Geochemistry and Mineralogy of the Upper Cretaceous-Paleocene Marine Series, Iullemmeden Basin, Niger Republic

Major and trace elements, mineralogy as well as the total organic content of the Upper Cretaceous-Paleocene Marine Series of the Iullemmeden basin in Niger were determined in order to assess their composition, the chemistry of the paleoenvironment and the nature of the parent crystalline rocks that they evolved from. The major and trace elements were analyzed using a Bruker Tracer IV energy-dispersive hand held X-ray fluorescence spectrometer, the mineralogical analyses were done using a Bruker AXS D8 Discover, while the total organic carbon was calculated by subtraction of inorganic carbon from the total carbon, determined using a CM5014 coulometer (UIC, Joliet, IL, USA) and a LECO SC-144DR instrument. Geochemical indices calculated from major oxides such as CIA, PIA, ICV and CIW have been computed and the results show that the Marine Series are made essentially of clay minerals of the smectite group (montmorillonite), quartz and other accessory minerals like rutile and calcite. Computed ratios of some trace elements such as V/(V + Ni) and V/(V + Cr) showed that the paleoenvironment was dysoxic-oxic and the low values (<2) of the Total Organic Carbon found in the sediments are justified by those poor preservative conditions. Based on Al 2 O 3 /TiO 2 ratios, the sediments were derived from intermediate to acidic igneous rocks surrounding the basin.

Iforas, the Hoggar and Air Mountains; to the south by the Eburnean Basement; to the east by the Air-Damagaram Mounio axis and communicating to the west with the Taoudeni Basin ( Figure 1). The marine Series is stratigraphically found between the Continental Intercalaire group at the base and the Continental Terminal group at the top. Formed during a series of Upper Cretaceous-Lower Tertiary marine transgressions, it is characterized by an alternating sequence of claystones/shales and limestones globally attributed to climatic oscillations rather than sea level fluctuations: long periods of deposition of shales with secondary gypsum formed as a result of sulfide evolution, separated by relatively brief episodes of oxygenation . Previous studies on the series include those of Boeckh (1965), Greigert (1966Greigert ( , 1979, Greigert & Pougnet (1967), Brodbeck et al. (1987), Dikouma et al. (1987), Dikouma (1990, Kogbe (1981), Meister et al. (1992), and Alzouma (1994). Most of these works placed emphasis on the biostratigraphy and facies description of the formations.
Less has been published on the geochemistry and mineralogy of the rocks, specifically in the Dakoro region, where the basal part of the Marine Series occupies a Cenomanian-Turonian cuesta. In this region, a toxic gas has been reported (Greigert, 1979) to emanate from the Marine Series, resulting to the death of some people in the course of digging water wells. Brodbeck et al. (1987) Figure 1. Location map of the iullemmeden basin (Modified from Alzouma (1994)). advanced the notion that the gas could be hydrogen sulfide (H 2 S) as a result of bacterial sulfate reduction, methane (CH 4 ) from anaerobic decomposition of organic matter, or carbon dioxide (CO 2 ) from aerobic oxidation of organic matter and pyrite. Moumouni & Fryar (2017) investigated on groundwater from some dug wells and boreholes and sediments of the Marine Series. They found out the presence of high acetate concentrations, low SO 4 -S relative to total S, and/or a rotten-egg odor, all of which are consistent with reduction of 2 4 SO − to H 2 S. This study aims at assessing the composition, depositional paleoenvironment of the Marine Series and deduces the parent rock types that they originated from. This is carried out through determination of major and trace elements, mineralogy. The total organic carbon (TOC) is also determined in order to link the preservation of the organic matter with the chemistry of the sediments. Many factors influence sediment composition, including source rock composition, chemical weathering, climate, transport, burial, and diagenesis. Pettijohn (1975) and Graver & Scott (1995) stated that shales retain most of the mineral constituents of the source and their bulk chemistry preserves the near-original signature of the provenance much better than any other siliciclastic rock. Provenance, depositional palaeoenvironments, and paleoweathering conditions of siliciclastic rocks have been studied using geochemical ratios and indices such as Al 2 O 3 /SiO 2 , Al 2 O 3 /TiO 2 , K 2 O/Al 2 O 3 , K 2 O/Na 2 O, TOC/S, CIA (chemical index of alteration), PIA (plagioclase index of alteration), ICV (index of compositional variability), and CIW (chemical index of weathering). Examples include Dickinson & Suczek (1979), Bhatia (1983), Bhatia & Crook (1986), Roser & Korsch (1986), McLennan & Taylor (1991), Cox et al. (1995). Suttner & Dutta (1986) successfully used SiO 2 versus (Al 2 O 3 + K 2 O + Na 2 O) to assess the paleoclimatic conditions of fluvial sandstones in the Cutler Formation (Permian) and Fountain Formation (Permian-Pennsylvanian) in Colorado and in the Gondwana Supergroup (Permian-Triassic) of Peninsular India.

