Foraminifera study for the characterization of the Campanian/Maastrichtian boundary in Gebel Owaina, Nile Valley, Egypt

The present work aims to provide a high-resolution stratigraphic range of the planktic foraminiferal group during the Campanian/Maastrichtian boundary (C/M) of Gebel Owaina, Nile Valley. We use the base of Planoglobulina acervulinoides (CF7c Subzone) to mark the C/M boundary, where the planktic foraminiferal assemblages in the subzone are proposed as indicative of the early Maastrichtian age. Five zones (CF9-CF5) and three subzones (CF8a, CF8b, and CF8c) were recognized in the present study. A morphogroups scheme for the calcareous benthic foraminifera used in this work comprises five morphogroups, observed in the Owaina section. Three planktic morphogroups can be recognized and indicate open marine conditions with a good connection to the Tethys Ocean due to the highly diverse Maastrichtian assemblage and high P/B ratio (% P > 95).

The C/M boundary GSSP coincides with the LO of the planktic foraminiferal species Rugoglobigerina scotti and Contusotruncana contusa.It is hypothesized that the LO of Contusotruncana contusa could be concurrent with the LO of Rugoglobigerina scotti, as reported in the GSSP for the C/M boundary [13].
The upper Campanian-Maastrichtian succession of the Dakhla Formation (i.e.Sharawna Shale Member) in the Gebel Owaina section (Fig. 1) is still poorly documented.Dakhla Formation termed by [38] is represented by thick siliciclastic facies which are underlain by the Duwi Formation (phosphate deposits) and overlain by the Tarawan Formation (carbonate facies).The Campanian/Maastrichtian boundary was recorded at the middle part of the middle Sharawna Marl unit (Fig. 2).Biostratigraphic correlation based on planktic and benthic foraminifera has received little attention due to the scarcity of low diversity of microfossils in these, mostly shallow marine sequences.
El Naggar (1966) [40] made a detailed study on the stratigraphy and planktic foraminifera of some Upper Cretaceous-Lower Tertiary sections along the Esna-Idfu regions, the Nile Valley.He stated that the Esna Group is widely distributed in Egypt extending from the Dakhla-Kharga-Kurkur scarp into the Nile Valley, resting over the Phosphate Formation and underlying the Thebes Formation.He recognized several zones from the base upwards, as follows; Globotruncana fornicate Zone, Globotruncana gansseri Zone, the Globotruncana esnehensis Zone, El-Younsy et al. (2017) [42] reviewed the Upper Cretaceous-Lower Paleogene succession at the Gebel Qareiya area and they investigated several distinctive benthonic Fig. 1 Geological map of the studied area (After the EGPC/ CONOCO, 1987, Luxor sheet) [131] foraminiferal assemblages, these foraminiferal assemblages, and their paleobathymetric interpretation are as follows; the pre-Campanian Shale of the Quseir Formation is characterized by the presence of Haplophragmoides calculus benthonic assemblage, which points to the inner neritic paleodepths (~ 10-50 m) setting, the lower Campanian shale and marl of the Duwi Formation are characterized by the presence of Lagena sulcata and Tritaxia midwayensis benthonic assemblage, which points to inner to middle neritic paleodepths (~ 40-80 m) setting, the Upper Campanian-Maastrichtian of the Hamama Marl Member is characterized by the occurrence of Anomalina umboniferus, Rectuvigerinastriata, and Bolivina midwayensis benthonic assemblage, which points to outer neritic-upper bathyal paleodepths (~ 150-600 m) setting, coincides with an increase in the P/B ratio of ~ 30-80%.[43] studied Maastrichtian-early Paleocene foraminiferal palaeobathymetry, palaeodiversity, and vertical facies changes of Gebel El Sharawna, south Luxor, Egypt, and determine the depositional sequences, their relationships to global records and/ or tectonic signatures.Five benthonic assemblages are recorded and replicated in the present study to reflect fluctuation in palaeo-water depth from restricted marginal marine to outer shelf palaeoenvironments.Four sequence boundaries that coincide with the Campanian/ Maastrichtian, intra-early Maastrichtian, Early/Late Maastrichtian, Cretaceous/Palaeogene (K/ Pg), and intra-Danian were recognized based upon sharp vertical facies changes, foraminiferal assemblage changes, hiatuses, mineral hard ground, and reworking.

