Miocene sediment mineralogy of the lower Chelif basin (NW Algeria): implications for weathering and provenance

* Correspondence: a_marok@yahoo.fr

The present study examines the detailed mineralogy of the Miocene sediments of the Algerian margin (lower Chelif basin, northern Algeria) and attempts to constrain their chemical weathering and provenance.It completes the results obtained on clay and nonclay minerals in the Mediterranean basins, in particular Miocene basins of Morocco, Tunisia, France, and Spain.Three representative sections of the northern and southern margins of the basin were selected for this purpose.
In this basin, the lower Miocene facies record important changes in space and time.The sedimentary sequence, which is mostly marine, is constituted by bluish marls that laterally change to sandstones, purple hard marls, and conglomerates (e.g., Perrodon, 1957;Bessedik et al., 2002).According to studies of Neurdin-Trescartes (1992, 1995) and Arab et al. (2015), the marine lower Miocene was deposited during a compressional episode in a piggyback position on top of the still-moving Tellian allochthon.
Finally, for the Upper Miocene (Tortonian-Messinian), the northern and southern margins of the Chelif basin are distinguished by both marine and continental sedimentation (blue marls, diatomites, and evaporites) showing a significant variation in thickness.
In the present study, three sections were investigated on the northern (Ouillis and Amarna) and southern (Tiraouet) margins of the basin (Figure 1a).

Tiraouet section
The Tiraouet section is located southeast of Chlef village (ex.El Asnam), around 15 km north of Sendjas (Figure 1a).This section consists of marls assigned to the lower Miocene (Burdigalian) by Perrodon (1957) and to the uppermost Burdigalian-Langhian by Bessedik et al. (2002) (Figure 1b).The association of planktonic foraminifera is represented by the Globigerinoides trilobus and Globoquadrina baroemoensis species, which confirm the Burdigalian-Langhian age.The studied section is 73 m thick (Figures 2a and 2b) and consists of: -blue marls (39 m thick) changing to sandy marls to the top.This stratigraphic interval is overlain by a tephra bed that is 0.80 m thick (Figure 2c).
-purple hard marls (34 m thick) without macrofossils overlain by a conglomerate bar.

Ouillis section
This northern section is located in the southwestern part of the Dahra massif, north of Sidi Bel Attar (Figure 1a).Taken at the Ouillis quarry, this section is represented by the upper part of the diatomite formation (19 m), which can be subdivided into two lithologically distinct units (Figure 3a), Member A and Member B.
Member A (14 m thick) is represented by centimetric to decimetric alternating layers largely dominated by the diatomite facies.From bottom to top (Figure 3b): -9 m of dilated and monotonous alternation of diatomites and marls, with 10 cm of tephra bed at the base.The massive, centimeter-to-decimeter diatomite beds are white in color.The base and top of these beds are beige and more or less marly.The marls (0.15 to 0.70 m thick) are massive and lenticular, brown at their base to black, and grading gradually to diatomites.Samples collected from this section have a siliceous microfauna represented by radiolarians (Mokhtar Samet, 2013).
-5 m of 11 closely alternating sequences including laminated diatomite-marls and laminated diatomitemarls-gypsum.Gray laminated diatomites sometimes contain fish scales.Marls are black and 0.10 to 0.70 m thick, sometimes having laminar structures.There are five intervals of gypsum each 1 to 2 cm thick.
Member B (5 m thick) is characterized by a rhythmic alternation of marls and diatomite beds (Figure 3c).It ends with centimeter-scale limestone beds incised into the underlying marls.In the Ouillis section, the diatomite formation age was based on micropaleontological arguments and lithostratigraphic correlations with neighboring regions (Mokhtar Samet, 2013).From this point of view, a single association of benthic foraminifera could be defined towards the top of the study section (Member B).It is composed of the following species: Bulimina subulata, Bulimina aculeata, Bolivina dilatata, Bolivina dentellata, Uvigerina sp., and Rectuvigerina cylindrica.This association gives a Messinian age (biozone Globorotalia mediteranea) correlating with neighboring regions.In addition, Member A, which does not contain foraminifera, can be dated by correlation with diatom assemblages in the neighboring regions (Mansour et al., 2008).The correlation confirms the Messinian (Upper Miocene) chronological interval.

