1. Introduction

The Upper Cenomanian-Turonian successions covered large provinces in north Egypt. These successions are represented by siliciclastic and carbonate marine facies which rich with diverse macrofaunal assemblages. Several biostratigraphic and palaeoecological studies have been carried out on these faunal assemblages [1-11]. However, some localities in the North Eastern Desert are still uninvestigated until now; one of these localities is the Um-Artah area (Figure 1). So, the most important goal of the present study is to fill the gap in information on the Um-Artah section, whereas there is no information about this area. Therefore, this study firstly introduced details stratigraphic studies about the Upper Cenomanian-Turonian transition at the Um-Artah area. At the same time, it aims to establish a detailed biostratigraphic and sequence stratigraphic framework for the Upper Cenomanian-Turonian successions of the Southern Galala Plateau, and hence, correlated its depositional sequence with local and international ones.

2. Materials and methods

Two stratigraphic sections were described, measured in detail in the Upper Cenomanian-Turonian strata at the Southern Galala area (Figure 2) with a focus on their macrofossil content (especially ammonites). The first outcrop is located in the southern part of W. El Dakhl section (at latitude 28° 39' 48" N and longitude 32° 24' 58" E) and the second section is at W. Um-Artah section (at latitude 28° 34' 58" N and longitude 32° 23' 01" E). These sections were described in detail. The stratigraphic ranges of the macrofossil content were identified and traced, and therefore, biostratigraphy records were established. These macrofossils were photographed. The integrated stratigraphic data were interpreted in the sequence stratigraphic framework.

3. Lithostratigraphy

Two rock units are analyzed: Galala (at the base) and Maghra El-Hadida (at the top) formations. These two formations represent the Upper Cenomanian-Lower Turonian transition. They are unconformably underlain by the Malha formation and overlain by Matulla formation. Due to the vertical lithologic variations between Malha/Galala and Maghra El-Hadida/Matulla formational boundaries, these formational contacts are easily detected in the field. The unfossiliferous cross-bedded sandstones of the Malha formation vertically change to fossiliferous shale of the Galala Formation (Figures 2-4). At the same time, there is a vertical facies change between the uppermost part of Maghra El-Hadida formation (carbonate facies) and the Matulla formation (clastic facies). These formational contacts are characterized by unconformity surfaces (Figures 2-4).

3.1.  Galala formation

It attains about 40 m at W. El Dakhl section and about 43m at Um-Artah section. It covers the Late Cenomanian time interval. At W. El Dakhl, it is composed of shallow-marine deposits of siltstone and silty marl with limestone intercalations, followed by thick green shale intercalated with hard limestone. On the other hand, at the W. Um-Artah section, it is composed of shales, limestones, siltstones, and nodular marl with oyster limestone intercalations in the lower part; change upward within the upper part into hard silty limestone. It is unconformably overlain by Maghra El-Hadida formation. This formational contact is marked by vertical facies variation from carbonate facies (uppermost part of Galala) to clastic facies (lowermost part of Maghra El-Hadida) [13].

3.2. Maghra El-Hadida formation

It attains about 26m and 89m thick at W. El Dakhl and W. Um-Artah sections respectively. It represents the latest Cenomanian-Turonian time interval. At W. El Dakhl section, it is composed of sandstone, shale, and limestone interbedded, followed by nodular bioturbated limestone and marl. At W. Um-Artah section, Maghra El-Hadida formation consists of siltstone, nodular marl, shales with thin, well-bedded limestone intercalations, while the upper part is composed of sandstone and nodular marl intercalated with limestone and sandy dolomitic limestones [14].

4. Biostratigraphy

These sediments of the early-Late Cenomanian (Galala formation in the present study) are deposited during a major sea-level transgression [15]. Therefore, the Upper Cenomanian Galala formation represents the initial marine transgression over the study area, and hence it is marked by abundant of macrofaunal (cephalopods, bivalves, gastropods, and echinoids) contents. The identified index ammonite species led to the establishment of a biostratigraphic framework for the study area (Figure 3-4), which is used for regional and inter-regional chronostratigraphic correlations [16-21] (Figure 5). The distribution charts of the identified macrofossils are illustrated in Figures 3 and 4 and photographed and shown in Figures 6-7. Based on the index ammonite taxa, five ammonite zones of the Upper Cenomanian-Turonian interval are identified, which are stratigraphically arranged from older to younger and described as follows:

4.1. Neolobites vibrayeanus zone (Late Cenomanian)

D’Orbigny [22] defined this zone as the total range of it’s nominate taxon. Neolobites vibrayeanus Zone is represented by the Galala formation. It attains ~43m at W. Um-Artah section and ~40m at W. El Dakhl section. Neolobites vibrayeanus Zone includes the cephalopods: Pseudaspidoceras pseudonodosoides (Choffat) and Angulithes  mermeti   (Coquand); the bivalves: Exogyra conica (Sowerby), Pycnodonte (Phygraea) vesiculosa (Sowerby), Ilymatogyra africana (Lamarck), Granocardium  productum (J.Sowerby), Rhynchostreon suborbiculatum (Lamarck), Ceratostreon flabellatum (Goldfuss) and Pterotrigonia (Scabrotrigonia)  scabra   (Lamarck); the gastropods: Pterocera incerta (Sowerby), Tylostoma pallaryi (Peron and Fourtau), Colombellina (Colombellina) fusiformis (Douville) and Nerinea olisiponensis (Sharpe); the echinoids: Heterodiadema libycum (Agassiz and Desor) and Hemiaster cubicus (Conrad). The same zone is recorded in Eastern Desert [21] and in Sinai [23]. In the Tethyan region (Figure 5), this zone as herein defined corresponds partly to the N. vibrayeanus Zone recorded in Tunisia [24] and Algeria [25].

4.2. Vascoceras cauvini zone (Latest Cenomanian)

Chudeau [26] defined this zone as the total range of Vascoceras cauvini. It is represented by ~6m and ~8m of the lowermost part of the Maghra El-Hadida formation at W. Um-Artah and W. El Dakhl sections respectively. Vascoceras cauvini Chudeau is associated with the cephalopods: Pseudaspidoceras pseudonodosoides (Choffat); bivalves: Ceratostreon flabellatum (Goldfuss) and Granocardium productum (Sowerby). The same zone is recorded in the Eastern Desert [3, 21] and Sinai [5]. Also, this zone is recorded from western Tethys, where this species is widespread during the latest Cenomanian in Niger [27] and Nigeria [28].

4.3. Vascoceras proprium zone (Early Turonian)

Reyment [29] defines this zone as the total range of its nominate taxon. The lowest occurrence of the Vascoceras proprium is represented the biomarker for the Turonian base. This zone is conformably overlain by the Early Turonian Choffaticeras (Choffaticeras) segne Zone. It attains about 11m and 2m of the lower part of the Maghra El-Hadida formation at W. Um-Artah and W. El Dakhl sections, respectively. It corresponds to Vascoceras proprium Zone recorded in Sinai [18] and in the Eastern Desert [30] and Pseudaspidoceras flexuosum Zone recorded in Tunisia [31]. The Vascoceras proprium is used as the biomarker for the Turonian base instead of Pseudaspidoceras flexuosum, depending on the wide spreading of Vascoceratidae (this is recommended by [32]), which is widely distributed from Africa, South America, and Boreal Europe. The lowest occurrence of Vascoceras proprium is equivalent to the lowest occurrence of Watinoceras devonense (in Egypt, [33]; in Jordan, [17]). So, this species is the reliable bioevent marking the Turonian base.

4.4.  Choffaticeras (Choffaticeras) segne zone (Early Turonian)

Solger [34] defines it as the total range zone. It is overlain by a barren interval. It is recorded within the middle part of Maghra El-Hadida formation and covers ~34m at W. Um-Artah section and ~5m at W. El Dakhl section. This zone is recorded by Abdel-Gawad et al. [3] in the north Eastern Desert. Also, it is equivalent to Choffaticeras securiforme-Choffaticeras quaasi and Choffaticeras segne zones in the Eastern Desert [6]. In Sinai, it is equivalent to Choffaticeras segne, Choffaticeras securiforme and Choffaticeras luciae zones [5] and Choffaticeras segne Zone [35]. Inter-regional scale, this zone is equivalent to Choffaticeras interval (or Choffaticeras spp. Interval zone) in Tunisia [31].

4.5.  Barran intervali

The upper part of the Maghra El-Hadida formation is barren of macrofossils including ammonites. This interval is represented by ~13m at W. Um-Artah section and ~7m thick at W. El Dakhl section. This interval is underlain by Choffaticeras (Choffaticeras) segne Zone of the Early Turonian age and overlain by Coilopoceras requienianum Zone of the Late Turonian age. Therefore, it belongs to the Middle Turonian age.