Farouk et al. (2019)
This paper aims to document the morphology of the foraminifera and stratigraphic range of the planktic foraminiferal group during the Campanian/Maastrichtian boundary that has prevailed during the sedimentary deposition across the Campanian/Maastrichtian boundary and evaluate the paleoecology of the Gebel Owaina section.The new detailed subzonation of the standard Gansserina gansseri Zone [5,13,[44][45][46][47] involves the consecutive origination of rugoglobigerinids and multiserial heterohelicids.

Geological setting and lithostratigraphy
During Campanian-Maastrichtian, Egypt was part of a vast epicontinental shelf at the edge of the southern Tethys Ocean on the northwestern margin of the Arabian-Nubian massif [48].Tectonics played an important role in controlling the configuration of the Egyptian Campanian-Maastrichtian depositional environment [48][49][50][51]; leading to great vertical and lateral complexities in the facies distribution and thicknesses.The Campanian-Maastrichtian succession in the studied area shows great lateral changes in both lithofacies and biofacies.
The shelf area of Egypt, during the Late Cretaceous and Early Paleogene times, can be divided into the Stable Shelf surrounding the basement exposures of the Arabo Nubian craton and the Unstable Shelf [38].The Late Cretaceous-Early Eocene sediments in central and southern Egypt are characterized by gradual facies [52].
Gebel Oweina area (Latitude 25°15'N; Longitude 32°45' E) is located on the eastern side of the upper Nile Valley, about 25 km SE of Esna town and 8.5 km NE of the Sibaiya railway station.It rises about 450 m above sea level [38] and has a height: of ~ 154 m.
The detailed lithostratigraphic description of the encountered Upper Cretaceous rock units in the studied area is given below.The samples examined in the present study were obtained from one stratigraphic surface section (Fig. 2)., and about 72 samples were collected from the Nile Valley facies (Quseir, Duwi, and Dakhla formations).
In the south of the Western Desert, [54,55]  The Dakhla Formation in the studied succession has a thickness of about 154 m and is made mainly up of shale, light grey at the top, and green at the base, including marl bands.Generally, the Dakhla Formation overlies and underlies the Duwi and Tarawan formations, respectively, with two unconformity surfaces at the base and the top.
The Dakhla Formation was assigned to the early Campanian-middle Paleocene, [57], in the studied area the twothirds part of the Dakhla Formation attains a thickness of 117 m and is assigned to the late Campanian-early Maastrichtian (Fig. 3).
Lithologically, the Dakhla Formation consists of two members; Sharawna Shale Member (Cretaceous) and Owaina Shale Member (Paleocene) [55].The Sharawna Shale Member consists of three distinct units; a lower shale unit (about 57 m thick), a middle marl unit (12 m thick), and an upper shale unit (about 48 m thick).The Sharawna Shale Member [58] can be correlated with the Hamama Member [59] in the Qena-Luxor region.The middle marl unit is composed of biscuit-colored, hard, and highly fossiliferous.This unit can be correlated with the Sharawna Marl unit [55,60].
The Owaina Shale Member [55] of the Dakhla Formation (Paleocene) is composed of dark grey, greenish grey, green, greyish violet, and greenish-yellow shales with clay ironstone bands, it is intercalated with gypsum vines.The famous disconformity surface between the Cretaceous and Paleocene occurs near the base of the Owaina Shale Member and is represented by a 50 to 120-cm thick phosphatic conglomerate layer.The Owaina Shale Member can be correlated with the Beida Member [60].This shale member is disconformably overlain by the Paleogene sediments.

