotherms. On the other hand, the accumulation of the clayey sediments and some geosoils (basal part of the Alexandria Formation) occurred in less agitated, intertidal water conditions, as deduced from the occurrence of planktic foraminiferal species as Globigerinoides trilobus trilobus, G. trilobus immaturus, G. obliquus extremus, G. obliquus obliquus, Globorotalia inflata, and Orbulina universa.


INTRODUCTION
Deposits related to Miocene-Pleistocene times mostly cover the northern part of the Western Desert of Egypt (Fig. 1). The Miocene sediments (Marmarica Formation) form an extensive plateau known as the Marmarica Plateau. The surface of this plateau is of Middle Miocene age and is composed of a shallowmarine biogenic carbonate sequence with some shale and marl intercalations. Along parts of the Mediterranean coastal plain, the Middle Miocene rocks are unconformably overlain by a mantle of thin shallow-marine carbonate beds or by small isolated hills or elongated ridges of conglomeratic and oolitic limestones of Pliocene to Pleistocene age (Alexandria Formation). Substantial variations in depositional environments and climatic conditions through Miocene-Pleistocene times are distinguished.
In general, little research has been carried out on the biostratigraphy of the Miocene-Pleistocene rocks in the northern part of the Western Desert, with the exception of that by Mansour et al. (1969), Omara & Ouda (1968) and Ouda (1971). Recently, Ouda (1998) has studied the biostratigraphy and palaeoecology of the Middle and Upper Tertiary deposits. He selected 24 wells drilled along the entire northern stretch of the Western Desert, and concentrated on global planktic foraminiferal palaeoevents and larger benthic foraminiferal datum planes considered to be correlative and isochronous over the Mediterranean region.
The previous investigations were restricted mainly to the exposed Miocene-Pleistocene rock sequence along the northern part of the Western Desert. The present study aims to provide biostratigraphic information about part of the sub-surface Miocene-Pleistocene succession underlying the limestone ridges.
The present work deals with the study of the foraminiferal content in the sub-surface Miocene-Pleistocene stratigraphic rock sequence of the El-Dabaa area (Fig. 1), and defines their vertical distribution among the different rock units. Unfortunately, this rock sequence is barren of palynological elements (spores, pollen grains and dinoflagellates). The foraminiferal study has been undertaken in an attempt to interpret the environmental conditions prevailing during the deposition of these sediments and to determine the age of the recorded rock units.
from Alexandria to Salum (Alexandria Formation) (Shata, 1955;Shukri et al., 1956;Butzer, 1960;Hassouba, 1995). The El-Dabaa area occupies a small portion of the western Mediterranean coastal zone ( Fig. 1) and lies midway between Alexandria and Mersa-Matruh (156 km from Alexandria). The coastal plain in this area slopes gently in a northward direction and merges gradually southwards into the great Marmarican homoclinal plateau. The surface is characterized by a number of beach-dune calcareous ridges of oolitic and biogenic components running parallel to the present coastline and separated by lowland areas (depressions) filled with lagoonal-sabkha deposits. Both the first and the second ridges are prominent, but the older ones are partially to highly eroded and are less morphologically distinct (El Shami et al., 1969).

The Marmarica Formation (base)
This formation is represented by different fossiliferous carbonate rocks. According to its colour, lithology, and fossil contents, this rock unit is sub-divided into four informal lithological units from old to young as given by Mansour (1999).

Grey-white oncolitic/shelly limestones (Unit V; Beds 22-20).
Grey, moderately hard, compacted algal dolomite to highly dolomitic limestone; very rich in white oncolitic grains with some bivalve, gastropod shell debris, bryozoan skeletal fragments and larger foraminifera. The rocks contain some argillaceous material and gypsum. This unit attains a thickness of 22-25 m.
White chalky/shelly dolostones (Unit III; Beds 13-3). White to creamy, hard, moderately porous fossiliferous chalky dolomite; rich in coarse to medium shell debris of bivalves and gastropods in addition to miliolid and alveolinid foraminifera and bryozoan shells; about 22-42 m thick.
Grey shelly dolostone (Unit II; Bed 2). Light to dark grey, very hard, vesicular shelly dolomite, highly fossiliferous with bivalve and gastropod shell debris, in addition to some badly preserved foraminiferal tests, echinoid, algal and coral fragments. This unit attains a thickness of 5-15 m.

