Mineralogy, geochemistry, petrography, and depositional environment of Gebel El-Qurn, Early Eocene, West Luxor, South Egypt

Geologic setting

The stratigraphic succession at Gebel El-Qurn, west Luxor, comprising the Esna Shale and Thebes Formations, reaches about 400 m thick (Fig. 2), which ranges from the upper Paleocene to the lower Eocene.

The Thebes Formation is about 350 m thick and commences with fine-grained micritic limestone and marl rich in bands and nodules of chert, probably deposited at bathyal depths (Snavely et al. 1978), and ends with repetitive skeletal limestones and oyster banks formed in a shallow water setting. Said (1962) classified the Thebes Formation into four members according to variations in lithology and weathering characteristics (Fig. 2).

Member IV was composed of about 250 m of an alternating series of dolomitic limestones, marls, shales, and nodular limestones with interbedded chert nodules and chert bands.

They are regarded as individual shallowing upward cycles of deposition. In addition, they are richly fossiliferous, with cephalopods (Nautilus sp.), gastropods (Turritella sp.), pelecypods (Ostrea sp.), and anodont bivalves (Lucina thebaica), and various nummulites and Operculina sp. They were deposited in shallow water (10–100 m) on an extensive carbonate platform (Wüst 1995).

Mineralogy

X-ray patterns revealed that the dominating minerals in the studied samples are mainly calcite as well as dolomite, quartz, halite, pyrite, and clay minerals. The EDX analysis data for the bulk samples show that this phase of dolomite is nearly stoichiometry (Mg: Ca ≈ 1:1) and the size of the dolomite crystals is up to 80 μm without any zoning. Also, present of halite which conformed with the X-ray analysis. Dolomite crystals are unimodal and polymodal; planar-e and planar-s as well as some are interlocked (Fig. 3). Also, the photo is showing patches from disorder from halite crystals, which is conformed with X-ray results.

Petrography

The faunal assemblage that characterizes the studied succession consists of forminifera, Nummulites, pelecypods, miliolids as well as echinods fragments, and others. Petrographically, five major microfacies were encountered in the studied section:

Nummulitic biomicrite wackestone/mudstone microfacies

This microfacies is the most prevailing microfacies in the studied section of Gebel El-Qurn. It consists mainly of 35% fossils and 4% quartz grains embedded in micrite and microsparite matrix. Some of the microcrystalline matrix is stained by iron oxy-hydroxides and exhibits re-crystallization to microsparite. Allochems are mainly represented by abundant nummulites species, Nummulite gizehensis and Nummulite discorbinus, as well as amounts of gastropods, pelecypods, miliolids, bryozoa, and others (Figs. 4 and 5). This microfacies shows considerable variation in the size of the allochems. The large nummulite size possibly indicates higher salinity and adequate paleoecologic conditions (Plumley et al. 1962) and presumably characteristic of warm open marine condition (Wilson 1975). The sand content is monocrystalline, fine to medium, subangular to subrounded quartz grains scattered in the matrix.

Nummulitic biosparite (grainstone) microfacies

The allochems content are represented by nummulites (50–70%), and other fossils are embedded in sparite matrix (grainstone). Nummulites vary from 10 to 25 mm in diameter (Fig. 6). The nummulites are leached and later on filled with sparry calcite or micrite, and some are stained with iron oxy-hydroxides. Those retaining their original fibrous structure are occasionally present.

Nummulitic rudstone

The rudstone microfacies is made up of nummulites (30 to 40%), foraminifera (3%), and quartz grains (5 to 6%). Allochems are embedded and cemented with sparry calcite (Fig. 7). Nummulites reach up to 35 mm in diameter. Some foraminifera and miliolids are enclosed in microsparite cement. The allochems (fossils) are mostly filled with sparry calcite, but a few miliolids and algae are micritized and have a micritic envelope (Fig. 8). Monocrystalline and polycrystalline, medium to coarse, subangular to rounded quartz grains are embedded in the matrix.

Pelecypod biomicrite (wacke/packstone) microfacies

This microfacies is present in the upper part of the studied section in Thebes Formation. The allochems are represented mainly by pelecypod shells (35%), and few algae and micritic peloids are present. This association is texturally considered as a biomicrite (wacke to packstone) in a matrix (Fig. 9). The sediments are loosely packed inferring that cementation occurred before significant compaction. Micrite envelope outlined the skeletal grains of pelecypods, with microsparite pore-filling.

