During the last 10 years, the Late Cretaceous timescale has been improved by the integration of floating Astronomical Time Scales (ATS), higher resolution biostratigraphic frameworks and highresolution carbon-isotope stratigraphy dates [1-5] of the Coniacian/ Santonian and Santonian/Campanian Stage boundaries. The Coniacian/ Santonian boundary has now been formalized, with a Global Boundary Stratotype Section and Point (GSSP) at Olazaguta, northern Spain [6] the defining marker is the lowest occurrence of the inoceramid bivalve Cladoceramus undulatoplicatus (Römer), which occurs widely in the northern hemisphere. However, detailed correlation with deepwater Tethyan successions (which generally lack inoceramid bivalves), where the base of the Santonian is traditionally taken at the lowest occurrence of the planktonic foraminifera Dicarinella asymetrica [7], is not currently possible. There is no formal agreement on either a marker or a type-locality for the Santonian/Campanian boundary GSSP, for which a series of potential markers have been discussed [8], including the Highest Occurrence (HO) of the crinoid Marsupites, the Lowest Occurrence (LO) of the planktonic foraminifera Globotruncanita elevata (Brotzen), and the LO of Dicarinella asymetrica.

Here, we present a correlation of new high-resolution planktonic datasets through the Coniacian and Upper Campanian of Gabal Duwi (Figure 1) and provide the first attempt to calibrate the new Regional sea level curve of Egypt [9] and Global Eustatic curves [10] to the Duwi Basin. This study brings new insights into the presence of condensed levels and potential hiatuses in existing Egyptian records and provides evidence of the position of the Santonian/Campanian boundary in the standard Tethyan biostratigraphic record of Gabal Duwi.

Figure 1: Geological map of Gabal Duwi range.

The Duwi Formation has not been formally defined, through various proposals have been made. For example, this formation has been assigned to Late Campanian age [11,12]. Meanwhile, it assigned to Maastrichtian age by Hegab [13], later on, assigned to Late Campanian- Maastrichtian as suggested by Schrank and Perch-Nielsen [14] Baiumy and Tada [15]. But up till now, there is no agreement on the age assignments of the fine subdivision of the Campanian rock units in the region.

In order to solve the problem of the age of the Duwi Formation, the planktonic foraminiferal assemblages have been analyzed and identified to allow us to recognize the faunal turnover, biostratigraphy and sea level changes during the Duwi Formation deposit.

The Coniacian-Santonian time interval is the inferred time of oceanic anoxic event 3 (OAE 3), the last of the Cretaceous OAEs. A detailed look on the temporal and spatial distribution of organic-rich deposits attributed to OAE 3 suggests that black shale occurrences are restricted to the equatorial to mid-latitudinal Atlantic and adjacent basins, shelves and epicontinental seas like parts of the Caribbean, the Maracaibo Basin and the Western Interior Basin, and are largely absent in the Tethys, the North Atlantic, the southern South Atlantic, and the Pacific. Here, oxic bottom waters prevailed as indicated by the widespread occurrence of red deep-marine CORBs (Cretaceous Oceanic Red Beds). Widespread CORB sedimentation started during the Turonian after Oceanic Anoxic Event 2 (OAE 2) except in the Atlantic realm where organic-rich strata continue up to the Santonian. The temporal distribution of black shales attributed to OAE 3 indicates that organic-rich strata do not define a single and distinct short-time event, but are distributed over a longer time span and occur in different basins during different times. This suggests intermittent and regional anoxic conditions from the Coniacian to the Santonian [16,17].

Palaeontological and geochemical evidence increasingly indicates that the Coniacian–Santonian interval represents the transition from the mid-Cretaceous extreme greenhouse characterized by elevated temperatures, increased volcanic activity, high sea-level and regional to global ocean anoxic events, to more temperate climatic conditions in the Campanian-Maastrichtian [18-20]. Coincident with this climatic transition, planktonic foraminiferal assemblages underwent major compositional changes due largely to a pulse of diversification.

Causes of the Santonian turnover, however, are still a matter of debate. Hypotheses proposed to suggest that it could be related to the following:

(i) tectonically forced changes in surface-water and deep-water circulation [7]; (ii) the onset of the Late Cretaceous cooling trend during the late Santonian Patrizzo, [21] combined with taxa competition within particular depth habitats [22]; and (iii) the development of minor and regional anoxic events (i.e. Oceanic Anoxic Event 3; e.g. [23-25] in the Atlantic and adjacent epicontinental sea, which resulted in the enlargement of the ecological niches in the more oxygenated Tethys, Pacific and Indian oceans [16].

Long-term cooling during the remainder of the Late Cretaceous Friedrich et al. [17]; Linnert et al. [20], where this cooling trend accelerated during the beginning of the Campanian [17,20], but its mechanisms and dynamics are not yet well understood.