The Alexandria Formation (top) (Unit I; Bed 1)
Carbonate aeolianite which is intercalated with geosoil horizons and about 5 to 15 m thick. A thin layer of fine-grained carbonate sediment with a high content of siliciclastic material is observed at the base, with common planktic foraminifera.

MATERIAL AND METHODS
The present work is based on the study of core samples collected from thirteen shallow boreholes (maximum depth is about 100 m) drilled by the Qattara Project Authority on the surface of the second limestone ridge at El-Dabaa area (Figs 2, 3). Fifty-six samples representing the different rock types of the Marmarica and the Alexandria Formations were selected for microfaunal investigation. About 50 g of each rock sample was processed in steps including gentle mechanical disaggregation, oven drying, soaking in dilute solution of H 2 O 2 (10%), wet sieving through a 63 µm screen and drying at 40(C.

FORAMINIFERAL BIOSTRATIGRAPHY
The sub-surface rock sequence under investigation in the El-Dabaa area, represents part of the Miocene to Pleistocene.
The boundary between the Marmarica Formation of Miocene age and the Alexandria Formation of Plio-Pleistocene age can be defined micropaleontologically.
Biostratigraphic zones based on planktic and larger foraminifera, covering the Miocene in the northern part of the Western Desert were proposed by Ouda (1998): the Praeorbulina sicana, P. glomerosa zones (for the lower part of the Marmarica Formation), the Heterostegina costata zone (also for the lower part of the Marmarica Formation), and the Borelis melo zone (for the upper part of the Marmarica Formation) (Figs 4,5).
In the present study, larger foraminiferal species are also used as zonal markers in some intervals due to the lack of planktic foraminifera. The studied sequence of Miocene to Plio-Pleistocene age is divided into four known biozones and an unnamed biozone representing the upper part of the Alexandria Formation. These biozones are discussed in ascending stratigraphical order as follows.  Author. Ouda (1971), emended by Ouda (1998).
Age and correlation. The present zone is equated with the lower part of the Heterostegina costata Zone of Ouda (1998) of independent Middle Miocene, Langhian age. The genus Miogypsina, which shows its last occurrence at the top of the lower Miocene Burdigalian (Berggren et al., 1995;Ouda, 1998) has not been recorded.
Characteristics. The identified foraminifera from this zone are mostly benthic and include the taxa Triloculina sp., Quinqueloculina sp., Marginulinopsis sp. and rare Textularia sp., in addition to larger foraminifera such as Amphistegina sp. and Operculina sp. The observed planktic foraminifera include Praeorbulina sicana, P. transitoria and Orbulina universa (Fig. 4).
Occurrence. Lower part of the yellow marly/shelly dolomite; part of Marmarica Formation, lower part of Unit IV.
Age and correlation. The present zone can be equated with both the Praeorbulina sicana and P. glomerosa Zones of Ouda (1998) and Subzone M5a & b of Berggren et al. (1995) of Middle Miocene, early Langhian age.

Borelis melo melo Zone
Definition. Biostratigraphic total range zone of the marker species. The lower limit of this zone coincides with the FAD of Orbulina suturalis and Globorotalia fohsi fohsi.

Globigerinoides obliquus extremus Zone
Definition. Stratigraphic interval of the zonal marker from the LAD of the Middle Miocene marker species to the LAD of the nominated species.
Author. Cita (1973). Occurrence. Geosoil horizons (fine-grained clayey sediments) at the base of the second limestone ridge; lower part of Alexandria Formation.

Characteristics
Age and correlation. Late Pliocene according to Sprovieri (1993). Along the northern Western Desert, the Early Pliocene is characterized by open marine deposits and is restricted to the Gulf of Salum in the northwestern corner and exhibit  Ouda (1998) and Berggren et al. (1995).

