Geochemical analysis for evaluating the paleoweathering, paleoclimate, and depositional environments of the siliciclastic Miocene-Pliocene sequence at Al-Rehaili area, Northern Jeddah, Saudi Arabia

Geochemical data and their various approaches are useful to evaluate the climatic control on the depositional environments. This study aims to evaluate the paleoweathering and plaeoclimate condition that have controls on the depositional environments of the Miocene to Pliocene siliciclastic sequence at Al-Rehaili area, north of Jeddah. To achieve this aim, selected sandstone samples were geochemically (major and trace elements analysis) and petrographically examined. The results of these analyses reveal that the examined sandstones were deposited in non-marine (fluvial/alluvial-lacustrine) environments and suffered from weak to intermediate chemical weathering and intense physical induration under semi-arid to semi-humid climatic conditions.


Introduction
Geochemical analysis with different techniques is very significant in evaluating the paleoweathering and the climatic control on depositional environments (Obasi and Madukwe 2016;Madukwe et al. 2016). To better understand the erosional processes and weathering in the source areas, the mineralogical and chemical compositions of clastic sedimentary rocks are essential to be studied. The lithology, mineral composition, and the chemical weathering of the source areas along with the postdepositional alteration during the diagenesis are the main controlling factors that govern the geochemistry of clastic sedimentary rocks (Johnsson 1993;Armient et al. 1998). In addition, these factors are generally controlled by the tectonic, base-level fluctuations, climatic changes, and sediment caliber (Álvarez 2005). Therefore, the major and trace elements can be used to identify the climatic control on clastic sediments during deposition and weathering (Barbera et al. 2006).
Several scattered outcrops of the Miocene-Pliocene siliciclastic successions are exposed in the northeastern part of Jeddah at Al-Rehaili area alongside the coastline plain of the Red Sea. The composition of the Red Sea syn-rift clastic sedimentary rocks is controlled mainly by tectonic, sea-level fluctuation, and climate. Although the lithology and depositional environments of these deposits have been studied and determined by Ghandour and AL-Washmi (2013) and Alqahtani and Khalil (2015), very few studies have used the geochemical data to evaluate the factors that control the formation and depositions of these deposits using geochemical analysis (Alqahtani and Khalil 2019). Alqahtani and Khalil (2019) used geochemical data to determine the tectonic control on depositing the clastic sedimentary rocks alongside the Red Sea. Similar results are also presented by Ghandour and AL-Washmi (2013). The present study aims to examine the climate control, weathering, and depositional environments of the Miocene Pliocene succession (Bathan Formation) exposed at Al-Rehaili area using geochemical data.

Geological setting
The study area is situated in Al-Rehaili region, the northeastern part of Jeddah. It lies at latitude of 21°1′ N and longitude 39°01′ E (Fig. 1a, b). The Red Sea is interpreted as a young rift system and the sequences within this rift are subdivided into two main subdivision: (1) Pre-rift successions, which are interpreted as continental/fluvial clastic deposits, wherever the oldest deposits are found in Salaman Gulf, a few miles from Jeddah to the north; and (2) syn-rift successions, which are dominated by thick evaporite-siliciclastics sequences (Schmidt and Hadley 1984). The tectonic and structural settings of the Red Sea rift are extensively studied (Townsend et al. 2009;Xu et al. 2012).
The present study concentrates on the Bathan Formation, which is 100 m thick and contains shale, fine grained, and medium-to coarse-grained sandstones with dispersed pebbles and gravels. It is interpreted to be deposited in non-marine (fluvio-lacustrine) environments (Ghandour and AL-Washmi 2013;Khalil 2015, 2019). Moreover, the Bathan Formation is believed to be deposited during the major rifting phase, which created horst graben structure. This phase records the first major rapid uplift of the Red Sea escarpment (Ghandour and AL-Washmi 2013). The age of Bathan Formation was difficult to be precisely determined due to the lack of biostratigraphic data. However, the exposed successions are unconformably overlain by basaltic rocks of Harrat Rahat. These basaltic rocks are believed to be 5-11 Ma old (Chen et al. 2016;Moufti and Németh 2016). Moreover, Schmidt and Hadley (1984) interpreted equivalent succession alongside the coastal plain of the Red Sea to the north of Jeddah to be 12 to 15 Ma old. Therefore, and based on its stratigraphic situation, the Bathan Formation is interpreted to be deposited during Miocene-Pliocene. In addition, Alqahtani and Khalil (2015) divided the non-marine successions of Bathan Formation into three main sedimentary units (units I,II,and III;Fig. 2). Similar units and deposits are also found and described by Ghandour and AL-Washmi (2013). Alqahtani and Khalil (2015) created several stratigraphic composite columnar sections covering four outcrops at Al-Rehaili area (Fig. 2). These stratigraphic columns were correlated throughout the study area using lateral tracing, soil zones, and the marked erosional surfaces. In addition, the lithology, sedimentary facies, and depositional environments of the main sedimentary units were described and interpreted. Furthermore, Alqahtani and Khalil (2019) collected 17 samples from several sandstone beds and units for geochemical analysis (Figs. 2, 3 and 4). These samples were crushed for 60 s using Herzog Gyromill (Simatic C7-621). In addition, 2-mm thick pellets  Fig. 1) showing the sandstone samples (black dots) taken for geochemical analysis (adopted from Alqahtani and Khalil, 2015) were prepared from the pulverized sample by grinding 20 g of it with 0.4 g of stearic acid for 60 s and were loaded into each sample holder of the X-ray machine (Phillips PW-1660) for analysis.