The Campanian is also characterized by significant fluctuations of the sea level [10,26,27], a major shift in the δ15 N of marine organic matter [28], clay mineralogical changes, and the occurrence of positive and negative carbon isotope events: the Santonian/Campanian Boundary Event (SCBE) [1,27], the Mid Campanian Event (MCE) [1,3,27], the Conica Event [29], the Late Campanian Event (LCE) [1,3,4,5,27], the Epsilon Event (EE) (also called C1-Event) [3,30], and the Campanian/Maastrichtian Boundary Event (CMBE) [3,5,30].

The aim of the present study is to present an illustrated record for the planktonic foraminifera of the study area which has never been attempted before and to discuss the chronostratigraphy of the foraminiferal interval of the investigated locality in other Egyptian occurrences in the light of the application of up to date biostratigraphical standards. The study of the biostratigraphy of Campanian planktonic foraminifera in Egypt started in connection to the solution of the problem of tracing exactly the Santonian/Campanian boundary in the region.

The northwestern Red Sea-Gulf of Suez rift consists of four distinct sub-basins as half graben separated by complex accommodation zones [31]. Each sub–basin is asymmetric and bounded on one side by a major NW trending border fault system, Duwi sub-basin is the largest one (Figure 1).

The pre-rift sediments of the northwestern margin of the Red Sea are filled these hanging wall synclines by Paleozoic, Cretaceous, Paleogene sediments [32,33]. The basement complex is unconformably overlain by the 500-700 m thick of Pri-Rift sediments. The lower part of these sediments is of siliciclastic Nubian facies, overlain by the 350- 570 m thick of interbedded shales, sandstones and limestones of the Quseir, Duwi, Dakhla, Tarawan, Esna and Thebes formations [34,35]. Duwi area can be considered as tilted faulted blocks of the Red Sea tectonics dissected by major and minor faults (Figure 1). A major phosphogenic episode took place during Late Cretaceous of shallow to the epicontinental shelf of the southern Neo-Tethys Ocean [36]. The common association of phosphatic strata with chert and organic-rich sediments (black shales in Duwi) in both the Middle East and Egypt has been interpreted as an indication that phosphorite accumulation association with highly productive surface waters possibly caused by upwelling currents [37].

The Duwi (Phosphate) Formation (Figure 2) was laid down in extremely shallow Epicontinental-Neritic seas which flanked the southern margin of the Tethys sea in Egypt that extended across the northern margin of the Arabo-Nubian Craton and deeper towards the Tethyan seaway to the north.

Figure 2: Lithostratigraphic subdivision of the Late Cretaceous. Duwi Formation, Red Sea, Egypt.

The present study has focused on the 77 m thick of the total interval of the Duwi Formation at Gabal Duwi Range. This succession is well exposed at the type section which is easily accessible with its contact with Lower Quseir Formation and the Upper Dakhla Formation.

The entire interval includes the transition from the Coniacian stage to the Campanian stage of the Duwi Formation. Each marl, shale, silt and soft bioclastic carbonate beds are sampled for foraminiferal analyses, while the very hard rocks as bioclastic and silicified limestones, chert and phosphatic beds are not sampled. Throughout the Duwi Formation interval, a total of 80 samples are collected from Pre- Coniacian-Late Campanian. The sampling system is doomed to rock nature where the general hardness nature of the rocks throughout all intervals exacerbated the problem.

The samples were soaked in H₂O₂ and water with some heating for several days and then washed through a 63 μm sieve with tap water. Planktonic foraminiferal species were picked from the residues, identified, mounted on covered micro slides for a permanent record.

The present work (77 m thick) yielded more than 70 species belonging to 27 genera of planktonic foraminifera suitable for the establishment of chronozones for worldwide correlation. The identified species are listed in Figure 3. Only the very important, as well as the index species, are scanned and illustrated (Figures 4-6). The material has been deposited in the Museum of the Geology Department, Menoufia University.

Figure 3: Coniacian-late Campanian planktonic foraminiferal species distribution chart, upper part of the Duwi Formation, Gabal Duwi, Egypt. Specimens in bold are index fossils for the identical biozones.

Figure 4: SEM Images for foraminiferal species; 1a, 1b: Concavatotruncana concavata; 2a-2d: Dicarinella asymetrica; 3a, 3b: Globotruncanita elevata; 4a, 4b: Globotruncana ventricosa; 5a, 5b: Contusotruncana plummearae; 6: Radotruncana calcarata; 7a-7c: Globotruncana aegyptiaca.

Figure 5: SEM Images for foraminiferal species; 1: Dicarinella primitive; 2: D. canaliculata; 3: Marginotruncana paraconcavata; 4: Marginotruncana caronae; 5: Marginotruncana sinuosa; 6: Marginotruncana renzi; 7: Globotruncanita stuarti; 8: Praeglobotruncana kalaati; 9: Globotruncanita stuartiformis; 10: Globotruncanita insignis; 11: Gansserina cf. repanda; 12: Contosutruncana morozova; 13: Contosutruncana fornicate; 14a,14b: Globotruncana bulloides; 15: Globotruncana arca.

