Introduction

The black shales have attracted the interest of many researchers primarily because of their economic importance in terms of hydrocarbon development potential (i.e., source rocks) as well as metal concentrations (Wignall 1993; El Kammar 1993; Armstrong-Altrin et al. 2013). Several black shales are particularly metalliferous, mostly enriched in U, V, Cu, Ni, and Zn (Schultz 1991). The black shale could be used also as an energy donator in an electric power plant and/or in production of cement. The composition of the inorganic fraction may vary from a shale where clay minerals are predominant, to carbonates with subordinate amounts of clay and other minerals. The organic fraction is mainly an insoluble solid material (kerogen) which is entirely comparable to the organic matter present in many petroleum source rocks. The abundance of organic rich layers was confirmed in place because parts of the Duwi mine are still burning after they caught fire some years ago. The organic matter is derived from dominating marine organisms under strong reducing conditions (El Kammar et al. 1990; El Kammar 1993).

The black shales in Egypt belong to two stratigraphic formations namely Duwi at the base overlain by the Dakhla of Campanian–Maastrichtian age. Both formations persistently extend along the southern escarpments which bound the Western Desert from Dakhla to Kharga Oasis, to south Kharga at Sinn El-Kaddab escarpment, further to the western reaches of Kom-Ombo, then traced northward along the Nile Valley from Idfu to wadi Qena and the Galala plateau, as well as along the Red Sea Coast between Quseir and Safaga forming a thick belt.

Black shales are argillaceous sediments containing high percent of total organic carbon. Their depositional controls are not clear, but are widely viewed as a complex interplay between surface water productivity, rates of oxidation of organic matter, and/or dilution by non-organic material (Bottcher et al. 2006). The mineralogy and chemistry of black shales have been affected, therefore by numerous factors including composition of source material, duration of weathering, transportation mechanisms, bulk sediment accumulation rate, prevailing organisms, chemical composition of water, and diagenesis (Loukola-Ruskeeniemi 1991; Hayashi et al. 1997; Armstrong-Altrin et al. 2004, 2013). Shales that are the most abundant type of detrital sediments in sedimentary basins (Pettijohn 1975) are represented by the average crustal composition of the provenance area much better than any other siliciclastic sedimentary rocks (Nagarajan et al. 2007), since they preserve the original signature of the source rocks and diagenetic history (Mondal et al. 2012). Chemical composition of the clastic sediments provides important information on provenance, weathering conditions, and sediment recycling (Basu et al. 2016; Borghesi et al. 2016; Campodonico et al. 2016; Selvaraj et al. 2016; Tawfik et al. 2016; Tobia and Shangola 2016; Armstrong-Altrin et al. 2015a, b). Also, the geochemical parameters have been used by various authors to interpret the paleoenvironment and paleooxygenation conditions of ancient shales (Nagarajan et al. 2007; Abou El-Anwar et al. 2017).

Significant contributions have been made by several studies in relation to the regional geology, sedimentology, mineralogy, geochemistry, and economic potentiality of the Upper Cretaceous Duwi black shales exposed at Western Desert district, Nile Valley district, and the Red Sea coastal zone (Sediek and Amer 2001; Ghandour et al. 2003; Ibrahim et al. 2004; Schulte et al. 2011, 2013; El Kammar 2014; Ghanem et al. 2016; Abou El-Anwar and Gomaa 2016; Abou El-Anwar et al. 2017). In contrast, no detailed studies that focused on provenance, paleoweathering, and paleoenvironments of the Upper Cretaceous Duwi black shales have been documented. Therefore, the present work aims to examine the mineralogy and geochemistry of the Upper Cretaceous Duwi black shales at five sites at El-Nasr company open-pit exploited phosphate mines, Nile Valley district (Kom-Mir, El Sebaiya, Um Salamah, Badr-3, Elgididh-6, Figs. 1 and 2) in Aswan Governorate, Egypt, to identify the source rock characteristics, palaeoweathering, and paleoenvironment of the source area.