Foraminiferal studies
All collected 72 samples were dried for at least 24 h in an oven at 60˚C.About 50 g from each sample were soaked in Hydrogen Peroxide solution (10% H 2 O 2 ), then washed over a 63 μm mesh sieve, and then dried and sieved into fractions greater than 250, 125, and 63 μm.The washed residue from each sample was studied under a binocular microscope.The foraminifera tests were picked into faunal slides.About 250-300 foraminifera specimens were picked from a random sample in the rich samples but 50 individuals in the poor samples [61].The following parameters were calculated; TFN = Total Foraminifera Number, number of planktic foraminifera (P), number of benthic foraminifera (B), Planktic ratio P % = (P/ (P + B) × 100), Agglutinated Foraminifera/Calcareous Foraminifera (A/C) %, Epifaunal % = Epifaunal/Epifaunal + Infaunal × 100 (Epi% = E/E + I X 100), diversity = number of species per sample, Fisher's alpha, and Shannon-Weaver.The most important foraminifera are photographed using scanning electron microscopy (JEOL JSM-5500 LV) of Alexandria University (Figs. 4, 5 and 6).

Biostratigraphy
Several zonal schemes for the Late Cretaceous planktic foraminifera (Campanian-Maastrichtian) were proposed in many parts of the world.The planktic zones used here are based on the schemes of [9,13,62,63].Based on the first occurrence (FO), and last occurrence (LO) of the identified index planktic foraminiferal species, eight planktic foraminiferal zones, and subzones are recognized in the Campanian-Maastrichtian sequence.The range charts of the identified planktic foraminiferal species of the studied section are shown in Fig. 7, while the distribution of the planktic and benthic foraminiferal species counted in Campanian-Maastrichtian is shown in Figs. 8, and 9.The following is the description of the established planktic foraminiferal zones arranged from older to younger.

Campanian-Maastrichtian zones
The predominant zones recognized in this section are five zones only; CF9, CF8, CF7, CF6, and CF5, and three subzones CF 8a, CF 8b, and CF7c.The following zones and subzones discuss from older to younger:
Definition: The base of the Globotruncanella havanensis Zone designated by [6] is traced at the last occurrence of Radotruncana calcarata.
Thickness: It attains about ~ 9.5 m of the Dakhla Formation from samples 38 to 39 in the Gebel Owaina section (Fig. 7).
Definition: It is defined as the interval from the (FO) of Globotruncana aegyptiaca Nakkady to the (FO) of Gansserina gansseri.Thickness: It attains about ~ 19.5 m of the Dakhla Formation from samples 40 to 41 at the Gebel Owaina section (Fig. 7).
Benthic Assemblage: nearly the same as in the previous zone, in addition to the presence of Osangularia plummerae.