Figure 6: SEM Images for foraminiferal species; 1: Globotruncana rosetta; 2a, 2b: Globotruncana duwi; 3: Archaeglobigerina cretacea; 4a, 4b: Globotruncana neotricarinata; 5: Ventilabrella eggeri; 6: Ventilabrella austinana; 7: Neoflabellina suturalis. 8: Costallegrina bulbosa; 9: Pseudoguemblina cotulatal; 10: Planoheterohelix moremani; 11: Planogloulina carseyae; 12a, 12b: Rugoglobigerina rugosa; 13a, 13b: Rugoglobigerina macrocephala.

Stratigraphy

In the Quseir-Safaga District, the Cretaceous–Eocene deposits occupy the troughs of synclinal–like folds within the crystalline basement hill ranges. The Cretaceous sections especially Duwi Formation are described by many authors focusing on the phosphate formations and black shales [33-35,37,38].

At its type locality at Quseir, Youssef [34] give to the Duwi Formation a Campanian age. In the south of the Western Desert, Luger [39], Farouk [40] consider that the Duwi Formation is of Late Campanian to early Maastrichtian age. Stratigraphically, the Duwi Formation is underlain by the Quseir Formation (Quseir Variegated Shale) and overlain by the Dakhla Formation.

The rocks of the Duwi Formation overlie marginal marine to shallow marine shales of the Quseir Formation which overlie the Nubian Formation [33] and underlie deeper-marine marls and carbonates of the Maastrichtian Dakhla Formation [33]. Duwi Formation is considered the first of fully marine transgression of the Egyptian Late Cretaceous.

The lithostratigraphic subdivision of Gabal Duwi emerged after long series of investigations which started from the beginning of the century [33,34,37,41].

The following litho-stratigraphic subdivisions (Figure 2) for the Pre-Coniacian-Late Campanian in Gabal Duwi are proposed, from the base to the top as follow:

Lower phosphate unit (Pre-Coniacian): It attains 13.5 m thick named Abu Shigeila or Hamadat “C” horizon [33]. It is made of interbedded dark colored thinly bedded siliceous fine-grained claystones with scattered mega-visible phosphate peloids. It is underlain by Quseir Formation of Nubian facies which attains 11.5 m thick of dark varicolored shales with very thin two phosphate lamina.

Middle phosphate unit (Pre-Coniacian): It is about 32 m thick named Main Duwi Beds or “ B ” beds. The lower 8 m thick is made of unfossiliferous silicified limestone with phosphate beds and sandy phosphate. The upper 24 m is made of thick builds up of the oyster bank, cross-bedded bioclastic limestone covered phosphatic limestone, silicified limestone, marly, sandy shales, some chert bands, ochreous shale and black shale bands.

Upper phosphatic unit (Coniacian-Late Campanian): It is named locally Atchan phosphate or “A” Beds, its thickness 20 m of more marly limestone, marls and shale beds, the dolomitic oyster band near the top. It is capped by a conglomeratic phosphate bed (50 cm thick) marked the top of the Duwi Formation. The direct overlying shale beds are belonging to the Dakhla Formation.

In general, the Duwi Formation is mainly hard varieties of silicified phosphatic oyster beds builds up with chert bands and black shales, few and thin intervening throughout scattered marly, shale and marly limestone bands.

The Duwi Formation had a large number of vertebrate remains, coprolites, fish teeth and heteromorph ammonites [33]. Dominik and Schall [42] assigned these faunal assemblages to the Bostrychoceras polypolocum Zone.

Biostratigraphy

All of the samples collected from the informal member (A–Beds) of about 20 m thick of the Duwi Formation section forming several assemblages of moderate to well preserved taxa including small biserial and large multiserial heterohelicids, trochospiral keeled, unkeeled and very rare planispiral Globigerinelloides.

The lower beds (about 57 m thick) of the Duwi Formation are barren interval including (B-beds and C-Beds) (Figures 2 and 7a), (Oxygen, Anoxic Environment) including rhythmically bedded dark shales and black shales facies with phosphatic beds and lamina [15,37,41]. The upper part which overlying the B-beds had black shale and marl interval (7.7 m thick) (Figures 7d), which may coincide with the Coniacian–Santonian time interval OAE 3 of [23-25,43].

Figure 7: Comparison of chronostratigraphic assignment of the Coniacian–Late Campanian Tethyan biozones recorded in Tunisia, Europe–Mediterranean, Italy and present study.