Fig. 1
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Geological map of the study area showing locations of studied sites (modified after Conoco 1987)

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Fig. 2
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Correlation chart of the Duwi Formation at the studied locations: (1) Kom-Mir, (2) El Sebaiya, (3) Um Salamah, (4) Badr-3, and (5) Elgididh-6, Aswan Governorate, Egypt

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Geological setting

The study area lies between longitudes 32° 30′–32° 50′ E and latitudes 25° 05′–25° 30′ N, on the southwestern side of the Nile Valley (Fig. 1). The stratigraphic succession is of Late Campanian–Early Paleocene age and represents a part of the dominated sedimentary deposits of black and variegated shales that are widely distributed in Upper Egypt. This succession laterally extends from the New Valley in the Western Desert to Safaga–El Qusier region on the Red Sea Coastal plain through Aswan at Nile Valley. The study area contains occurrences of extensive black shale beds of Duwi Formation (Late Campanian–Early Maastrichtian) that overlie a fluvial variegated shale sequence in Quseir member of the older Nubia Formation, with an undulating erosion contact (El-Azabi and Farouk 2010). The entire succession is conformably overlained by the deeper marine laminated gray to black shales of the Dakhla Formation (Late Maastrichtian–Early Paleocene) (Fig. 2).

The Quseir shale member is composed of different variegated shales, reddish brown, gray, and yellow in color. The Duwi Formation composes of a lower phosphorite member represented by phosphorite bed intercalated with thin lenses of gray shales of thickness ranging between 0.2 and 0.6 m averaging 0.4 m, and the middle shale member is mainly composed of gray shale, cracked, and filled with gypsum. Meanwhile, the Upper Phosphorite Member is composed of thin phosphatic beds. The Dakhla Formation is composed of gray to black shale, papery, cracked, and filled with gypsum and ferruginous stains. Obviously, the deposition represents an initial stage of the Late Cretaceous marine transgression in Egypt (Glenn and Arthur 1990; El-Azabi and Farouk 2010).

The northwestern margin of the Red Sea consists of Precambrian crystalline basement together with Mesozoic, Cenozoic pre-rift sediment, and the late Oligocene–Miocene to recent syn-rift sediments (Said 1990). The pre-rifting (Late Cretaceous–Middle Eocene) deposits of a 500–700 m thick occupy the troughs of synformal-like folds within the basement hill ranges. The lower part of the pre-rift section is the 130-m massive thick-bedded siliciclastic Nubia Formation which overlain by 220–370-m-thick sequence of interbedded shales, sandstones, and limestones of Quseir, Duwi, Dakhla, and Esna formations (Khalil and McClay 2009). The uppermost pre-rift strata consists of 130–200 m of competent, thick-bedded limestones and cherty limestone of the lower to middle Eocene Thebes Formation. The marine upper Eocene and Oligocene deposits are absent, indicating that the region must have undergone elevation changes during these two epochs (Said 1992). The late Oligocene to recent syn-rift strata unconformably overlie Thebes Formation and vary in thickness from less than 100 m on shore to as much a 5 km in offshore basin (Heath et al. 1998). Faulting with a dominant NW trend is the main feature in the region and forms complicated horsts and grabens with outcropping basement rocks covering the major part of this region.

Samples and methodology

Twenty-five representative samples of Duwi black shale were collected from the Duwi phosphate mines belonging to El Nasr mining company namely Kom-Mir, El Sebaiya, Um Salamah, Badr-3, and Elgididh in Aswan Governorate, Nile Valley district (Fig. 1). The petrographic characteristics of 10 selected samples were investigated by thin section microscopic observations. The mineralogy of all Duwi black shale samples was determined by X-ray diffraction (XRD) analysis using both smear-on glass slide and powder press techniques (Hardy and Tucker 1988). The analysis was done by a Philips X-ray diffractometer model PW/1710 (CuKα radiation with 40 kV, 35 mA, and 2°–70° 2-theta). Clay minerals were identified by their characteristic reflections (Moore and Reynolds Jr 1997). Also, the samples were scanned by scanning electronic microscope (SEM, 3.5 nm of resolution) equipped with energy-dispersive spectrometer (EDS), to determine the chemical composition during SEM observations. XRD and SEM-EDS analyses were performed at the laboratory of the Central Metallurgical Research and Development Institute, Egypt.