Explanation of Plate 1
Each taxon is followed by the well number and depth of sample. Magnification is represented by a bar scale under each figure. Fig. 1

Explanation of Plate 3
Each taxon is followed by the well number and depth of sample. Magnification is represented by a bar scale under each figure. Fig. 1. Globigerinoides trilobus trilobus (Reuss) (D143, 66 m). Fig. 2. Wall ultrastructure of figure 1 showing its dolomitic composition of euhedral dolomite rhombs.  Fig. 6. Wall ultrastructure of figure 5 showing the dolomitization of the test.

Explanation of Plate 4
Each taxon is followed by the well number and depth of sample. Magnification is represented by a bar scale under each figure. Fig. 1. Globorotalia fohsi lobata Bermudez (D143, 66 m). Fig. 2. Wall structure of figure 1 showing its dolomitic composition of unhedral to subhedral dolomite rhombs. deep-water facies. To the east, the Upper Pliocene deposits rest unconformably on the Middle Miocene limestone, with variable thickness and exhibit a shallow-marine facies. The Late Miocene was a period of regional emergence and a marked drop in sea-level, and active subaerial erosion in the entire northern Western Desert based on palaeontological criteria, as documented by Ouda & Masoud (1993) and Ouda & Obaidallah (1995). Following Ouda (1998), the occurrence of Globigerinoides obliquus extremus and Globorotalia inflata in the present sediments indicate a Late Pliocene age. Also, the finegrained clay-rich carbonate sediments comprise a shallow-water, benthic foraminiferal assemblage as Elphidium, Ammonia, Cibicides, Discorbis, as well as various miliolids which are observed in different localities in the northern Western Desert (Hammad et al., 1976;Boukhary et al., 1976) and ranging in age from the latest Pliocene to the Pleistocene. Consequently, it is suggested that this fine-grained clay-rich carbonate bed can be assigned to the Late Pliocene (Fig. 5).

Un-named Zone
Definition. Stratigraphic belt of rock that is rich in long-ranging benthic foraminifera but barren of any planktic taxa.
Characteristics. This zone is characterized by the same benthic foraminiferal assemblage as that found in the underlying geosoil horizons.
Occurrence. Second limestone ridge; upper part of Alexandria Formation or the carbonate aeolianites.

Age and correlation.
For the carbonate aeolianites, no index planktic foraminifera were found. However, from their stratigraphic position, they can be considered to be Pleistocene in age. Ouda (1998) also recorded this shallow-water benthic assemblage of an inner shelf-type from the Mersa Matruh and Burg El Arab-1 wells, confined to the oolitic limestone which caps the open-marine Pliocene clays and ranging in age from the late Pliocene to the Pleistocene. Accordingly, no planktic foraminifera referable to the Globorotalia inflata Zone (latest Pliocene) have been encountered anywhere in the onshore sediments of the Medditerranean coastal plain, and the sediments younger than the Globigerinoides obliquus extremus Zone are dominantly shallow-water facies.