The planktonic foraminiferal assemblages of studied upper 20 m thick of the Duwi Formation are diverse and from low to middle abundance (Figures 3-6). Several low latitude Tethyan zonal schemes for the Upper Cretaceous have been previously as [7,44-46]. The following 6 standard Tethyan biozones covering the sedimentary sequence of the upper 20 m thick of the interval (A-beds) are from older to younger as follows (The Upper Cretaceous (CF) numerical zonal scheme of Li and Keller [47,48] is used.

Dicarinella concavata zone (CF 14)

Definition: Interval zone from the FO of Dicarinella concavata (Brotzen) to the FO of Dicarinella asymetrica (Sigal).

Author: Sigal [49] as Globotruncana concavata Zone.

Age: Coniacian to early Santonian.

Thickness: 4.3 m equivalent to the lowermost marl beds of the A-Beds that overlies the barren interval of may propose Pre-Coniacian interval (Figures 3 and 8) of Coniacian–Santonian time interval of Oceanic Anoxic Event 3 (OAE 3).

Figure 8: Field photographs; (a) General photo showing some of stratigraphic boundaries of Gabal Duwi; (b) Field zonal boundaries, Contusotruncana plummearae, Radotruncana calcarata, Globotruncana aegyptiaca; (c) Field boundary between B-Beds and A-Beds of the Duwi Fm. and Zone; (d) Contact between last mined phosphatic bed of B-Beds and the direct overlying Coniacian-Santonian sequence of A-Beds; (e) Relation between Dicarinella asymetrica Zone and the overlying Globotruncanita elevata Zone.

Assemblage: Dicarinella concavata is not abundant in its individuals. Some individuals of Whiteinell baltica are present within this zone might probably of Turonian span. Species as Marginotruncana rensi, M. marginata, M. schneegansi, M. paraconcavata, Globigerinelloides paragottisi, G. esceri, G. cf. lobatus, G. bolli, G. asper, Muricohedbergella fandrini, Murico. planispira, Murico delrioensis, Heterohelix globulosa, Hx. papula, Hx. sephenoides, Pseudotextularia deformis, P. elegans, Dicarinella canliculata, Contusotruncana fornicate, Ventilabarella austiniana and others.

Remarks: According to the diverse of shell shapes as marginotruncanids, dicrinellids and concvatotruncanids groups evolved from Praeglobotrucana [50,51], cited concavata species within the genus Dicarinella where the species of Dicarinella have an umbilically convex shell with closely positioned keels near the dorsal side. The extinction of C. concavata within the nearly lowest appearance of Dicarinella asymertrica correlates with observations of Petrizzo [52] in southern Tanzania, South Spain, Petrizzo [53] in Exmouth Plateau, Indian Ocean, in the Tethyan Tunisia [54] and in southern Tibet [55].

Dicarinella asymetrica zone (CF 13)

Definition: Total range of Dicarinell asymetrica (Sigal).

Author: Postoma [56] from Tethys region as Globotruncana concavata carinata Zone.

Age: Santonian

Thickness: 3.4 m equivalent to marly limestone beds that overlie the Coniacian marls (Figure 7e).

Assemblage: The nominate taxon is consistently present but not abundant. Most taxa of the C. concavata Zone are present in D. asymetrica Zone except C. concavata or any form of Sigalia spp.

Still, the occurrence of Contusotruncana fornicate, Whiteinella baltica and W. bornholmensis and all Margiotruncana spp. The first appearance of Muricohedbergella silteri, Globotruncana bulloides, G. arca and Ventilabarella eggeri are marked this zone. The total range of Huberella hubris, laeviheterohelix pulchra and Planoheterohelix moremani, Heterohelix papula and Hx. sphenoides are extinct in Santonian.

Remarks: El Gammal [57], Obidalla and Kassab [58], Farouk and Faris [59] and many others reported the occurrence of the Santonian marker Dicarinella asymetrica of the Matulla Formation in Southern and Northern Sinai [9]. Aref and Ramadan [60] recorded D. asymetrica also in the Matulla Formation of Esh El Mallaha Range, Gulf of Suez.

Globotruncanita elevate zone (CF 12)

Definition: Partial range zone, recognized from the last occurrence of Dicarinella asymetrica and the first occurrence of Globotruncana ventricosa White.

Author: Dalbiez [61].

Age: Early Campanian

Thickness: About 4.5 m thick equivalent to marl interval overlie the Santonian marly limestone (Figure 8e).

Assemblage: It is characterized by abundance and more species diversification of both globotruncanids and heterohelicids. Taxa as Globotruncanita stuartiformis, Globotruncana bulloides, G. arca, G. neotricarinata, G. lapparenti, Contusotruncana fornicata and Marginotruncana marginata are very rare. It is marked also by the extinction of both Muricohedbergella flandrini and Dicarinella asymetrica.