Twenty-five shale samples were grinded to < 63 μm and dried at 110 °C and treated with lithium metaborate and tetraborate to make pressed powder pellets. They were analyzed using X-ray fluorescence Pnalytical Axios Advanced XRF equipment for major and trace element geochemistry. XRF analyses were performed at the laboratory of the Central Metallurgical Research and Development Institute, Egypt. The accuracy of the analytical method was evaluated using the standard MESS-3 and was 100 ± 3% for all oxides and trace elements. Loss on ignition (LOI) was estimated by heating sample at 1000 °C for 2 h. Major element data were recalculated to an anhydrous (LOI-free) basis and adjusted to 100% before using them in various diagrams. The total iron is expressed as Fe2O3. The correlation coefficient has been carried out for the chemical data by using the Davis method (1986).

Results

Petrography

The shales consist mainly of foraminiferal argillaceous matrix (Fig. 3a). In some samples, the argillaceous matrix is color-laminated (Fig. 3b). This lamination resulted from the alteration of organic matter-rich and iron oxide-rich laminae. Sand-sized grains (average 31%, Table 1) are embedded in argillaceous materials (Fig. 3c, d). Quartz grains are generally fine to very fine, angular to subangular, monocrystalline, poorly sorted, and exhibit either uniform or undulose extinction (Fig. 3c, d). Feldspars (average 4%, Table 1) are represented mainly by Na-plagioclase (albite) and K-feldspar (microcline). Iron oxides (average 1%, Table 1), are formed of very fine material or dark patches replacing the clay matrix (Fig. 3e). This indicates that these iron oxides are authigenic and were either precipitated by moving fluids or resulted by the degradation and breakdown of iron-rich minerals and detrital ferromagnesian silicates.

Fig. 3
figure 3

Photomicrograph and SEM of the Duwi black shales showing a highly foraminiferal argillaceous matrix with thin bands of gypsum/anhydrite (G), a laminated foraminiferal shale (note minor scattered silt-sized quartz grains (Q) in argillaceous matrix); b fine to very fine quartz grain (Q) embedded in argillaceous matrix (note rhombic dolomite crystals (D) and aggregates of framboidal pyrite (Py); c quartz grains (Q) embedded in argillaceous matrix (note pore-filling euhedral pyramidal syntaxial overgrowth (QO); d feldspar grains embedded in calcite (Ca) cement and argillaceous matrix (note feldspar overgrowth (FO)); e argillaceous matrix are partially replaced by iron oxides (I); f detrital particles of smectite (Sm); g detrital platelets of kaolinite (K) (note irregular edges of smectite (Sm); and h detrital particles of smectite (Sm) and kaolinite (K) embedded in calcite (Ca) cement

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Table 1 Semi-quantitative mineralogical composition of bulk studied samples
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Mineralogy