Equivalent and age:
This zone is probably equivalent to the Rugoglobigerina hexacamerata zones of [9,64].Therefore, this zone is assigned to late Campanian.
Remarks: The (FO) of Globotruncana aegyptiaca was considered the marker of the base of the earliest Maastrichtian age while others considered the (FO) Globotruncana aegyptiaca to lie within the late Campanian.
In Tunisia, [65] used this zone as a marker for the late Campanian age, but [6,66,67] assigned this zone to the early Maastrichtian.Globotruncana aegyptiaca Zone is conformably overlain by the Gansserina gansseri zone.
Definition: This subzone is defined as the interval from the (FO) of the R. hexacamerata (Brönnimann) to the (FO) of Gansserina gansseri (Bolli).
Thickness: It attains about ~ 3.50 m of the Dakhla Formation from samples 42 to 43 at the Gebel Owaina section (Fig. 7).
Remarks: This zone is equivalent to the upper part of Globotruncana lapparenti tricarinata Zone of [70], the upper part of Globotruncana tricarinata Zone of [71], the upper part of Globotruncana tricarinata Zone of [72], the upper part of G. stuartiformis Zone of [1], the upper part of G. falsostuarti Zone of [5,73], the upper part of G. havanensis Zone of [74], the upper part of Globotruncana aegyptiaca Zone of [5,6], the upper part of G. aegyptiaca Zone (CF8) of [8,62].Also, it is equivalent to the Rugoglobigerina hexacamerata Subzone (CF8b) of [9].
Definition: This zone is defined from the (FO) of Gansserina gansseri (Bolli) to the (FO) of Abathomphalus mayaroensis.
Definition: This subzone spans the interval between the (FO) of Planoglobulina acervulinoides and the (FO) of Contusotruncana contusa at the top.
Thickness: It attains about ~ 7 m of the Dakhla Formation from samples 44 to 46 at the Gebel Owaina section (Fig. 7).
Equivalent and age: This subzone is probably equivalent to the Gansserina gansseri [6,70].
Age: Early Maastrichtian.Remarks: Remarks [79] utilized the FO of Planoglobulina acervulinoides as a biostratigraphic marker and located it within the upper part of the G. gansseri Zone.It was also recorded at a relatively similar position in several localities including Kalaat Senan in Tunisia [45,78]; Black Nose [63]; Zumaia, northern Spain [13]; [80] and Exmouth Plateau of the northwestern Australian margin [81].Ref. [82] assigned the FO of this species marks the upper part of the early Maastrichtian.Here we use the Planoglobulina acervulinoides to mark the C/M boundary [78], where the planktic foraminiferal assemblages in this subzone are indicative of the early Maastrichtian age.
This subzone can be correlated with the upper part of the standard G. gansseri Zone, and the heterohelicids show a gradual diversification expressed by the emergence of complex multiserial and coarsely striate forms (e.g., Racemiguembelina powelli) [8,79].
Definition: It is defined from the (FO) of Contusotruncana contusa at the base and the (LO) of the Globotruncana linneiana (D' Orbigny) at the top of the biozone.This zone is equal to the CF6 Zone of [8,9,62].
Thickness: It attains about ~ 10 m of the Dakhla Formation from samples 47 to 53 at the Gebel Owaina section (Fig. 7).
Equivalent and age: This zone is probably equivalent to the Contusotruncana contusa Zone of [8,44,62,62,84,85].Thus, this zone is assigned to the late early Maastrichtian.
Definition: This zone is defined from the (LO) of the Globotruncana linneiana at the base to the (FO) of Racemiguembelina fructicosa at the top, [79].
Thickness: It attains about ~ 31.50 m within the Dakhla Formation from samples 54 to 71 at the Gebel Owaina section (Fig. 7).
Remarks: Ref. [9] used the top of this biozone to place the early/late Maastrichtian boundary in Tunisia.In this study, this zone is equivalent to the upper middle part of the G. gansseri Zone of [5,6,65,86], the Ps.intermedia Zone (CF5) of [8,9,62].

Benthic parameters
Even more surprisingly, the total foraminifera number (TFN) values reflect the state of water depth, usually the increase of the water depth, the increase in TFN.The benthic foraminifera number (B) is a useful proxy to estimate oxygen content and organic matter flux in the past [e.g., 93,94].The (B) generally decreases in oxygen-depleted sediments [95,96].In contrast, a higher (B) is observed with increasing organic matter flux to the seafloor [e.g., 94,97,98].Consequently, the (B) is controlled by both oxygen content and organic matter flux.According to [99], the benthic Foraminifera Number is inversely related to water depth.The Benthic foraminiferal parameters of the studied successions would be briefly discussed below: In the Owaina section, samples 38-41 (CF9 Zone-CF8a Subzone) the TFN values increase from 265 to 306 individuals.TFN values decrease from 265 to 261 individuals in the CF8b Subzone and CF7 Zone then increases again upward in CF6 and CF5 Zones to reach a maximum value of 367 individuals in sample 61 and at the end of the CF5 Zone.
The benthic number (B) ranges from 3 to 305 individuals in samples 38-70, while samples 1-37 are barren.In samples, 38-41 (B) values increase from 182 to 233 individuals (CF9 Zone-CF8a Subzone).(B) values decrease from 233 to 66 individuals in the CF8b Subzone and CF7 Zone then increases again upward in CF6 and CF5 zones to reach a maximum value of 305 individuals in sample 56 at the end of the CF5 Zone (Fig. 10).

Diversity Indices
In the present study, biodiversity is measured by many diversity indices, such as species richness, Fisher's alpha, and Shannon-Weaver.In general, the species diversity increases with paleo-water depth [100][101][102].