4.6. Coilopoceras requienianum zone (Late Turonian)

D’Orbigny [22] defines this zone as the total range zone. It is represented by the uppermost part of the Maghra El-Hadida formation, covering ~25m at W. Um-Artah section and ~4m at W. El Dakhl section. The same zone is recorded in the Eastern Desert [36] and in Sinai [37-38].

5.  Sequence stratigraphy

    The sequence stratigraphy of the Upper Cenomanian-Turonian strata has been attempted by many authors, such as Bachmann and Kuss [39] and Wilmsen and Nagm [40]. Four 3^rd depositional sequences are recorded in the study area, based on the integrations of the field observations, lithologic and macrofaunal analyses (Figures 3 and 4). The calibration of the depositional sequences using the biostartigraphic zones is helpful for correlating the stratigraphic surfaces with those in other regions (Figure 8). These depositional sequences and its stratigraphic surface are described as follows:

5.1. Depositional sequence 1 (DS1)

The first DS1 comprises ~43m and 40m of Galala formation at W. Um-Artah and W. El Dakhl, respectively (Figures 3 and 4). It is of Late Cenomanian age.

The Galala/Malha formational boundary represents the lower sequence boundary (SB Ce 4). This SB Ce 4 is an unconformity surface which is easy to identify in the field by vertical facies variation of Malha (non-marine deposit)/Galala (marine deposit) contact. The SB Ce 4 is herein placed at the base of Neolobites vibrayeanus Zone, which is equivalent to Calycoceras (Proeucalycoceras) guerangeri Zone at European basins. So, the SB Ce4 in the present study is mismatching with Ce 4 at the European basins [41] due to the absence of Acanthoceras jukesbronei Zone (or its equivalent zone in this region). Also, it is nearly coincidence with the SB Ce 4 at the North Eastern Desert of Nagm and Wilmsen [21]. The SB Ce 4 (the base of DS 1) coincides with the transgressive surface (TS).

The transgressive systems tract (TST) of this sequence attains about 37m thick of shales, siltstones, and silty marl with limestone intercalations at W. Um-Artah section. The TST at W. El Dakhl section attains about 32m thick of siltstone and silty marl with limestone intercalations. The high density and diversity of oysters, gastropods and echinoids with bioturbated limestone reflect a shallow open lagoon environment within the TST. A thick limestone bed that contains abundant of ammonite Neolobites vibrayeanus and nautiloid Angulithes mermeti associated with abundant macro-faunal content is marked the maximum flooding surface (MFS).

The highstand systems tract (HST) of this sequence attains ~5m of silty limestone with limestone intercalations at W. Um-Artah section and ~8m of shale and limestone at W. El Dakhl section (Figures 3 and 4).

5.2. Depositional sequence 2 (DS2)

DS2 covers the upper Cenomanian-lower Turonian transition. The SB Ce 5 (base of the DS2) is recorded at the notable lithological variations from fossiliferous limestone and green shale of the uppermost part of the Neolobites vibrayeanus Zone to siltstone and sandstone of the lowermost part of the Vascoceras cauvini Zone. This SB Ce5 is equivalent to the SB Ce5 at North Eastern Desert [21], Ce 5 at European basins [41], SB at Tunisia [44] (Tunisia) and SB at Anglo-Paris basin [43] (Figure 8). The absence of Metoicoceras geslinianum zone documented this unconformity surface (SB Ce5).

The depositional sequence (DS2) is represented by ~51m at W. Um-Artah and ~15m at W. El Dakhl sections. The TST of DS 2 is ~29m of siltstone, nodular marl, shale, marl and limestone intercalations at W. Um-Artah and ~11m of sandstone, shale, silty limestone and topped by limestone at W. El Dakhl section with abundant of the fossil content in the upper TST marking the MFS, which overlain by the HST.

This HST is ~ 22m of shale with limestone intercalations at W. Um-Artah and ~4m of limestone at W. El Dakhl sections (Figures 3 and 4). The HST includes the latest Cenomanian ammonite Vascoceras cauvini and the Early Turonian Vascoceras proprium and Choffaticeras (Ch.) segne zones. The Vascoceras cauvini Zone rests over the SB Ce 5, indicating a stratigraphic gap due to the missing of Metoicoceras geslinianum Zone of middle-Late Cenomanian at the study area. Nagm et al. [7] noted that this zone is mostly missing in Egypt.