The XRD results indicate that the Upper Cretaceous Duwi black shales consist mainly of quartz, calcite, smectite and kaolinite clay minerals, phosphate, dolomite, feldspar, with little gypsum, anhydrite, iron oxides, and pyrite (Table 1; Fig. 4). Iron oxides (mainly hematite) are recorded in Sebaiya, Badr-3, and Elgididh-6 black shales (Table 1). It increases from east to west. Smectite and kaolinite (average = 20.48%) constitutes the most abundant clay minerals. Their values increase from the west (Badr-3, Elgididh-6, and Um Salamah) to the east (Sebaiya). The bulk rock mineralogy of the shales indicates the presence of phyllosilicates with abundant quartz, calcite, phosphate (fluorapatite), dolomite, and feldspar. Quartz is the most abundant non-clay minerals (average = 31%) followed by calcite (average = 20.9%), fluorapatite (average = 10.4%), dolomite (average = 6.9%), feldspar (average = 4%), gypsum (up to 3.5%), anhydrite (up to 1.5%), iron oxide, and pyrite (up to 1.5%). The mineralogical composition indicates a high MMI (mudrock maturity index; MMI = 100 × phyllosilicates / (phyllosilicates + quartz + feldspars; Bhatia 1985), with an average value of 36 (Table 1).

Fig. 4
figure 4

X-ray diffraction patterns of the Duwi black shale samples at (1) Kom-Mir (sample KM5), (2) El Sebaiya (sample Sb3), (3) Um Salamah (sample US5), (4) Badr-3 (sample B2), and (5) Elgididh-6 (sample Eg5), Aswan Governorate, Egypt

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Clay minerals

Smectite (Fig. 3c–e, g–i) constitute the dominant clay mineral content of the studied shale samples. It ranges from 0.8 to 63.7% (Table 1). The abundance of smectite, the low content of kaolinite, the presence of pyrite rhombs, and the complete absence of illite in the shales refer to a deposition under fluvio-marine environment under alkaline-reducing conditions (Sediek and Amer 2001). Also, the absence of any volcanic precursor, such as tuff or glass in the studied shales, proves that smectite is mainly of detrital origin. Generally, the clay mineral associations in the studied shales with its smectite dominance and low kaolinite suggest a terrestrial provenance that are degraded from chlorite and illite after deposition and had not attained intensive weathering, under a warm and semi-arid climate, and the resulted materials were carried by fluvial action, which finally interfered and admixed with marine environments (marginal marine, low energy, and reducing conditions of Duwi Formation) (Hendriks et al. 1990).

Kaolinite (Fig. 3h, i) constitutes the second dominant clay mineral in the studied black shale of Duwi Formation. It ranges from 0.2 to 1.3% (Table 1). Kaolinite in the marine deposits of Upper Cretaceous age are products of terrestrial weathering and represent continental products of a warm and at least seasonally humid climate, being eroded and transported toward the sea by rivers (Hendriks et al. 1990; Hallam et al. 1991). In addition to a detrital origin, smectite and kaolinite may also develop by diagenetic processes due to the circulation of acid solutions (Ghandour et al. 2003).

Non-clay minerals

Quartz (avg. = 31%, Table 1) was present as an important non-clay mineral in shales. Quartz grains (Fig. 3c, d) are fine to very fine grained, angular to subangular, mostly monocrystalline, with normal and undulose extinction.

Carbonate minerals detected are calcite and dolomite. Calcite (Fig. 3e, i) was recorded in all studied sites except El Sebaiya (Table 1). It ranges from 0.0% to about 94%. On the other hand, dolomite (Fig. 3c) was recorded only in shales of Kom-Mir and Elgididh-6 phosphate mines. It reaches up to 76% (Table 1). The presence of calcite and dolomite in shales in addition to the presence of foraminifers’ fossils may indicate the deposition of this formation in a marine environment.

Sulfates detected by XRD are gypsum and anhydrite (Table 1 and Fig. 3a). Anhydrite was found only in El Sebaiya black shale, while gypsum was recorded in all studied sites except Elgididh-6 black shale. The anhydrite content reaches up to 13%, while gypsum content reaches up to 45%. The evaporites form from saline-rich fluids-brines. Brines may be generated by concentration of sea water, by evaporation or freezing, or as residual connate fluids in the subsurface (Selley 1988).