Species richness (S)
In the Owaina section, the species richness of benthic foraminiferal tests ranged from barren to 46 species per sample.It is worth mentioning that samples, 1-38, and 72-74, are barren intervals but other samples range from 2 to 46 species/sample.The species richness of benthic foraminifera in samples 39-42 (CF9 Zone, CF8a, CF8b subzones) ranges from 19 to 26 species per sample.In samples, 43 to 53 (CF7, CF6 zones) species richness of benthic foraminifera tests is increasing upward from 18 to 25 species/sample.In samples, 54 to 71 (CF5 Zone) species richness of benthic foraminifera are fluctuating from 14 to 28 species/sample at the end of the CF5 Zone (Fig. 10).

Fisher α values
In the present study, the Fisher α values are ranging from 0.856 to 15.32 in samples 38-70.The highest values are ranging from 7 to 15.32 in samples 39 to 43 (CF9 Zone, CF8a, CF8b, CF7c subzones).The lowest values are ranging from less than 1 to 7 in samples 45 to 71 of zones CF6 and CF5 (Fig. 10).

Arenaceous/Calcareous (A/C %)
The A/C% benthic foraminiferal ratio of the present study is illustrated in (Fig. 10).Ref. [103] stated that the dominance of calcareous foraminifers indicates deposition largely above the CCD line, in an area high in calcium carbonate, well-oxygenated, and characterized by normal salinity and/or high temperature.The calcareous foraminiferal tests are dominated by irregular distribution ranging from dominant ranging from 63 to 100%.

Epifaunal/Infaunal Ratio (E/I):
Several studies have demonstrated the relationship between the different microhabitats of benthic foraminifera and the epifaunal % [e.g., 94,104,105].The Epifaunal % (E/I) is ranging from 8 to 94% in samples 38 and 128.The dominance of epifaunal species often indicates high oxygen conditions and/or low nutrient concentration [106].

Benthic foraminiferal Morphogroups
The benthic foraminifera distribution, abundance, and evolution are governed by several parameters such as depth, the flux of organic nutrients to the seafloor, oxygen levels of the bottom, salinity, food quality, and quantity, substrate, temperature, and others [35-37, 102, 107, 108].The beds of the Owaina section yielded two agglutinated genera and more than nine calcareous benthic genera.The morphogroups scheme for benthic foraminifera used in this work was adopted after [108], where five morphogroups are observed in the Owaina section (Fig. 11).

Paleoecology of benthic foraminifera
The presence of Valvulineria in the CF5 Zone inhabited inner shelf to upper slope environments [109], where the abundance of Cibicidoides in the CF5 Zone as epifaunal form and recognized in the middle-outer shelf to upper slope deposits [95,110,111].Lenticulina presents in all zones and represents epifaunal to deep infaunal microhabitats and is observed in the sublittoral to upper bathyal [110,112].Orthokarestenia and Bulimina.suggests that they are shallow to deep infaunal, and live in the neritic to the upper bathyal environment [111,112].These faunas are indicative of a well-oxygenated environment and are usually determined by increased organic flux [113,114].
Gaudryina and Spiroplectinella, these taxa are in shallow to deep infaunal conditions, with low levels of oxygen, and are found in a wide range of marine environments from the inner shelf to upper bathyal [106].

Planktic percent (P%)
Planktic percent (P%) or the index of oceans of a foraminiferal population (Gibson, 1989) indicates an inner shelf (10-50 m) characterizes by rare planktic (< 8%) with low species diversity.The relative P% of 8-25% characterizes the middle shelf (50-100 m) with increased species diversity.On the other hand, the outer shelf environment (100-200 m) is characterized by a high P% of 70%, while the P% of 90% is attributed to the middle slope (400-800 m) [115].
The planktic number (P) is ranging from 1 to 312 individuals in samples 39-70.In samples, 39-41 (P) values decrease from 83 to 60 individuals (CF9 Zone-CF8a Subzone).(P) values increase from 96 to 195 individuals in the CF8b Subzone and CF7c Subzone then decrease again upward in CF6 and CF5 zones to reach a minimum value of 2 individuals in sample 70 and at the end of CF5 Zone, it is barren except sample 72 (110 individuals).