Geologic History and Setting
During the Upper Cenomanian-Mid Turonian/Senonian, the first Cretaceous marine transgressions invaded the area from the north and the calcareous sandstones, fine sandstones, and gypsiferous, glauconitic, and fossiliferous shales of the Marine Series were deposited. These deposits show rapid facies changes and typify a detrital continental-platform style of sedimentation, following a general transgression interrupted by a brief regressive episode (Kogbe, 1981). Each individual transgression overlapped the preceding one in a south-westerly direction ( Figure 2). The detritic fraction (here sandstones) might have occurred from the alteration of the underlying continental deposits. As described by Brodbeck et al. (1987), the lower part of the Marine Series in the Dakoro area consists of ~10 m of thick gray to dark silty shales locally rich in pyrite, or gypsum and crowned with decameter-thick beds of calcareous shells (lumachelles). Greigert (1966) defined six marine transgressions: three in the Cretaceous and the other three in the latest Cretaceous and earliest Tertiary. Greigert & Pougnet (1967)

Majia Group
The Marine Series is well exposed in the Tahoua area of Niger as seen below ( Figure   3).
The general geological map of the Iullemmeden basin established by Greigert (1966) and modified by successive researchers is presented below (Figure 4).    from Tahoua) were analyzed using a Bruker AXS D8 Discover at the KGS. Cox et al. (1995) and Moosavirad et al. (2011) stated that the major element composition of fine siliciclastic rocks is usually controlled by clay minerals and non-clay silicate phases. Table 1  In order to assess the composition, maturity and weathering of the sediments, geochemical indices such as the CIA, PIA, ICV and CIW have been computed as described by Nesbitt & Young (1982), Roser & Korsch (1988), Cox et al. (1995), Cullers (2000), and Hofmann et al. (2001) (Table 3). These CIA values could be underestimated as XRF has a very hard time measuring elements of low energy electrons like Na.

Composition, Maturity and Palaeoweathering of the Marine Series
hence, the XRD results could be more reliable here. However, Cox et al. (1995) stated that values of K 2 O/Al 2 O 3 ratio of clays are less than 0.3 and those of feldspars range from 0.3 to 0.9. The values of K 2 O/Al 2 O 3 ratio of the Marine Series vary from 0.01 to 0.23 ( Table 2) that is less than 0.3. These values also indicate preponderance of clay minerals over K-bearing minerals such as K-feldspars and micas (Cox et al., 1995).
In terms of maturity, CIA values plotted against those of ICV show that most of the samples are immature and have been subjected to weak weathering ( Figure 6).   This could be also applied to mudrocks as a measure of compositional maturity because minerals show a relationship between resistance to weathering and ICV. Compositionally immature mudrocks that contain a high proportion of non-clay silicate minerals or that are rich in clay minerals such as montmorillonite and sericite will have high values of this index (>1). On the other hand, compositionally mature mudrocks that are poor in nonclay silicates or dominated by minerals such as those of the kandite family (kaolinite, halloysite and dickite) will have low values. Table 2 indicates that the values of ICV of the Marine Series range between 1.13 and 4.2 (>>1), consistent with compositional immaturity.

Paleoenvironment and Tectonic Setting
A binary diagram of SiO 2 versus (Al 2 O 3 + K 2 O + Na 2 O) shows that the Marine Series was deposited under wet/humid conditions (Figure 7). This is consistent with the findings of Pascal et al. (1991) and agrees with the hypothesis of generally high rainfall during the Cenomanian-Turonian (Parrish & Curtis, 1982), in accord with proximity to the equator (Rat et al., 1991).
Similarly, Zhou & Jiang (2009) and Pi et al. (2014) stated that V is more effectively fixed in sediments containing organic matter in anoxic environments compared to Ni and Cr. Therefore, variations in the V/(V + Ni) and V/(V + Cr) ratios can be used to indicate oxygenation of the depositional environment, and higher V/(V + Ni) and V/(V + Cr) ratios indicate more strongly reducing conditions. According to Galarraga et al. (2008), a V/Ni ratio higher than 3 indicates that the organic matter was deposited under reducing conditions, while V/Ni ratios ranging between 1.9 and 3 indicate deposition under dysoxic to oxic conditions (mixed terrigenous and marine organic matter) and a V/Ni ratio < 1.9 indicates predominantly terrigenous organic matter.
From the analytical results of the Marine Series (Table 4), all the V/Ni values are less than 1.9 except for samples K61 (V/Ni = 2.12) and DJ9 (V/Ni = 10.56).
All the values of Cu/Zn have their ratio lower than 1.9. The ratios V/(V + Ni) and V/(V + Cr) are either negative or positively low. All the above indicates dysoxic-oxic depositional environment. Rather, all the values of TOC (Table 4)       The source of the sediments can be traced from the contiguous Aïr crystalline basement rocks in the north and the Zinder-Nigeria cristalline basement rocks in the South. Indeed, three periods of volcanism had been identified in previous studies (Yahaya, 1992;Wagani, 2007, etc.) namely Carboniferous, Permian and Jurassic. Karche & Vachette (1976), Sempéré (1981) and Gerbeaud (2006)  Shield as at that period of time, the Air was uplifted upstream of the paleocurrents that fed the Iullemmeden basin. The presence of smectite clays such as montmorillonite (main mineral in bentonite) can be supported by the argument made by Fisher & Schminck (1984) who observed in USA that pyroclastic materials from high volcanic eruptions can be transported and deposited as far as 1000Km. Bentonite is a rock formed of highly colloidal and plastic clays and is produced by in-situ devitrification of volcanic ash. Its presence in the area of study had been earlier mentioned in previous studies by Gourouza et al. (2011;2013) and in similar Cretaceous lithology (the Pindiga Formation) by Arabi et al. (2017) in the Benue Trough.

Conclusion
The present work shows that the Marine Series are made predominantly of clay minerals of the smectite group (montmorillonite), quartz and other accessory minerals like rutile and calcite. The sediments are immature and were deposited under dysoxic-oxic conditions which may justify the poor preservation of organic matter in the environment. In terms of origin, the sediments were derived from intermediate to acidic igneous rocks found adjacent to the basin. The occurrence of montmorillonite in the sediments by previous works is also confirmed in this study and the origin may be linked to the volcanic ash that evolved from volcanic eruptions that occurred in late Paleozoic-Mesozoic period in the North and South crystalline basement surrounding the basin. The bentonite potential and its source may be considered as a perspective investigation in the basin with the recent oil discovery success in Niger.