Anhydrite and gypsum was probably syn-precipitated under reducing conditions primarily as sulfides with shale. After shale compaction, the sulfides oxidized to sulfates as a result of biogenic activity; the sulfates react with the calcic cement (produced by weathering of carbonate) to form gypsum. Due to the compaction of clays, the sulfates will be expelled and concentrated along the bedding planes as gypsiferous bands which can be seen in the field.

Phosphate minerals encountered in Duwi black shales are carbonate fluorapatite (francolite). It is recorded in all studied sites except El Sebaiya. It varies from 0 to 69% (Table 1). The carbonate and phosphate minerals, in the shales, are indicating their deposition in a marine environment (Temraz 2005).

Feldspars detected by XRD are represented mainly by Na-plagioclase (albite) and K-feldspar (microcline) in the shales (Fig. 4). It varies from 0 to 30% (Table 1). Microcline is relatively more common than plagioclase. The absence of feldspar in most samples may indicate intensive degree of chemical weathering.

Iron oxides detected by XRD in Duwi black shale samples are represented mainly by hematite (Fig. 4). The hematite content reaches up to 15% (Table 1). It is commonly present as very fine material replacing the clay matrix or as dark patches (Fig. 3f). Iron oxides can be formed either by pedogenic processes or by the precipitation of iron oxides from laterally flowing surface or groundwater (Ollier and Galloway 1990).

Pyrite spheres and framboids were recorded in most samples of the Duwi Formation except El Sebaiya black shale (Table 1 and Fig. 3c). It is recorded only in Um Salamah, Badr-3, and Elgididh-6. The pyrite content reaches up to 4% (Table 1). The presence of pyrite in shales indicates the prevalence of a reducing environment during deposition. The reduction of sea water sulfate bacterial activity leads to pyrite formation in sediments (Berner 1982).

Major and trace element concentrations

The major element concentrations of the black shales of the Upper Cretaceous Duwi Formation are reported in Table 2. SiO2 is the dominant constituent of the Duwi black shales in all studied sites (avg. = 41.69%, Table 2). Al2O3 (avg. = 12.12%) and Fe2O3t (avg. = 10.93%) are the second abundant elements in studied samples. The SiO2/Al2O3 ratios vary from 5.05 and 10.8 (avg. = 7.32) higher than that for pure kaolinite (1.18) and smectite (2.81–3.31). This indicates that the shales consist mainly of a mixture of smectite and kaolinite. Cao (avg. = 5.89%) and P2O5 (avg. = 2.83%) are the third abundant elements in shale samples (Table 2). The average values of Cao and P2O5 are relatively comparable to the average marine black shales of Nile Valley (Temraz 2005). The shale samples are less abundant in K2O and MgO contents and are depleted in Na2O, TiO2, and SO3. Depletion of Na2O in shales suggests either lesser amount of plagioclase detritus in the shales and/or comparatively intense chemical weathering at the source and during fluvial transportation of the detrital material of the shales. Depletion of TiO2 and low K2O content indicates the presence of relatively lesser quantities of phyllosilicate minerals in the shales (McCann 1991; Condie et al. 1992; Armstrong-Altrin 2015). The SEM-EDS study reveals that the Duwi black shale samples are rich in Si, Al, K, and Fe contents, which may suggest the abundance of quartz and clay minerals (Fig. 5). On the SiO2/Al2O3 versus Fe2O3/K2O geochemical classification diagram, the shale samples are classified mainly as Fe-shale (Fig. 6; Herron 1988), which is also consistent with the petrographic data.