Marmarica Formation (fossiliferous carbonate rocks)
The Miocene is represented by the Marmarica limestone plateau of Middle Miocene age, which is composed of shallow-marine, fossiliferous, partly chalky carbonate intercalated with a few marl and shale horizons.
Generally, the grey-white oncolitic/shelly limestone rock unit (Unit V) is moderately rich in benthic foraminifera. The upper more clayey 'oncolitic beds' contain more foraminifera than the underlying shelly carbonate beds. The identified foraminiferal taxa mostly belong to the rotaliid group. No planktic foraminifera were observed (Planktic/Benthic ratio=c. 0).
The yellow marly/shelly dolostone rock unit (Unit IV) is slightly fossiliferous with rare specimens of foraminifera. The recognized foraminifera through the rocks of this unit are the smaller benthic types, in addition to larger foraminifera such as Borelis melo melo, Amphistegina sp., Operculina sp. Planktic foraminifera are rarely observed (P/B ratio=c. 0.01) (Figs 4, 5).
The distribution of the foraminifera varies through the white chalky/shelly dolostone beds (Unit III) where the coarse-grained rocks are richer in foraminifera than the fine-grained ones. Generally, the foraminiferal assemblage in this unit resembles that of the overlying bed with the same foraminiferal groups but with low frequency. Rare planktic foraminifera were observed (P/B ratio=c. 0.01) including Globigerinoides trilobus trilobus, G. trilobus immaturus, G. ruber and Orbulina suturalis.
The grey shelly dolostone bed (Unit II) contains a reasonable number of foraminifera. Benthic specimens are dominant, with very few planktic individuals (P/B ratio=c. 0.01). The identified benthic foraminiferal assemblage belongs to the following groups: miliolids (42%), alveolinids (30%), rotaliids (20%) and lagenids (7%). This bed contains an assemblage of benthic foraminifera slightly different from that of the overlying oolitic limestones and geosoil (Unit I) (Fig. 4). It is characterized by the presence of Anomalina, Marginulinopsis and Borelis, which are not found in the aeolianite sediments, in addition to the presence of large numbers of Triloculina spp. Planktic foraminifera are represented by only a few specimens of Globigerinoides trilobus trilobus, Globorotalia fohsi robusta, Orbulina suturalis, Orbulina sp. and Sphaeroidinellopsis disjuncta.
It can be noted that common benthic foraminiferal specimens, with rare planktic ones, characterize the fossiliferous carbonate rocks of the Marmarica Formation. The planktic/ benthic ratio in this rock unit is very low (0-0.01), indicative of a shallow-water, inner shelf environment (Murray, 1991). The distribution of the foraminiferal fauna is varied throughout the different units. It increases in the upper grey bed and decreases gradually downward in the white and yellow rock units. The most abundant fossils observed in these units belong to the genera Triloculina, Quinqueloculina, Marginulinopsis, Anomalina and Borelis, with rare Nonion, Nonionella, Peneroplis, Operculina and Amphistegina. This foraminiferal assemblage is characterized by the presence of some larger foraminifera, which inhabited warm shallow-water environments in the proximity of reefs (Murray, 1991). According to Haynes (1981) and Murray (1991), the above foraminiferal assemblage indicates clear carbonate environments, characterized by a surface water temperature of 25(C, subject to some current activity, with salinity ranging from normal to slightly hypersaline (35-50‰), and coral and algal substrates in inner-shelf or lagoons (0-20 m palaeodepth). The decreasing trend in the number of tests downward could be due to the prevailing strong currents which break down most of the shells, as in the white chalky beds, or due to the effect of clastic materials, as in the yellow marly beds.
In the lower oncolitic beds, an increase in foraminiferal tests was observed and the identified fossils belong to the genera Elphidium, Cibicides, Ammonia, Nonion, Eponides, in addition to the presence of some large foraminifera as Heterostegina, Operculina and Amphistegina (i.e. high percentages of rotalids and rarity of miliolids) indicating a slightly deeper-water middle neritic environment (El-Deeb, 1995). It can be concluded that the sediments of the oncolitic/shelly rocks were deposited in slightly deeper water than the overlying fossiliferous rocks and in less agitated conditions.