Data sources and methodology
Alqahtani and Khalil (2019) determined several major oxide elements such as SiO 2 , Al 2 O 3 , CaO, Fe 2 O 3 , K 2 O, MgO, MnO, Na 2 O, P 2 O 5 , TiO 2 , and SO 3 using x-ray fluorescence spectrometry (XRF) and trace elements via ICP-MS (Inductively Coupled Plasma Mass Spectrometry) ( Table 1). In addition, thirty-one trace elements including Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cu, Ga, Ge, Hf, La, Li, Mo, Nb, Ni, Pb, Rb, Sb, Sn, Sr, Ta, Th, u, V, W, Y, Zn, and Zr were identified (Table 2). In the present work, we used all the data described above to achieve the objective of this study.

Chemical composition of the examined sandstones
According to Alqahtani and Khalil (2019), the general chemical composition of the examined sandstones displays wide variances (see Alqahtani and Khalil (2019) for more information; Table 1). These chemical compositions were plotted into several diagrams. The binary plot of Al2O3 against TiO2 is usually used to discriminate sandstones from mudstones (Moore 1992). Plotting the examined samples on this plot shows that all samples are sandstone (Fig. 5a). The binary diagram log of Na 2 O/K 2 O versus log SiO 2 /Al 2 O 3 which was used and developed by Pettijohn et al. (1987) and Madukwe   5b).
Using the ternary plot of Fe 2 O 3 +MgO -Na 2 O -K 2 O, which was established and modified by different authors such as Blatt et al. (1972) and Madukwe et al. (2016), has been used to differentiate the silicate types. This ternary plot illustrates that the vast majority of the examined sandstones fall in the field of sodic sandstone and few samples fall in the field of ferromagnesian potassic sandstones (Fig. 5c). In addition, plotting the ternary diagram of SiO2 -Al2O3 -Fe2O3 which was developed and modified by Dury (1951), Jing et al. (2013), and El-Desoky et al. (2015) has been used in this study to illustrate that the examined sandstone falls mostly in the ferruginous silicates field, except one sample falls in the silica (quartz) field (Fig. 5d).

Source area weathering and mineral composition
Shao et al. (2012) developed a ternary diagram of Al 2 O 3 -CaO -K 2 O to deduce degree of weathering. Using this plot, the degree of weathering of the examined sandstones ranges from weak to intermediate chemical weathering (Fig. 6a). Furthermore, K 2 O against Al 2 O 3 binary plot developed and modified by Kohonen (1995) and Obasi and Madukwe (2016) was used to determine the mineral contents. This binary plot shows clearly that the examined sandstones fall close to the feldspar field rather than the muscovite field, which reflects weak to moderate weathering (Fig. 6b). Nesbitt and Young (1982, 1989 used the ternary plot of Al 2 O 3 -(CaO+Na 2 O) -K 2 O to deduce weathering trends to determine the relation between mineral composition, source rocks, and weathering paths. Plotting the examined samples on this ternary diagram displays an intermediate weathering history with gabbros, tonalite, and granodiorite trends (Fig. 6c). In addition, Fe 2 O 3 /SiO 2 versus Al 2 O 3 /SiO 2 binary plot can be used to distinguish source mineralogy (Herron 1988). Using this plot, the vast majority of the sandstone samples were consisted of quartz (Fig. 6d).