Table 2 Major element composition (wt%) of Duwi black shales, Aswan Governorate, Egypt
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Fig. 5
figure 5

SEM and EDS spectrum for the Duwi black shale samples. a El Sebaiya (sample US1). b Elgididh-6 (Eg5). Arrow mark shows the points where the EDS analyses were performed

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Fig. 6
figure 6

Geochemical classification diagram using log (SiO2/Al2O3)–log (Fe2O3/K2O) (after Herron 1988)

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The trace element concentrations of the Upper Cretaceous Duwi black shales for the five investigated sites are listed in Table 3. Figure 7a, b shows the distribution of major and selected trace element contents of the shales normalized to PAAS (Post Archaean Australian Shale; Taylor and McLennan 1985). Compared to PAAS, the Upper Cretaceous Duwi black shales are highly enriched in SiO2, Al2O3, CaO, Fe2O3, P2O5, Sr, Ba, V, Ni, Cr, Zn, Cu, and Rb and slightly enriched in MgO, K2O, U, Th, and Cd and Sc, Zr, Pb, and Co contents and highly depleted in TiO2, Na2O, Hf, and Cs content. The similar trace element content in shales may be related to the same source rock compositions. Strontium is the most abundant trace element in the studied samples (average = 600 ppm). The high Sr content may be due to the presence of aragonitic fossils and shells. Vanadium is the second abundant trace element in black shale samples. The concentration of V (avg. = 376 ppm) is more than those of Turekian and Wedepohl (1961) and Vine and Tourtelot (1970) may be due to oxidation and weathering of the organic matter. Zinc is the third abundant trace element in studied shales (255 ppm). The studied Duwi black shale samples also contain high concentrations of Cr, Ba, Zr, Cu, and Ni (158, 156, 153, 48, and 47 ppm, respectively). This enrichment is due to the contribution by intermediate and mafic source rocks.

Table 3 Trace element composition (ppm) of the studied samples
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Fig. 7
figure 7

Distribution of PAAS normalized abundance of Duwi black shales. a Major elements. b Trace elements

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The positive correlation between Al2O3 and SiO2, TiO2, and Zr (r = 0.59, 0.69, and 0.55, respectively, n = 25, Table 4) can be explained by a terrigenous origin. This may be due to the presence of a considerable amount of detrital clays. There is a weak positive correlation between Fe2O3 and the SiO2, Al2O3, MgO, TiO2, Na2O (r = 0.18, 0.15, 0.17, 0.21, and 0.55, respectively, Table 4) and a significant correlation between Fe2O3 and Ni (r = 0.82). This may be due to the association of Fe3+ with clay minerals. K2O shows a positive correlation with SiO2, Al2O3, and TiO2 (r = 0.51, 0.30, and 0.49, respectively). This indicates the association of K2O with aluminosilicate phases.

Table 4 Values of Pearson’s coefficient of correlation of major and trace elements in Duwi black shales
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Discussion

Source area weathering

The intensity and duration of weathering in clastic sediments can also be assessed by examining the relationship between alkali and alkaline earth elements (Nesbitt and Young 1982). The degree of weathering is quantified by various methods. The commonly used weathering indices like chemical index of alteration (CIA; Nesbitt and Young 1982), chemical index of weathering (CIW; Harnois 1988), and plagioclase index of alteration (PIA; Fedo et al. 1995) are widely used to deduce the intensity of the source area weathering (Armstrong-Altrin and Machain-Castillo 2016; Zaid 2017a, b; Abou El-Anwar et al. 2017). According to Fedo et al. (1995), the CIA, CIW, and PIA values of ~ 60 indicates low weathering, ~ 60–80 moderate weathering, and more than 80 indicate intensive weathering. The CIA values of the Upper Cretaceous Duwi black shales varies from 82 to 86% (avg. = 84%). The average CIA is higher than the PAAS values (70–75; Taylor and McLennan 1985), suggesting an intensive degree of chemical weathering in the source area. The PIA values range from 92 to 95% (avg. = 94%) indicating a high degree of alteration of source material. CIW values range from 93 to 96% (avg. = 95%) also indicate high degree of weathering. Based on CIA, PIA, and CIW values (> 80%), it can be concluded that the litho-components in shales were subjected to intense chemical weathering and reflect warm/humid climatic conditions in the depositional basin (McLennan et al. 1993).

