Cenomanian—Turonian ostracods from Gebel Nezzazat, southwestern Sinai, Egypt, with observations on ∂13C values and the Cenomanian/Turonian boundary

96 surface samples from the Cenomanian—Turonian succession of Gebel Nezzazat, southwestern Sinai, Egypt were examined for ostracods. 45 species and varieties have been recognised with one new species ?Pterygocythere bisulcata sp.nov. and four left in open nomenclature. Most species have been recorded in rocks of the same age in the Middle East and North Africa, and some from West Africa, Europe and South America suggesting biogeographic relationships between these regions. Three local ostracod zonal assemblages are established, two in the Cenomanian and one in the Turonian. The ostracods and associated foraminifera and megafossils, suggest a shallow marine environment, sometimes with restricted marine water and in part brackish. The Oceanic Anoxic Event of the Late Cenomanian is recognised on the evidence of ∂13C values; ostracod diversity has a negative relationship to ∂13C values.


INTRODUCTION
This study deals with the systematic examination of Cenomanian-Turonian ostracods from Gebel Nezzazat, southwestern Sinai (Fig. 1). The Cenomanian-Turonian marine exposure is represented herein by three lithostratigraphic units:

Raha Formation (Early Cenomanian)
This is divided into the Mellaha sand Member, sandstone with some shale and limestone intercalation (35m), and the Abu Had Member, shale, marl and limestone intercalation (32m) overlying the Malha Formation of Early Cretaceous age.
The age determination of these formations is based on the foraminifera1 and megafossil content (Shahin 1988). The present study is carried out to complete the picture of the Cenomanian-Turonian ostracods and to shed some light on their habitat and palaeogeographic distribution.
Few studies have been carried out on the Cenomanian-Turonian ostracods of Egypt. Bold (1964) described 33 species from the Cenomanianxampanian of the Abu Roash area. Colin & El Dakkak (1975) studied the Cenomanian ostracods of Gebel Nezzazat and recorded only 5 species. Boukhary et al. (1977) recorded 10 species from the Cenomanian of northern Galala, Eastern Desert.
The systematics and terminology follow that of the Treatise (Moore, 1961). The abbreviations, L, H, W in the descriptions refer to length, height and width (in p) respectively. All material is deposited in the Department of Geology, Faculty of Science, Mansoura University, Egypt under the catalogue number SHN.   France (Babinot, 1970(Babinot, ,1980, the Turonian of Spain (Reyment, 1984) and Israel . Bairdia sp.C Bold (1964) from the Turonian of Egypt is very similar to this species and could be considered synonymous.

Derivation of name.
From the sulcus present on each valve, just posterior to the eye spot.
Holotype. -SHN 117, Paratypes-SHNl18. Remarks. Asciocythere polita was described from the Turonian of France by Damotte (1962) and recorded from the Cenomanian-Turonian of central Spain (Breman, 1976) and the Turonian of France (Babinot, 1980). This species is similar to that of Damotte but differs in having a concave ventral margin and less pointed posterior end. It is also similar to Ovocytheridea reniformis Bold in outline and smooth surface. Bold (1964) stated that some species of the genus Asciocythere could belong to Ovocytheridea. The two genera differ in the structure of the median hinge element.  (Bassoullet and Damotte, 1969) and from the Cenomanian of Israel (Rosenfeld & Raab, 1974). Damotte,197 1 (P12, fig. 15) 1971 Dolocytheridea crassa Damotte: 4 Remarks. This species was described from the Cenomanianof France (Damotte, 1971 andBabinot, 1980) and the Early Turonian of Spain (Breman, 1976).

Veeniucythereis rnaghrebensis
Veeniacythereis jezzineensis closely resembles V . rnughrebensis except for the extent of development of the main ridges on the surface of the carapace (see the description of Bischoff, 1963 andBassoullet &Damotte, 1969 (Honigstein, 1984).

figs 1-5
Cythereis creturiu acutu Honigstein, 1984 (Pl. 4, fig. 2 Honigstein (1984) distinguished this subspecies from Cythereis creturiu creturiu Bold by its acute middle part of the posterior end. He recorded this species from the Santonian of Israel. The specimen described herein is similar to Honigstein's except for the nearly smooth posterolateral area. Bischoff, 1963 (PI.  Bischoff (1963) described this species from the Albian of Lebanon. Bismuth et al. (1981) and Athersuch (1988) recorded C. cf. fahrioni from the Albian-Xenomanian of Tunisia and Albian of Oman respectively, which are here considered to be true members of the species.