Climate and effects of diagenesis
The applicability of the ratios of SiO 2 /(Al 2 O 3 +K 2 O+ Na 2 O) for paleo-climatic condition during deposition of the sediments is well recognized by many authors (Wronkiewicz and Condie 1987;Madukwe et al. 2016).
Plotting the examined samples on binary diagram of SiO 2 / (Al 2 O 3 +K 2 O+Na 2 O) displays reasonable semi-arid to semi-humid climatic circumstances in the study area during the Miocene-Pliocene times (Fig. 7a). Postdepositional variation can significantly alter geochemistry of sedimentary rocks. Among the most important diagenetic processes are silicification and K-metasomatism which might result from hydrothermal metasomatism or syn-depositional interaction with water at low temperatures (Jacobson et al. 2003). The binary discernment plot of log (SiO 2 /Al 2 O 3 ) versus log (K 2 O/Na 2 O) refers to postdepositional processes affected the examined sandstones throughout syn-depositional interaction with water at low temperatures (Fig. 7b).

Environments of deposition
The binary diagram of Fe 2 O 3 versus MgO which was developed and modified by Ratcliffe et al. (2007) and  characterizes and differentiates marine from non-marine and deltaic sandstones. Plotting the examined samples on this diagram, it shows clearly that all examined samples are non-marine and deltaic sandstone except two samples which fall in the field of marine origin (Fig. 8a). The relation between log K 2 O/Al 2 O 3 versus log MgO/Al 2 O 3 was used to discriminate marine from non-marine sandstones (Roaldest 1978;El-Desoky et al. 2015). Applying this relation in the present study shows that most of the examined samples fall in non-marine field, while a few of them (3 samples) fall within the marine field (Fig. 8b). Furthermore, the ternary diagram of MgO -Fe 2 O 3 -(SiO 2 /Al 2 O 3 ) developed and modified by Ratcliffe et al. (2007) and  categorizes and discriminates the marine from non-marine and deltaic sandstones. This plot confirms that all the examined Fig. 4 Paleosol made up of fine-grained sandstone and siltstone facies. Quartz grains are ill-sorted, sub-angular to sub-rounded samples fall in the field of non-marine and deltaic sandstones (Fig. 8c). Ni/Co versus V/Cr binary plot was also used to evaluate the paleoredox conditions (Roaldest 1978;Noori and Rais 2014). This binary plot shows that the examined sandstones were deposited in an environment that has oxic conditions (Fig. 8d).

Paleosols and climate
Paleosols have several geologic applications ranging from landscape reconstruction to paleo-climatic studies (Kraus 1999). The paleo-climatic importance of paleosols stems from studies of quaternary soil development, which have shown that some pedogenic features can be quantitatively related to soil-forming factors such as climate. Continental successions with multistory paleosols can provide a continuous record of ancient climatic conditions and climatic changes through time (Kraus 1999). In addition, paleosol-landscape analysis can produce a clearer, more complete picture of the environmental conditions and processes operating in ancient continental basins (Purser and Bosence 1998). Paleosols are not only used to interpret ancient climatic but can also be used to estimate paleo-precipitation and paleo-temperature (Kraus 1999).
Detrital clays are common in immature paleosols developed in climates that are unfavorable for pedogenesis and typically include minerals resistant to chemical weathering (Singh et al. 2016). For example, igneous quartz, muscovite, secondary microcrystalline quartz, smectite, vermiculite, illite,  (Tabor and Myers 2015). In the study area, some paleosol horizons were developed (Figs. 3 and 4). Figure 3 shows medium-to coarse-grained pebbly sandstone facies. It is made up of quartz, rock fragments, feldspars within detrital clay, and iron oxide matrix. These detrital clays refer to weak to moderate chemical weathering in semi-arid to semi-humid climatic conditions with strong and sporadic rainfall (Tabor and Myers 2015). Figure 4 shows paleosol made up of fine-grained sandstone/ siltstone facies. Quartz grains are ill sorted, sub-angular to sub-rounded.