The maturity of shale can be identified by the ratio values of SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O, which increase when maturity increases. The ratio values > 6, > 5, and < 0.5, respectively, may indicate high maturity of sediments (Cox et al. 1995). The SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O ratio values of the Duwi black shales in all studied sites (~ 5.0–10.8, ~ 13.4–23.6, and ~ 1.7–2.8, respectively) suggest moderate compositional maturity.

The index of compositional variability (ICV; Cox et al. 1995) is considered as an important parameter to evaluate the maturity of sediments (Cullers 2000). ICV values are high (> 1) for detrital ferromagnesian minerals and feldspars and low (< 1) for the clay minerals subjected to intensive weathering processes (Cox et al. 1995; Cullers 2000). The ICV values of the Duwi black shales range from 0.92 to 1.23% (average = 1.13 ± 0.06, n = 25) are slightly more than 1 (Table 2), indicating the presence of detrital ferromagnesian minerals beside the abundant clay minerals. Compositionally submature to mature sediments with slightly high ICV values are partially recycle deposits derived by intensive weathering of source rocks (Zaid 2017a, b).

Provenance

The chemical analyses of the clastic sediments has been widely used to identify the source rock characteristics (Armstrong-Altrin et al. 2015a, b; Abou El-Anwar and Gomaa 2016; Zaid 2017a, b). In order to infer the provenance of siliclastic rocks, several major and trace element-based discrimination diagrams have been proposed by many investigators (Roser and Korsch 1988; Floyd et al. 1989; McLennan et al. 1993; Verma and Armstrong-Altrin 2013, 2016; Verma et al. 2016). On the major element-based provenance discrimination diagram of Roser and Korsch (1988), the studied Upper Cretaceous Duwi black shales plot exclusively in the quartzose sedimentary provenance fields (Fig. 8). The Al2O3/TiO2 ratio and SiO2 contents can be used to identify the source rock (Hayashi et al. 1997). The Al2O3/TiO2 ratio of the Duwi black shales vary from 9.92 to 49.6 (avg. 23.6), and SiO2adj contents vary from 51.0 to 55.7 wt% (avg. 53.47 wt%). These values suggest that the source rocks for the shales are intermediate and mafic igneous rocks. K2O/Na2O ratio can be considered as a simplified chemical provenance indicator (Potter 1978). The K2O/Na2O ratio (avg. = 2.52, Table 2) reflects derivation from intermediate or basic rocks.

Fig. 8
figure 8

Provenance discrimination diagram for shales (after Roser and Korsch 1988). Discriminant function 1 = (− 1.773 × TiO2%) + (0.607 × Al2O3%) + (0.76 × Fe2O3T%) + (− 1.5 × MgO%) + (0.616 × CaO%) + (0.509 × Na2O%) + (− 1.22 × K2O%) + (− 9.09). Discriminant function 2 = (0.445 × TiO2%) + (0.07 × Al2O3%) + (− 0.25 × Fe2O3T%) + (− 1.142 × MgO%) + (0.438 × CaO%) + (0.432 × Na2O%) + (1.426 × K2O%) + (− 6.861)

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The concentration of the transition trace elements can be used to identify the composition of the source rocks (Garver et al. 1996; Armstrong-Altrin 2009). The studied samples have higher contents of some transition metals such as V, Zn, and Cr (average of 376, 255, and 158 ppm, respectively) and slightly depleted in some others such as Cu, Ni, and Co (average of 48, 47, and 9 ppm, respectively) compared with PAAS. Figure 7 indicates that the source area was dominated by rocks of intermediate type with subordinate of mafic composition. The positive correlation between Cr and Ni (r = 0.98, Table 4) and the Cr/Ni ratios (3.86) revealed that few mafic components from the basement source rocks accumulated during weathering. They are probably derived from the basement terrains in the Red Sea Hills and transported via Wadi el-Shaghab and Wadi el-Miyah (Fig. 1).