BIOSTRATIGRAPHY
The stratigraphic ranges of the species described here are shown in Fig. 2.
The Cenomanian-Turonian succession of Gebel Nezzazat has already been subdivided into biostratigraphic zones on the basis of foraminiferal and macrofossil content (Shahin, 1988). Three ostracod assemblages can be distinguished from oldest to youngest as follows:

Cytherella-Veeniacythereis-Metacytheropteron Assemblage
The lower part of this unit is characterized by the dominance of Cytherella spp., Veeniacythereis spp., Herrigocythere cf. donzei and Bairdia spp. This assemblage declines or even disappears in the middle part of the unit, reappearing towards the top. Brachycythere spp., Dolocytheridea atlasica and Paracypris acutocaudata occur sporadically within this unit, and extend upwards into the next unit.
Most of this association is present in the Cenomanian of many parts of the world. It is equivalent to the Rotalipora brotzeni and R. reicheli Zones, of Early Cenomanian age (Shahin, 1988).
The presence of Cytherella and Veeniacythereis indicates open marine, moderately deepneritic conditions (Morkhoven, 1963), which are confirmed by the presence of Rotalipora and Thomasinella. Dolocytheridea spp. and Ovocytheridea spp. together with a few Xestoleberis derorimensis occur sporadically within this unit, perhaps suggesting littoral or even brackish water conditions at some horizons.

Cythereis spp. Assemblage
Most of the characteristic species of the previous assemblage disappear, although some of them (Paracypris acutocaudataRosenfeld, Bairdia spp., Dolocytheridea atlasica Bassoullet & Damotte and some Cytherella) extend within this unit. It is characterized by the dominance of Cythereis spp. and other trachyleberidids. The interval between samples 34 and 42 is devoid of ostracods, probably due to an anoxic event. The genera found below this barren interzone reappear above it where other important genera make their first appearance (see the range chart, Fig. 2).
This association is partly equivalent to the Rotalipora cushmani Zone and the lower part of the Heterohelix globulosa Zone of Late Cenomanian and earliest Turonian age respectively (Shahin, 1988).
Most of the aforementioned species abruptly disappear a few metres above the Cenomanian-Turonian boundary. However, some of them are long ranging and reappear in the Turonian. This disappearance is probably due to another anoxic event associated with the Cenomanian-Turonian boundary.
The abundance of Cythereis and related genera and Paracypris indicates open, relatively deep neritic condidions (cf. Morkhoven, 1963). However the presence of Ovocytheridea spp. again refers to shallow to restricted marine environments at least where it is present. These alternating marine conditions were also deduced from the foraminifera1 and megafossil content (Shahin, 1988).

Asciocythere polita-Neocyprideis vandenboldi Assemblage
Following the barren interzone, some species start to appear and extend upwards while others, which first appeared in the Cenomanian, are also reported (Fig. 2). This interval is equivalent tothe Coilopoceras sp. Assemblage Zone of the Late Turonian (Shahin, 1988). The sporadic occurrence of the brackish water Neocyprideis, Asciocythere and Xestoleberis together with a few deeper neritic genera Cythereis, Cytherella, Bairdia and Bairdoppilata indicate alternation of neritic, open marine and brackish environments. The top of this zone suggests brackish water conditions.

PALEOGEOGRAPHICAL DISTRIBUTION
During the Cenomanian a similar ostracod assemblage characterized a vast palaeogeographical province. This assemblage includes Metacytheropteron berbericus, Veeniacythereis jessineensis, V. maghrebensis and Cythereis namousensis. These are recorded from the Cenomanian of Libya, Tunisia, Algeria, Morocco, Israel, Lebanon, Iraq, Kuwait, Oman and Iran. Other elements of the fauna are present in France and Spain to the north, and Senegal and Tanzania to the south. Cytherella gr. ovata, Cytherella sulcata and Cytherellaparallela are recorded from the Cenomanian of France, Spain, England, North Africa and Israel.
The Turonian assemblage, including Brachyc ythere sapaucariensis and similar forms, is known from the Early Turonian of Brazil, Tanzania, Gabon, Nigeria and Tunisia. Asciocythere polita, Ovocytheridea reniformis and Bairdia cenomanica are recorded from the Turonian of France, Spain, Tunisia and Israel.
The Cenomanian-Turonian ostracods recorded here link the area with Europe, North and West Africa, the southern shelf of Tethys, South America and the Middle East. At that time Tethys covered much of the aforementioned regions where shallow to slightly deep marine sediments were deposited including similar ostracod assemblages. This is due mainly to the similarity of environmental conditions, and the ability of the fauna to be distributed throughout the Tethyan province. This also supports the idea of a trans-Saharan seaway and direct connection betweeen South America and West Africa (Furon, 1935 andChancellor, 1982 repecitively).