Biomicrite dolomitic lime-mudstone microfacies

The fossils are only allochems in this microfacies (20%). They are mainly of nummulites species, miliolids, and other shell fragments scattered in the micritic matrix (Fig. 10). This microfacies is characterized with fine anhedral crystals, closed packing, and intercrystalline porosity.

Diagenesis

Carbonate rocks are particularly susceptible to diagenesis partly because minerals are more soluble in water and so are subject to dissolution and re-precipitation. The main diagenetic features encountered in the carbonate rocks of Geble El Gurna are described below:

Geochemistry

Chemical analysis data of major, trace, and rare earth elements are listed in Table 1. Normative calcite content varies from 57.49 to 91.76%, averaging 84.12%, while the normative dolomite content ranges from 1.67 to 8.05%, averaging 4.85%. Calcite precipitated from seawater would contain from 5.9 to 27.1% MgCO₃ by weight. While calcite exposed to meteoric water diagenesis will lose most of its magnesium content (Moore 1989; Morse and Mackenzie 1990). The studied rocks contain 4.76% MgCO_3, in average, indicating that the studied calcite subjected to progressive diagenesis, which led to loose most of its magnesium content under meteoric water. MgO content ranges from 0.8 to 3.85%. It seems more eligible that the dolomitization process is controlled by the fracture and joint systems in the area, which maintain access to the dolomitizing fluids. Positive correlation (r = 0.41) between Ca and Mg indicated calcite is present as calcite and dolomite (Table 2), which agrees with X-ray results.

SiO₂ content in the studied samples ranges from 1.25 to 5.33% expected samples number 2 and 10, which represented limestones with nodular flint. The highest SiO₂ value is detected at the top bed of the succession indicating an upward increase in the influx of terrigenous constituent of the depositional basin.

The carbonate rocks of the studied succession record a low content of Al₂O₃ and K₂O (average 1.64 and 0.03%; respectively). The low content of both Al₂O₃ and K₂O can mainly be attributed to the low clay content. Si/Al ratio is about 3 (without S. Nos. 2 and 10), suggesting a three-layer clay structure. Moreover, the preponderance of K indicates possible smectite-illite mixed layer.

Fe₂O₃ content of the studied carbonate rocks ranges from 0.64 to 0.91%, averaging 0.81% (Table 1). The low values for the Geble El Gurna carbonate rocks indicate the formation of dolomite in the near-surface oxidizing environment (Choquette and James 1990). Positive correlation (r = 0.42, Table 2) between Fe₂O₃ and MgO indicates that iron may be associated with dolomite. Also, the positive correlation between Fe₂O₃ and Sr (r = 0.38) indicates that iron oxy-hydroxides may be scavenge and uptake Sr, which is an essential function of many microbes. This is in agreement with Krumbein (1983), Abou El-Anwar (2005, 2011, 2014, and 2016), and Abou El-Anwar et al. (2017, 2018).

Trace element data of sedimentary carbonate rock can be used to constrain the chemical composition of ancient seawater (Veizer and Mackenzie 2014; Kamber et al. 2014). Sodium is present in the carbonate rocks sodium co-precipitated with the carbonate minerals, which gives an indication of salinity of water from which the original carbonates were deposited (Moore 1989; Morse and Mackenzie 1990). Na₂O content of the studied samples ranges from 0.1 to 0.12%, averaging 0.15%. Na content may be attributed to the presence of halite which is conformed with the X-ray results. There are positive correlation between Na and Cl (r = 0.55, Table 2), which indicate high salinity during precipitation, so the studied area effected by dry climate, in agreement with the petrographic study. Sodium occurs in modern carbonate sediments to levels of about 3000 ppm and progressively drops with diagenesis to 200 ppm in ancient limestone (Scoffin 1987). The sodium content of the studied carbonates ranges from 800 to 1394 ppm, averaging 1100 ppm. This means that there is loss in Na content during diagenesis of the studied carbonate rocks. Therefore, the diagenesis processes may have taken place in an environment less saline than seawater (Loukina and Abou El-Anwar 1994; Loukina et al. 2001; Abou El-Anwar 2006; Abou El-Anwar 2011). This agrees with the environments of dolomitization in mixed marine/meteoric water (Badiozamani 1973).