Discussion
Sedimentary rocks formed under different climatic conditions have variable geochemical characteristics (Ratcliffe et al. 2007;Obasi and Madukwe 2016;Madukwe et al. 2016). Geochemical data and approaches are very useful to constrain and evaluate depositional environments of sedimentary rocks, weathering, and climate Roaldest 1978). Major elements provide valuable information on both the rock compositions and weathering of the sedimentary rocks (El-Desoky et al. 2015;Ratcliffe et al. 2007;Smaill 2015). In addition, these major  (Tabor and Myers 2015). Furthermore, major and trace elements and their various binary and ternary plots with discrimination functions are often applicable for climate and depositional environments of the sedimentary basins. However, these applications are not always usable for certain local settings, although some relationships between climatic conditions and geochemical composition of sediments could be documented.
The source of siliciclastic sedimentary rocks is partially or totally of igneous rocks. Crystalline rocks release their component minerals due to chemical, physical, and biological weathering as occurred in the study area. Chemical weathering is the main process accountable for obliteration of feldspars and construction of secondary clay minerals. During transportation, erosion and sorting of materials formed in weathering profiles yield sands and muds where the coarser portion is combined into sands and the secondary clay minerals are incorporated into muds. Textural, mineralogical, and bulk chemical analyses of these siliciclastic sediments offer vision into weathering circumstances affecting the source rocks. It also provides insight into the types of source rocks providing weathering materials for the deposited sediments. Weathering products form the source materials of soils and sedimentary rocks. The final composition of soil and terrigeneous sedimentary rock reflects the composition of the source rock. Many minerals of source rock are destroyed totally during weathering, while chemically resistant minerals are released from the decaying and disintegrating rock and accumulate as residues. Throughout this process, new minerals such as ferric oxides and clay minerals may form in situ in the soils. Therefore, soils are made up of persistence accumulations of minerals and rock fragments resulted from the parent rocks plus new minerals formed at the weathering position. Accordingly, soil composition is controlled not only by the parent-rock composition but also by the nature, strength, and duration of weathering and soil-forming processes. Al 2 O 3 -(CaO+Na 2 O+ K 2 O)-(Fe 2 O 3 +MgO) plot designates a non-homogenous source between the weathering trends of basaltic and granitic rocks and moderately affected by the chemical weathering. Al 2 O 3 versus SiO 2 plot refers to the absence of the biogenic contribution to the examined sandstone samples. Climate controls both the degree of chemical weathering profiting and the rate of weathering profiles formation on the crystalline rocks. The strength of tectonism controls the rate of material denudation from source areas (Alqahtani and Khalil 2019). The mineralogical and bulk chemical compositions of sands and muds derived from these weathering profiles thus are controlled by, and reflect, relative rates of chemical weathering and denudation.
Evidences that the Arabian Peninsula was significantly less arid in the Pliocene-Early Pleistocene and probably back to Middle Miocene come from the Wadi Birk (Robin 2004; Bailey et al. 2015). It was part of a river system that formerly Additional evidences for moister conditions come from fossil localities in the An Nafud Desert in northwestern Saudi Arabia (Malaren et al. 2009). Their remains were found in lacustrine deposits. SiO 2 /Al 2 O 3 versus Fe 2 O3/K 2 O plot indicates Fe-sand. Both SiO 2 /Al 2 O 3 versus K 2 O/Na 2 O and log (Na 2 O/K 2 O) versus log (SiO 2 /Al 2 O 3 ) plots denote mostly to litharenitic sandstones. Log (SiO 2 /Al 2 O 3 ) versus log (K 2 O/ Na 2 O) plot designates arkoses sandstones, while log ((Fe 2 O 3 +MgO)/(K 2 O+Na 2 O)) versus log (K 2 O/Na 2 O) plot refers arkoses and litharenitic composition. (Fe 2 O 3 +MgO) -Na 2 O -K 2 O plot refers mostly to sodic and rarely to ferromagnesian potassic sandstones, while SiO 2 -Al 2 O 3 -Fe 2 O 3 plot indicates ferruginous silicates. SiO 2 versus (Al 2 O 3 +K 2 O+ Na 2 O) plot demonstrates reasonable semi-arid to semi-humid climatic circumstances in the study area during the Miocene -Pliocene times. Log K 2 O/Al 2 O 3 versus log MgO/Al 2 O 3 plot indicates mostly to non-marine depositional environment, while both Fe 2 O 3 versus MgO binary and MgO -Fe 2 O 3 -SiO 2 /Al 2 O 3 ternary plots demonstrate the non-marine and deltaic sandstones. Ni/Co versus V/Cr plot demonstrates oxic depositional conditions and Ni/Co versus (Fig. 8d). Al 2 O 3 x5 -SiO 2 -CaOx2 ternary plot shows the examined sandstones fall in the quartz field (sandstones) and deposited on mud flats (Fig. 8c). V versus Ni plot indicates the vast majority of the examined samples deposited in terrestrial to marine-terrestrial depositional environments.