Remarks: Some authors as [62,63] have equated the first appearance of Globotruncanita elevata with different levels of the Santonian Stage in North Africa and Mediterranean regions. Recent works show the base of the Campanian [64-66] and present work is marked by first appearance of Globotruncanita elevata taxon. In Egypt, many authors reported previously that Globotruncanita elevata is marked the base of the Campanian Stage in most localities [9,58,60,67,68,].

The Santonian/Campanian Boundary Event (SCBE) of Jarvis [1,27] is well recognized in both Dicarinella asymetrica Zone and Globotruncanita elevate Zone by the extinction of Heterohelix papula and Hx. sphenoides of Santonian and by the extinction of Muricohedbergella flandrini and Dicarinella asymetrica of Early Campanian.

Contusotruncana plummerae zone (CF 11)

Definition: The interval from the first occurrence of the Contusotruncana plummerae to the first occurrence of Radotruncana calcarata. Duration of about 3.2 Ma is inferred for this zone [69].

Author: Petrizzo [69] in tropical and subtropical areas.

Age: Middle Campanian

Thickness: About 3 m thick (Figures 7c and 7e).

Assemblage: C. plummerae, C. fornicata, Rugotruncana subcircumnodifer, Globotruncana ventricosa, G. arca, G. bulloides, Heterohelix striata, Liunella falklandica, Archaeoglobigerina blowi, A. cretacea, Rugoglobigerina rugosa and very rare individuals of Globotruncanita elevate.

Remarks: [7,44,46,54,69] proposed the Globotruncana ventricosa Zone as interval between Gta. elevata and first appearance of Radotruncana calcarata in Tethyan Campanian foraminiferal zonation. Petrizzo [69] discussed unreliability of Globotruncana ventricosa for several reasons and introduced Contusotruncana plummerae as first appearance as zonal marker for tropical and subtropical regions instead of G. ventricosa to first occurrence of Radotruncana calcarata. Besides that, the exact position of G. ventricosa stratigraphically is imprecise [53,70], the Conica Event [29]. Farouk and Faris [59] recorded G. ventricosa over the Globotruncanita elevata in the shallow marine carbonate platform in the Mitla Pass, Northern Sinai.

Radotruncana calcarata zone (CF 9 and CF 10)

Definition: Total range zone of Radotruncana calcarata taxon.

Author: Dalbiez [61].

Age: Early Late Campanian

Thickness: About 2.4 m thick (Figure 8c).

Assemblage: In general, the Late Campanian was identified as a time of planktonic foraminiferal turnover [71]. No major change is observed in planktonic communities and no evolutionary trend of any noticeable environmentally influenced change in the composition. It is considered a stable interval in terms of paleoecology despite the general trend of cooling of Cretaceous Greenhouse. The distinctive morphology of R. calcarata most certainly suggests an adaptation to environmental changes may relate to hydrodynamic regime [72].

Remarks: Radotruncana calcarata biozone is considered the narrowest biozone of the present study. Georgescu [73] suggested this case and support the hemipelagic to pelagic setting [74]. In the present material of the Duwi Formation, Globotruncana aegyptiaca Nakkady is recorded at the upper level of the R. calcarata as primitive forms where Praeglobotruncana (Globotruncanella) havanensis is not seen. Therefore, this biozone encompassed interval of both R. calcarata and Praeglobotruncana havanensis zones.

Globotruncana aegyptiaca zone (CF 8a)

Definition: Interval zone, from the last occurrence of Radotruncana calcarata at the base to the first appearance of Rugoglobigerina hexacamerata Bronnimann at the top.

Author: Caron [44].

Age: Late Campanian

Thickness: about the last 4.2 m thick of uppermost marl bed of the Duwi Formation (A-Beds) under the chert band which rests over the last conglomeratic siliceous phosphatic band (Figure 8c). The conglomerate bed shows the Late Campanian Event (LCE) [1,3-5,27].

Assemblage: Abundance of Heterohelix reussi, Hx. globulosa, Globotruncanella minuta, Rugoglobigerina rugosa, Globotruncanita insignis, Rugotruncana subcircumnodifer, Globotruncana duwi, flood of Globotruncana aegyptiaca and other globotruncanids, Globgerinelloides yacoensis and Pseudoguemblina costulata are present.

Remarks: The interval of the Globotruncana aegyptiaca Zone of the Upper Cretaceous of the Gabal Duwi Range is of two parts (Figure 9b). The marly lower one belonged to the Duwi Formation and the other upper part is of lower shale beds belongs the Lower Maastrichtian Dakhla Formation (Figures 9a and 9b).

Figure 9: Field Photographs; (a) The Dakhla Formation, lower Maastrichtian and lower Paleocene; (b) Field relation between the Duwi and Dakhla Fm., two parts of the Globotruncana aegyptiaca separated by a last phosphate bed; (c) Oyster bank and oyster aggregates; (d) Part of the Nubian Sanstone sequence rounded the Duwi Basin.