The strontium content for the majority of the ancient carbonate is less than 500 ppm (Bathurst 1975). The Sr-contents of Geble El Gurna ranges from 400 to 520 ppm, averaging 483 ppm, which indicated that the dolomite crystals deposited in mixing zone of marine and meteoric water (Table 1). The depletion in Sr content can be attributed to the long diagenetic history particularly the dolomitization stages. This agrees with the findings of Brand and Veizer (1980); Loukina and Abou El-Anwar (1994). Strontium concentrations can be utilized for understanding the nature of the dolomitize fluid. Remnant inclusions of precursor carbonate (calcite or aragonite) and distribution coefficient of Sr (Dsr) between dolomite and diagenetic fluids are generally the major factors controlling the Sr concentrations in dolomite (Budd 1997). Therefore, the dolomites reflect mostly the nature of the dolomitize fluid. Estimates of Sr distribution coefficient (Dsr) for dolomite vary from 0.015 to 0.06 (Vahrenkamp and Stewart 1990; Banner 1995; Budd 1997). If the Sr content were mainly due to Dsr and Sr/Ca of the dolomitize fluid, the latter can be calculated from equation (Sr/Ca) dolomite = Dsr (Sr/Ca) fluid. Such calculations yield molar Sr/Ca for the Geble El Gurn dolomitization fluid of 0.00031 to 0.000974 with average 0.000984. These values are significantly less than the Sr/Ca ratio of present day seawater (0.0086; Drever 1988). Thus, the average Sr value suggesting that we are dealing with Sr-depleted dolomitizing fluids, likely due to partial or full meteoric water contributions (Abou El-Anwar 2010, 2012; Abou El-Anwar and Mekky 2013).

However, post-depositional diagenetic and metamorphic processes may modify the mineralogical compositions, concentrations of mobile trace elements (Webb et al. 2009). The fluid-immobile elements, such as the rare earth elements (REEs) and high-field strength elements (HFSE), remain largely unaffected by diagenesis (Webb et al. 2009); thus, they are useful for recognize the ancient environmental conditions. The mean values of the ΣREEs for the studied samples are 936 ppm. The studied carbonate rocks showed significant enrichment of LREEs and a well-depleted pattern of HREEs (Table 1). The total concentration of total REEs in average bulk continental crust is about 125 ppm (Rudnick and Gao 2003). Thus, geochemical pattern of the studied samples mentioned that high solubility and mobility potential of LREEs compared with HREEs, resulting in good fractionation and enrichment of these elements in the depositional environment. So, the concentration of the (LREEs) is higher than that of (HREEs), which is in conformance to the general distribution of REEs in shale or limestone (Gromet et al. 1984; Condie 1991; Ketris and Yudovich 2009).

The concentrations of REEs in ocean waters reflect different inputs, such as river and eolian flux (Zhao et al. 2009), hydrothermal alteration of oceanic crust (Michard and Albarede 1986), and diagenesis (Derry and Jacobsen 1990). Thus, the study of REE concentrations of the studied carbonate rocks is a useful tool to define the depositional environment; fresh water or marine conditions (cf. Bolhar and Van Kranendonk 2007), hydrothermal input (cf. Derry and Jacobsen 1990; Allwood et al. 2010), and redox conditions (De Baar et al. 1988; Elderfield 1988; Kato et al. 2002). The primary distribution pattern of REEs in many carbonates has survived irrespective of their age, provenance, post-depositional processes, and metamorphic grade (Bau and Möller 1993; Webb et al. 2009).

Positive values of La and Ga are a common feature of seawater (Bau et al. 1997; Alibo and Nozaki 1999) which suggested that the higher stability of them in seawater relative to neighboring REEs during scavenging. Thus, La and Ga content indicated that the investigated carbonate rocks were deposited in marine environment. Metals may be depleted resulting to REE oxidation state and mobility under different oxidation-reduction conditions (Wang and Liang 2014; Abou El-Anwar et al. 2018). A positive relation between Fe with Hf, Br, As, I, Ba, Nd, and W indicates that iron is increasing the buffer capacity of the studied carbonate matrix; therefore, these REE may be co-precipitated with Fe.

Thus, light REEs are scavenged preferentially from the water condition compared to the heavy REEs (De Baar et al. 1985; German and Elderfield 1990; German et al. 1991). Thus, this process in the studied carbonate rocks is resulting for dissolved REEs in oxic seawater under redox behavior (De Baar et al. 1985, 1988; Byrne and Sholkovitz 1996).

The total radioactivity measurements are ranging from 4 to 5.9 ppm for U and from 5.5 to 6.6 ppm for Th. The radioactivity measurements are less than the background level of carbonates, and they are in the allowed limits for carbonate rocks and representative of less contaminated samples. Thus, the investigated carbonate rocks can be used in cement industries and as building stones (cf. Abou El-Anwar et al. 2017; Abou El-Anwar 2018).