Alexandria Formation (carbonate aeolianite sediments)
Pliocene deposits are distributed along the coastal plain of the Western Desert and mostly represented by soft clays and soft non-porous, fine-grained or partly oolitic limestone sometimes with a basal conglomeratic band. The Quaternary deposits are represented by the calcareous limestone ridges parallel to the shoreline, the lagoonal, sabkha, beach and coastal dune deposits (Shata, 1955;Shukri et al., 1956;Said, 1962;El Shami et al., 1969;Ouda, 1998).
A good and well-preserved assemblage of benthic foraminifera was detected from the sediments of the second limestone ridge, including the oolitic limestones and the intercalated geosoil horizons. Some of these foraminiferal genera are present in large numbers while others are very rare and may be represented by only one specimen. The frequency of the most common benthic genera in this rock unit is shown in Figure 6.
The more clay-rich limestone and some of the geosoils are characterized by higher numbers of benthic foraminifera than in the white oolitic rocks, but the two rock units have almost the same assemblage. The more clay-rich rocks are characterized by the presence of Discorbis sp., which is not found in the white oolitic rocks. On the other hand, Sorites sp. is more common in the white oolitic rocks than in the geosoil and the clay-rich sediments (Fig. 6).
Very rare planktic foraminifera were observed in the fine-grained clay-rich sediments at the base of the second ridge (P/B ratio=c. 0.02). The identified planktic foraminifera includes Globigerinoides trilobus trilobus, G. trilobus immaturus, G. obliquus extremus, G. obliquus obliquus, Globorotalia inflata and Orbulina universa. These species are not recorded from the white oolitic limestone.
The above foraminiferal assemblage of the limestone ridge indicates the predominance of benthic foraminifera and the rarity of planktic ones. The presence of planktic foraminifera in low diversity indicates shallow, open-marine environments (Murray, 1973(Murray, , 1991. Also, the dominance of the families Miliolidae and Elphididae indicates shallow, warm marine water (Omara & Ouda, 1968) and a relatively quiet depositional environment (Cherif et al., 1975). According to Murray (1973), Cibicides lives on sea grasses in relatively quiet conditions. Hageman (1979) mentioned that Quinqueloculina seminula might flourish in open shallow-marine conditions, with a vegetated substrate. The species is slightly tolerant to reduced salinities, a water depth of 25-35 m, and an inner to middle neritic setting. Murray (1991) indicates that Elphidium and Quinqueloculina seminula are commonly dominant in shallow water of normal salinity environments in middle latitudes worldwide. According to Sherif (1991), Elphidium crispum inhabits open normal marine inner neritic to middle neritic (0-90 m) environments. The presence of Ammonia beccarii in high numbers indicates ordinary warm shallow-marine water (Omara & Ouda, 1968), in normal salinity inner to middle neritic environments (Hageman, 1979). According to Cherif et al. (1988), Ammonia beccarii seems generally to increase in coasts affected by influxes of freshwater coming from a river system or from a hinterland with some reasonable rainfall. This species favours lagoonal environments with salinity <10‰, 15-20(C and <10 m depth (Murray, 1991). El Deeb (1995) suggested that the presence of Quinqueloculina, Triloculina and Spiroloculina is indicative of littoral to shallowmarine conditions and an increase in their numbers indicates normal marine lagoons and carbonate platforms.
It can be concluded that the foraminiferal associations in the studied limestone ridge indicate a warm, clear, relatively agitated, shallow-marine depositional environment in a vegetated inner neritic platform or lagoon (10-25 m in depth). According to Murray (1973), the presence of Sorites is confined to shallow agitated intertidal to subtidal, tropical to subtropical marine conditions with 25(C surface water temperatures. The relative increase of Sorites in the white oolitic sediments indicates that the deposition of these sediments was under warmer water conditions during marine transgression. Moreover, the presence of Discorbis in the clay-rich oolitic limestones and some geosoils suggests less agitated, intertidal water conditions (El Nahass & El Fayumy, 1986). The increase in the clastic content and oolites in these sediments confirm that they were deposited near shoreline.

CONCLUSIONS
Forty-six benthic and 14 planktic foraminiferal species have been identified from sediments of the Marmarica (Middle Miocene) and Alexandria (late Pliocene to Pleistocene) Formations sub-cropping in the El-Dabaa area, northern part of the Egyptian Western Desert. Larger benthic foraminifera can be used as zonal markers in intervals which lack planktic taxa.
Five foraminiferal biozones are established: Heterostegina costata and Praeorbulina sicana zones (early Langhian), Borelis melo melo Zone (late Langhian-Serravallian), Globigerinoides obliquus extremus Zone (late Pliocene), and an un-named zone (late Pliocene-Pleistocene). The late Miocene and early-middle Pliocene was a period of regional emergence and active subaerial erosion in the entire northern Western Desert as indicated by palaeontological criteria.
In general, the foraminiferal assemblage recovered from the Marmarica Formation indicates warm, clear, shallow carbonate environments (0-20 m palaeodepth), about 25(C surface water temperature, with some current and salinity ranging from normal to slightly hypersaline (35-50‰). The Alexandria Formation was deposited in a warm, clear, relatively agitated, shallow-marine depositional environment. The presence of Discorbis and rare planktic species in the basal part of the Alexandria Formation suggests less agitated conditions.