Later works have documented G. aegyptiaca Zone as Late Campanian age [7,47,48,75,76]. The maximum abundance and diversification of globotruncanids of this biozone coincide with global abundance data presented by Premoli Silva and Sliter [77]. Smith and Pessagno [78], in their study on the Corisican Formation, Texas, USA, introduced Globotruncana aegyptiaca Zonule, as a lower biounit within their Gansserina (Globotruncana) gansseri Subzone of the Globotruncana contuse-stuartiformis Assemblage Zone. They differentiated the G. aegyptiaca Zonule from the underlying Early Maastrichtian Rugotruncana subcircumnodifer Subzone. Smith and Pessagno [78] defined the base of G. aegyptiaca Zonule by the first occurrence of the maker species with the limit distribution of Gansserina gansseri (Bolli) at the lower level. In Egypt, Aref [79] reported G. aegyptiaca Zone in the lower part of the Dakhla Formation of some Red Sea coastal areas, conformably overlain by Gansserina gansseri Zone (Duwi, Safaga and Esh El Mellaha sections) within the Matulla and partly Sudr Formation and unconformably underlain by the Globotruncana ventricosa. El- Deeb and El Gammal [80] recorded G. aegyptiaca Zone in southern Sinai and correlated its occurrence with Central Egypt and Red Sea regions. This zone is correlated to the Late Campanian Event (LCE) [1,3-5,27].

Stage boundaries

The present taxonomic analysis has revealed several traceable levels that show changes in the composition of the planktonic foraminiferal assemblages belonged to the upper 20 m thick of the Duwi Formation through the Coniacian–Santonian and Campanian Stages with comparable Tethyan events (Figure 10).

Figure 10: Sea–level changes vs. planktonic foraminiferal biozones, chronozones and biostratigraphic events, Coniacian-late Campanian succession of the Duwi Fm., Gabal Duwi, Egypt.
- Time scale adopted after.
- Assuming a constant sedimentation rate.
- Tethyan planktonic foraminiferal zones.
- Peaks and troughs on the regional sea level curves of Egypt.

Oceanic anoxic event 3: Oceanic Anoxic Event 3 (OAE 3) was designated by Arthur and Schlanger [24] and occupied the Dicarinella concavata Zone of the earliest event, the sharp contact between first lower marl bed of “Atchan Phosphate A-Beds” where the first appearance of Dicarinella concavata and its assemblage of the Coniacian markers. oyster phosphate thin bed (Figure 7b). The late Coniacian Event [25,81] has marked by the boundary between the Coniacian/Santonian Stages (Figures 9c and 9d). It is indicated by the first occurrence of both Dicarinella asymetrica and the first appearance of very rare benthic Neoflabelin suturalis [82]. Premoli Silva and Sliter [7,46] and others accepted the first appearance of Dicarinella asymetrica as a zonal marker of the Santonian Stage in low latitude globotruncanid zonal schemes. The disappearance of Concavotruncana concavata is reported at the same level of the first appearance of Dicarinella asymetrica but the highest occurrence of C. concavata cannot be used as marker event for the Asymetrica event [52].

The Coniacian–Santonian phase is followed by the extinction of Marginotruncana and Dicarinella in the Latest Santonian-Earliest Campanian, the two evolutionary steps are traditionally regarded as due to a broader, major turnover (the so-called Santonian turnover) that affected all trophic groups within the planktonic foraminifera but was especially important in the history of more oligotrophic, keeled taxa [62,77].

Santonian/Campanian Boundary (SCBE): The Santonian/ Campanian Boundary Event (SCBE) [1,27] is marked by major faunal turnover of margintruncanids were replaced by globotruncanids [46,83,84] confirm the cooling event of the lower Campanian. The first appearance of Globotruncanita elevata as indicated the marker of this boundary means prograding to oligotrophic episodes [85]. The lower Campanian shows the real diversity and radiation of globotruncanids, contusotruncanids and many foraminferal species in Northern Africa and Mediterranean regions [63,64], so at this boundary, many of mid Cretaceous planktonic foraminiferal taxa extincted globally [21,63]. The definition of Santonian/Campanian boundary is still mater of discussion [86-88].

Mid-Campanian Event (MCE): This event named the Mid Campanian Event (MCE) [1,3,27] and/or the Conica Event [29] at the base of the upper Campanian and recently as (Trunch event) by Thibault [86]. The Mid-Campanian Event at 78.7 Ma may equate to the high level of Globotruncana ventricosa Zone. The mid Campanian transgression therefore, corresponds to the Mid-Campanian Event [27] as leading to a Campanian sea level maximum to the top of Contusotruncana plummerae Zone of Middle Campanian. On another hand, some factors worked as a divide between the Boreal realm and the Tethyan basin [89] specifically the connection between the Boreal realm, the proto-Atlantic Ocean and the Tethyan basin became narrow gateways, progressively more restricted during the mid-Campanian possibly due to northward movement of Africa associated with the effects of the South Atlantic Ocean opening with consequent reduced intermediate/deep water mass exchange. This could cause a change in the Tethyan fauna as a result of changes the ecological supply of niche and large number of Campanian species of genera Planoglobulina, Contusotruncana, Rugoglobigerina, and Pseudoguemblina were adopted [90].