OSTRACOD DIVERSITY AND OXYGENATION LEVELS
Oxygen supply is an important ecological factor, and any reduction would lead to a reduction in the number of individuals. It is tentatively suggested therefore that a reduced ostracod fauna is the result of very low oxygen levels in the bottom water. When the ostracod diversity is plotted stratigraphically (Fig. 3), it is clear that there are repeated declines in the diversity of both platycopids and podocopids within the succesion.
A fluctuation of ostracod diversity coinciding with a13C values of normal marine salinity is clearly seen within the lower part of the Cenomanian. This change in diversityeis partly explained by the sedimentological variation in the Raha Formation from shale to sand to limestone bands. The increased diversity may reflect a higher proportion of sand-sized bioclasts in the sediment.
The peak of aI3C, accompanied by a drop in the diversity of ostracods, is clearly observed at the base of the Abu Qada Formation (Fig. 3). This major increase in aI3C values refers in part to high productivity caused by incoming nutrients associated with the Cenomanian transgression. This increased surface productivity leads to the development of amarked oxygen-minimum zone in the underlying water column because oxygen is utilized during the breakdown of organic matter as it sinks downwards from the surface water (Jenkyns, 1980 andSummerhayes, 1987). Hume et a [. (1920)  to the occurrence of some forms of hydrocarbon in a highly weathered state. Fischer & Arthur (1977) stated that in the case of a high rate of sedimentation and preservation of organic carbon in marine sediments, the rate of 8'C values increases in the oceanic reservoir and consequently in the biogenic calcite. Therefore, the high dI3C values in the Abu Qada Formation may be due to high productivity and the presence of organic carbon. Therefore, the accompanying drop in ostracod diversity below the Cenomanian-Turonian boundary is readily explained by bottom water oxygen level falling below the minimum respiratory requirements of individual species (Oceanic Anoxic Event), causing the disappearance or even extinction of many typical Cenomanian species (cf. Weaver, 198 1). The complete disappearance of the Cenomanian foraminifera1 forms Thornasinella and Rotaliporu in the earliest Turonian of G. Nezzazat is the most obvious feature around the Cenomanian-Turonian boundary (Shahin, 1988). Hart (1980) recorded that anoxic events, for example that of the Late Cenomanian-Early Turonian, coincide with levels of maximum faunal change. The unfavourable palaeoceanographic conditions seem to be the main reason for the observed fauna changes around this boundary. However, an increase of ostracod diversity in this anoxic interval, is also noticed a few metres below the boundary, and may be due to the tolerance of some platycopid and podocopid species to low oxygen levels.
The 8'C values again decrease just above the Cenomanian-Turonian boundary that lies within a region of already decreasing dIiC values. This decrease is accompanied by a continued drop in ostracod diversity (the podocopids are the only ostracods present). This is followed by a slight increase in the podocopids a few metres above the boundary, accompanied by an obvious increase in the planktonic foraminifera (Heterohelixglohulosa Abundance Zone) (Shahin, 1988). Arthur et al. (1987) stated that the earliest Turonian was a period of peak transgression, caused by a worldwide high stand of sea level. The large increase in the shelf sea areas caused by the transgression led to enhanced production of warm saline water, which sank to form bottom-water masses. This caused an increase in the rates of oceanic turnover because in the Cretaceous, oceanic circulation was salinity driven (Brass et al. 1982), and this triggered the anoxic event. This peak transgression led to the annihilation of most ostracod species (the barren interzone). Some of the ostracod species found in the Cenomanian reappear in the Late Turonian and other new species appear. It is clear that most of the ostracods are podocopids, corresponding to normal d13C values of normal marine salinity, accompanied only by very low numbers of small size benthonic forams. The increase of ostracod diversity in sample 90 partly reflects a high proportion of sand-sized bioclasts.
The presence of some hydrocarbons, the high positive d13C values, the sudden faunal change and the drop in ostracod diversity at the Cenomanian-Turonian boundary refer to an oceanic anoxic event at that time. This event may have caused a sequence of extinctions during the Late Cenomanian and the Early Turonian.