The trace element ratios such as La/Sc, La/Co, Th/Sc, Th/Co, and Cr/Th values of the Upper Cretaceous Duwi black shale are compared in Table 5, with those of sediments derived from mafic and felsic source rocks (Cullers 1994, 2000; Cullers and Podkovyrov 2000; Taylor and McLennan 1985). These ratios of the studied shale samples fall within the range of sediments derived mainly from intermediate and mafic igneous source rocks (Table 5). The Precambrian gabbro and metagabbro–diorite complex could have been the main source rocks with subordinate recycled Upper Cretaceous Nubia Formation for Duwi black shales.

Table 5 Range of elemental ratios of Duwi black shales in this study compared to the ratios in similar fractions derived from felsic, mafic rocks, and upper continental crust
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Depositional environment

The chemical analysis data can be used to evaluate the paleo-redox conditions of siliciclastic rocks during sedimentation (Armstrong-Altrin et al. 2015a; Armstrong-Altrin and Machain-Castillo 2016). The high enrichment of Sr, V, Ni, Cr, Zn, and Cd in the investigated Duwi black shale samples is comparable with those values of modern black shales directly due to mafic input. The average value of Fe2O3/TiO2 ratio is 21.42, which is consistent with mafic input as well (Abou El-Anwar et al. 2017). In addition, the V–Al2O3 and P2O5–Al2O3 bivariate diagrams can be used to discriminate depositional environments of shales. The V–Al2O3 bivariate plot (Fig. 9) shows that most of studied shale samples plot on shallow marine depositional environments.

Fig. 9
figure 9

Plot of V versus Al2O3 in the Duwi black shales for paleoenvironmental reconstructions

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The Ni/Co ratio values can be used as an indicator for paleo-redox conditions. The values of Ni/Co ratios below 5 indicate oxic environments, whereas values of this ratio above 5 suggest suboxic and anoxic depositional environments (Jones and Manning 1994). Ni/Co ratio of the studied black shales varies from 5.0 to 12.25 (avg. = 7.1, Table 3) suggest that the deposition of these shales is performed under suboxic and anoxic conditions. The values of V/Cr ratios are also considered as an indicator of paleo-redox conditions. The V/Cr ratio values ranges from 1.28 to 6.5 (avg. = 2.7, Table 3) which revealed that it was deposited under anoxic conditions (Hallberg 1976). The average value of Mo/Al2O3 ratios of the investigated samples is 0.5, which is slightly higher than the PAAS shale standard value (0.1; Taylor and McLennan 1985), suggest anoxic conditions for studied shales. Such conditions can only result from anaerobic bacterial activity.

Conclusions

The Duwi black shales are classified mainly as Fe-shale. Compared to PAAS, the shales are highly enriched in SiO2, Al2O3, CaO, Fe2O3, and P2O5 and highly depleted in TiO2 and Na2O content. The positive correlation between Al2O3 and SiO2, TiO2, and Zr indicate a considerable amount of detrital clays. K2O shows a positive correlation with both SiO2 and Al2O3, which indicates the association of K2O with aluminosilicate phases.

The CIA, CIW, and PIA values of the Duwi black shales suggest recycle deposits and intensive weathering in the source region and reflect warm/humid climatic conditions in the depositional basin. The SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O ratio values of the Duwi black shales in all studied sites suggest moderate compositional maturity. The ICV values of the Duwi black shales (avg. = 1.13) are slightly more than 1, indicating the presence of detrital ferromagnesian minerals beside the abundant clay minerals.

The geochemistry results indicate that the Duwi black shales were mainly derived from the Precambrian gabbro and metagabbro–diorite complex with subordinate recycled sedimentary rocks (Nubia Formation). The Fe2O3/TiO2, Ni/Co, V/Cr, and Mo/Al2O3 ratio values and the high concentration of Sr, V, Ni, Cr, Zn, and Cd in the investigated Duwi black shales indicate that the shales were deposited under anoxic reducing marine environments.