As in Duwi section, diversification of keeled species of genera like Globotruncana, Radotruncana, Contusotruncana show strong fluctuations and the assemblages became strong affinity with typical Tethyan assemblages [7,91]. The Late Campanian regression and subsequent sea level rise correspond to the present Radotruncana calcarata taxon of the Duwi section may suggest sediment starvation during sea level rise (note that the rate of sedimentation of R. calcarata Zone is calculated as 0.06 cm/kyr. (Figure 11), this supported by weakly phosphatized.

Figure 11: Age-thickness for the Coniacian–Late Campanian sedimentary sequence of the Duwi Fm., Gabal Duwi, Egypt; Sedimentation rates based on thickness of each biozone, its age (Ma) and corresponding chrones. Numbers of sedimentation rates correspond to each biozone.

Late Campanian Event (LCE): During the R. calcarata Zone, the Late Campanian Event (LCE) [1,3-5,27], the Epsilon Event (EE) (also called C1-Event) [3,30] in many Tethyan cases, Praeglobotrancanella (Globotruncanella) havanensis as an index species is not present in Duwi section, it stratigraphically downs into the hardgrounds indicating to maximum sea level rise. Radotruncana calcarata as in Tunisia and SW France [92] and may correlate with the Duwi section where the Globotruncana aegyptiaca Zone rests over R. calcarata directly marking the LCE. During the early late Campanian, a great transgression event occurred indicated by very large double–keeled foraminifera as Globotruncana aegyptiaca indicative of oligotrophic conditions. Jarvis [27] concluded that a broad agreement between the NW Europe and Egypt curves of Lüning [9] with generally rising sea levels throughout the Campanian, and major earliest Campanian and mid-late Campanian transgressions.

Egypt (even Duwi section) and in Tunisia [27], the Late Campanian Event coincides with the final phase of marl sedimentation at the base of the Gansserina (Globotruncana) gansseri Subzone, which directly overlying the Globotruncana aegyptiaca Zone.

In conclusion, of all three Campanian events in Europe and North Africa (Tunisia and Egypt) are mainly sea level changes appear to be synchronous indicating a dominance of eustatic over regional tectonic forcing [27].

Sedimentary basin environment

The Gabal Duwi Range is considered a sub-basin of Neo-Tethys started opening to the Tethys oceanic basin may since the Late Cretaceous starting from Coniacian Stage. The Coniacian deposition started with a vast shallowest platform receiving siliciclastic sediments of the Quseir Formation and lower parts of the Duwi Formation. With a tectonic normal subsidence of basin toward N-NW, a geodynamic evolution of the Duwi basinal area was related to the opening of gateways of Neo-Tethys depositing more basinal sediments from Coniacian up to Eocene Epochs, with significant sea level changes globally or periodic local regime.

Duwi is as asymmetric syncline system (Figure 12) of hanging a wall with extensional fault–related folding [31]. It is bounded by Precambrian basement rocks (Figure 1) where gentle, moderately east dipping Nubia Sandstones occur in the footwall (Figure 8d). These Pre-Coniacian (Hamadat C–Beds) accompanying eustatic sea level rise phosphorites are developed through intermittent winnowing of fine– grained phosphatic shales (OAE 3) (Oxygen Anoxic Environment) and biosiliceous sediments. Gradual sea level fall of the basin during Oyster banks of Duwi (B-Beds) (Figure 8c) with brackish back–reef sediments and deltaic sediments dominated the middle beds of the Duwi Formation all these sediments are barren of foraminifera.

Figure 12: Structural cross section A-À (on Figure 1) across the Gabal Duwi area demonstrating the asymmetric syncline system of hanging wall with extensional fault related folding as Duwi Basin as continental or interior sag basins.

The start of Coniacian Stage (Atchan A–Beds), the sea level rise cycle of inner to outer neritic marine conditions with strong waves and surface-bottom currents usually tend to distribute the local influx of terrigenous sediments over large areas [37], such terrigenous sediments rich in planktonic, benthonic foraminifera and ostracods. Such conditions often persist over long time intervals without being filled up to sea level [93]. This case is true for widely extended shallow marine basins as long as excess sediment volume in relation to space provided by gradual subsidence (averaged 0.96 cm/Kyr in case of upper beds of Duwi Formation). The margin of such basins is characterized by a kind of Ramp Morphology as Duwi basin case.