5.3. Depositional sequence 3 (DS3)

The DS3 covers the Middle Turonian interval and represented by the upper part of the Maghra El-Hadida formation at the two studied sections. This DS3 is barren of any ammonites. It belongs to the Middle Turonian age based on its stratigraphic position. The DS3 attains ~16m at W. Um-Artah and ~7m at W. El Dakhl sections.

The DS3 is bounded at the base by SB Tu 1 and topped by SB Tu 2. According to the zones comparison, the SB Tu 1 matches with Tu 1 at European basins [41], while this SB Tu 1 is mismatch with that of North Eastern Desert [21], due to missing of Watinoceras munieri/Mammites nigeriense zone at this study.

The TST of the DS3 attains ~9m at W. Um-Artah and ~4m at W. El Dakhl sections. This TST is composed of sandstones, silty shale, and siltstone at W. Um-Artah and nodular marl at W. El Dakhl sections.

The HST of the DS3 is composed of shale, marly shale and nodular marl in W. Um-Artah and nodular marl with limestone in the W. El Dakhl sections. It attains ~7m at W. Um-Artah and ~3m at W. El Dakhl sections (Figures 3, 4).

5.4. Depositional sequence 4 (DS4)

The fourth depositional sequence (DS4) is represented by Upper Turonian sediments of the uppermost part of the Maghra El-Hadida formation. This depositional sequence attains ~22m and ~4m at W. Um-Artah and W. El Dakhl sections, respectively.

The DS4 is bounded at base by sequence boundary SB Tu 2 and topped by sequence boundary SB Tu 3.  The SB Tu 2 in the study area is equivalent to Tu3 at European basins [41] and SB Tu 3 at the North Eastern Desert [21]. The boundary between DS 3 and DS 4 is marked by the vertical facies change from carbonate facies of the uppermost DS 3 to clastic facies of the lowermost DS 4. Also, the SB Tu 3 matches with Tu4 in the European basins [41] and SB Tu 4 at the North Eastern Desert [21].

The TST consists of ~8m of sandstone, nodular marl with limestone intercalations at W. Um-Artah section, and ~2m of limestone at W. El Dakhl section. It is topped by MFS which is marked by frequent occurrence of the ammonite Coilopoceras requienianum. The HST is composed of hard fossiliferous limestone and shale with limestone intercalations in W. Um-Artah section and limestone in the W. El Dakhl section. It attains ~14m at W. Um-Artah and ~2m at W. El Dakhl sections. The DS4 is topped by the SB Tu 3 which is marked by vertical facies change from marl or shale of the uppermost part of the Maghra El-Hadida formation (DS 4) to sandstones of the lowermost part of the Matulla formation.

6.  Conclusions

• The Upper Cenomanian-Turonian successions at Southern Galala Plateau, Eastern Desert, Egypt, have been studied in two exposures, namely: W. El Dakhl (northward) and W. Um-Artah (southward). Lithostratigraphically, these successions are subdivided into: Galala (Late Cenomanian) and Maghra El-Hadida (latest Cenomanian-Turonian).

• These successions are unconformably underlain by the Malha Formation of Lower Cretaceous-Middle Cenomanian age and unconformably overlain by the Matulla Formation of Coniacian-Santonian age. The shallow marine Galala Formation represents the first marine transgression over the study area, which consists of siltstone and silty marl with limestone intercalations topped by thick green shale intercalated with hard limestone at W. El Dakhl section and shales, limestone, siltstone and nodular marl with oyster limestone intercalations topped by hard silty limestone at W. Um-Artah section.

• Biostratigraphically, five ammonite zones are defined throughout the Upper Cenomanian- Turonian; Neolobites vibrayeanus and Vascoceras cauvini zones of Late Cenomanian; Vascoceras proprium and Choffaticeras (Ch.) segne zones of Early Turonian and Coilopoceras requienianum Zone of Late Turonian, while the middle Turonian is barren of ammonites. 

• Five sedimentary unconformity surfaces (Ce4, Ce5, Tu1, Tu2 and Tu3 sequence boundaries) have been recognized. These sequence boundaries outlined four third order depositional sequences (DS 1-4).

• These sequence boundaries are correlated with others in the Middle East, North Africa, and Europe regions. In comparison with the sequence boundaries at the European basins for example, the sequence boundary SB Ce 4 is mismatching with Ce 4 at the European basins. On the other hand, The SB Ce 5 matches with the SB Ce 5 at European basins. During the Turonian, the SB Tu 1 matches with Tu 1 at European basins. At the same time, The SB Tu 2-3 in the study area is equivalent to Tu 3-4 at European basins.