Planktic foraminiferal morphogroups
Planktic foraminifera tests are adapted to changes in temperature, nutrient supply, and oxygen content [116].We applied the classification of comparable Cretaceous morphotypes by [117][118][119][120][121], whereas Fig. 12 shows the distribution of planktic morphogroups in the Owaina section.In this matter, three groups can be recognized; (1) Shallow-water fauna (0-50 m) epipelagic is characterized by non-keeled tests of the genera, Rugoglobigerina, Globigerinelloides, Pseudotextularia, and Heterohelix with no peripheral keel, (2) Intermediate-water fauna (50-100 m) mesopelagic as a single-keeled and includes genera Globotruncanella, Globotruncanita, and Gansserina, with only one peripherally keeled, and (3) Deeper-water fauna (with adults normally below 100 m) bathypelagic and has a double-keeled test (Contusotruncana, and Globotruncana) [118,119].Ref. [122] reported that Globotruncana arca is more common in deeper water deposits.Ref. [123] found that Contusotruncana is a shallower dweller.During the Cretaceous time the warmest temperature is recorded in tests of rugoglobigerinids and globigerinellids [123,124].At the end of the Campanian, some planktic foraminifera became extinct, which might have been related to two volcanic events in the Atlantic Ocean, the Sierra Leone Rise, and the Maud Rise [127].These had been accompanied by a regional oxygen-reduction event [128] occurring throughout the Campanian, and was accompanied by third-order global sea level falls [9,120].

Campanian/Maastrichtian boundary
The beginning of the Maastrichtian was a period of high turnover, where the planktic foraminifera taxa were highly developed with morphological features such as double peripheral keels that allowed them to sink into deeper waters [129].The diversity and size of the foraminifera increased during the Maastrichtian pointing to warm temperatures and stable environments [130].

Conclusion
Here we use the Planoglobulina acervulinoides Subzone (CF7c) to mark the Campanian/Maastrichtian boundary.The planktic foraminiferal assemblages in the subzone are proposed as indicative of the early Maastrichtian age.This zone is probably equivalent to the Gansserina gansseri [6,68].
Globorotalia copressa/Globigerina daubjergensis Zone, and Globorotalia angulata Zone.El-Younsy et al. (2015) [41] studied the sedimentological and biostratigraphical detailed studies of the Campanian-Eocene succession in the Gebel Oweina area, Upper Egypt, and helped in reaching a high-resolution sequence stratigraphic classification that led to the recognition of the first depositional sequence in the studied Gebel Oweina stratigraphic section coincides with the Campanian of both the Duwi Formation (~ 33 m thick) and the Oweina Shale Member (~ 57 m thick) of the lower part of the Dakhla Formation.It is difficult to determine the precise age dating of this sequence because most of its sediment is barren of any diagnostic planktic foraminifera, except the basal part of the Oweina Shale Member, which contains planktic foraminifera of Globotruncana aegyptiaca.The second depositional sequence in the studied stratigraphic section is made up of both the Sharawna Marl Member (~ 16 m thick) and the lower part of the Oweina Shale Member (~ 52 m thick) of the middle part of the Dakhla Formation.It represents the Maastrichtian zones Gansserina gansseri and Pseudogumbelina palpebra that extends from ~ 70.4 to ~ 65.2 Ma [52].The third depositional sequence represents the Late Campanian G. aegyptiaca Zone (~ 9 m thick) followed upward by the Maastrichtian G. gansseri, P. hariaensis, P. palpebra, and P. Hantkeninoides zones (~ 21 m thick) that extends from ~ 70.4 to ~ 65.0 Ma [53].

Fig. 8
Fig. 8 Distribution of the planktic foraminiferal species recorded in Campanian-Maastrichtian

Fig. 9
Fig. 9 Distribution of the benthic foraminiferal species recorded in Campanian-Maastrichtian

Fig. 10
Fig. 10 Benthic and planktic foraminiferal parameters in the Owaina section

Fig. 12
Fig. 12 Distribution of planktic morphogroups in the Owaina section

Fig. 13
Fig. 13 A comparison of the most relevant planktic foraminiferal biozonations across the upper Campanian and Maastrichtian biozonations is proposed in this study 38,56,57]d the Duwi Formation is of the 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 [e.g.,38,56,57]and assigned to late Campanian-early Maastrichtian (Fig.3).