Sea level fluctuations

An erosional contact is present between the middle (B-beds) and upper (A–Beds) of the Duwi Formation. This contact is characterized by the long wall of bioclastic, silicified limestone of reefal facies (Oyster banks) or Lime–coquina and overlying silicified phosphatic bed of about 40 cm thick rich in glauconite (Figures 8a, 8b and 9c). The (A–Beds) sequence displays a transgressive nature with silty, marls and argillaceous limestone containing the migrated foraminiferal fauna from Coniacian to Late Campanian of the Duwi Formation. Fluctuations in relative sea level occurred in form of two chert bands and topmost conglomeratic phosphatic bed which separating the Duwi Formation and the overlying the Upper Campanian–Maastrichtian Dakhla Formation (Figure 9b). Also, very short–lived regressive episodes and drops in sea level are observed before the accumulation of thin black shales through all interval of the Duwi Formation especially the Coniacian–Campanian interval. There is no unconformity surfaces existed through the Coniacian–Late Campanian. In general, the sea level changes were quite marked no turbidity through Coniacian–Late Campanian. The succession may be explained as tectonically quiet as well as eustatic changes of sea levels.

Rates of sedimentations

According to authors Jarvis, Hart and Thibault N [1,94,95] it can be calculate or estimate the rate of sedimentation for the studied foraminiferal 20 m thick of the (A-Beds) from Coniacian to Late Campanian about 17 Ma [95] according to the Age-Depth Models described by Thibault (Figure 12) [95] allows to estimating the variations of sedimentation rates and age assignments of calcareous microfossils (planktonic foraminifera). It is clear that the lowest rate of sedimentation is of the Radotruncana calcarata Zone (0.06 Cm/ Kyr), the higher one is of the Globotruncana aegyptiaca (0.6 Cm/ Kyr) and other biozones are from 0.2 to 0.3 Cm/Kyr (Figure 13). The overall average is about 0.13 Cm/Kyr, indicating very calm and quiet, no tectonics or abnormal cases except only eustatic sea level basinal conditions comparing with the Tethyan Bottaccione and Contessa High Way sections, 1.16 Cm/Kyr [96]. Also, the average of basin subsidence is 0.96 Cm/Kyr, it means very gentle subsidence with a long time. These data may explain the small thicknesses of biozones comparing with deeper oceanic basins. Comparing our data with most Tethyan Cretaceous sections, the differences do not exceed ± 0.01 Cm/ Kyr. Through the Late Campanian, there is an increase in depth with an increase of flavoring and specification in planktonic components indicating a time of expansion of the Egyptian continental shelf and continued into the Maastrichtian.

Figure 13: Changes in the sedimentation rates with the eustatic sea level changes of Coniacian- Late Campanian, Duwi Fm., Gabal Duwi section.

1. The present study has focused on the Duwi Formation at Gabal Duwi Range. This succession is easily accessible with its contact with lower Quseir Formation and the upper Dakhla Formation. The entire interval includes the transition from the Coniacian stage to the Campanian stage.

2. The lithostratigraphic subdivisions for the Pre-Coniacian-Late Campanian in Gabal Duwi are proposed, from the base to the top as follow; Lower phosphate unit (Pre-Coniacian), which named Abu Shigeila or Hamadat “C” horizon; Middle phosphate unit (Pre-Coniacian), which named Main Duwi Beds or “ B ” beds and Upper phosphatic unit (Coniacian-Late Campanian), which named locally Atchan phosphate or “A” Beds.

3. About 6 standard Tethyan biozones covering the sedimentary sequence of the interval (A-beds) have been recognized and correlated to the world bioevent as follows; Dicarinella concavata Zone (CF 14) and Dicarinella asymetrica Zone (CF 13), where these two zones are correlated to the Coniacian– Santonian time interval of Oceanic Anoxic Event 3 (OAE 3). Globotruncanita elevate Zone (CF 12) is correlated with the Santonian–Campanian Boundary Event (SCBE). Contusotruncana plummerae Zone (CF 11) is correlated with the Mid Campanian Event (MCE). Radotruncana calcarata Zone (CF 9 and CF 10) and Globotruncana aegyptiaca Zone (CF 8a), these two zones are correlated to the Late Campanian Event (LCE). In conclusion, of all three Campanian events in Europe and North Africa (Tunisia and Egypt) are mainly sea level changes appear to be synchronous indicating a dominance of eustatic over regional tectonic forcing.

4. The Gabal Duwi Range is considered a sub-basin of Neo-Tethys started opening to the Tethys oceanic basin may since the Late Cretaceous starting from Coniacian Stage. Through the Late Campanian, there is an increase in depth with an increase of flavoring and specification in planktonic components indicating a time of expansion of the Egyptian continental shelf and continued into the Maastrichtian.

The authors are grateful to Dr. Friedrich van der Wart from the Netherlands for critically reading the manuscript and suggesting improvements. The authors thank the anonymous reviewers for their very helpful comments on the manuscript.