Amarna section
The Amarna section is located northeast of Djebel Diss, about 16 km north of Mostaganem (Figure 1a).The lithostratigraphic succession in this section includes only the Blue Marls Formation (64 m thick) (Figures 4a and 4b).It consists of yellowish sandy marls at the base and blue marls at the top.The planktonic association represented mainly by Neogloboquadrina acostaensis, Globorotalia scitula, Globigerinoides obliquus, Globigerinoides trilobus, and Globigerina bulloides gives a lower Tortonian age (biozone Neogloboquadrina acostaensis).The same age was given by Belhadji et al. (2008).This stratigraphic interval is capped by a 0.30-m-thick conglomerate bar.

Materials and methods
The results presented in this study are based on the processing and analyses of 47 samples of marl layers taken from Tiraouet (23), Ouillis (17), and Amarna (7) (Figures 2a, 3a, and 4a).X-ray diffraction identification of the crystalline phases was carried out on whole-rock and clay fractions with a PANanalytical XPERT-PRO diffractometer using CuKα (1.5418 Å) radiation with a ceramic copper anticathode X-ray tube under an accelerating voltage of 45 kV and a beam current of 40 mA.The positions and intensities of the observed diffraction peaks were compared with the PDF-ICDD (Powder Diffraction File) reference files for mineralogical phase identification.The RIR (Reference Intensity Ratio) included in the HighScore Plus software of the PDF-ICDD file allows the semiquantitative estimation of the corresponding detected phases when this is required and/or possible.The fine fraction (clay minerals) was extracted by wet process after removal of materials impeding their dispersion such as organic matter and carbonates.The obtained clay suspensions were spread on suitable supports for drying.Each oriented clay preparation was studied and then, depending on the need, was subjected to a heat treatment at 550 °C for 1

Bulk mineralogy
Table 1 shows the proportions of nonclay minerals for the Miocene sediments of the Chelif basin.
The Ouillis section shows dominance of illite (30%-60%) and a lack of chlorite except in layer O 2 (15%).Kaolinite, smectite, and I-S percentages are 15%-20%, 0%-20%, and 15%-25%, respectively (Figure 6b).Different forcing processes (climatic, tectonic, and lithologic) play important roles in the formation and preservation of clay minerals in these study sections (Figure 7a).The smectite-(kaolinite+chlorite)-illite ternary diagram was constructed to highlight the provenance of clay minerals of Miocene sediments and their weathering types (chemical weathering and mechanical erosion) (Figure 7b).In general, this plot clearly shows that sediments originated from a mixed mafic felsic source and were influenced by both physical erosion and chemical weathering.In detail, layers of the top of the Tortonian sediments in Amarna were affected essentially by a chemical weathering process and the top by a physical erosion process (Figure 7b).
For example, the chemical leaching proxy kaolinite/ (illite+chlorite) averages 0.54 in Member B and 0.37 in Member A. The humidity proxies kaolinite/illite and kaolinite/chlorite behave similarly.
The smectite/illite, smectite/(illite+chlorite), and (smectite+kaolinite)/(illite+chlorite) ratios show a decrease from Burdigalian-Langhian sediments to Tortonian and then an increase in the Messinian.The (smectite+kaolinite)/(illite+chlorite) ratio of 0.60-1.17The kaolinite/(illite+chlorite) chemical leaching ratio and the humidity proxies kaolinite/illite and kaolinite/ chlorite are high in Member B, indicating that leaching    leading to kaolinite formation was more important in the upper part of the lower Miocene deposits.

Conclusions
The clay mineralogy of the Chelif basin Miocene sediments was investigated in three sections in order to characterize the Burdigalian-Langhian, Tortonian, and Messinian sequences and to constrain their weathering and provenance.The mineralogy reveals important variations in the concentrations of minerals.These clastic sediments are marked by variable concentrations of quartz (25%-69%) and calcite (5%-49%) and by the presence of pyrite and halite in the Burdigalian-Langhian, and by anatase and gypsum in the Tortonian and Messinian units.Barytine and jarosite were only observed in the Messinian deposits where dolomite was absent.
Clay mineral assemblages of the lower Chelif basin include illite, chlorite, kaolinite, smectite, and I/S.The Miocene sediments indicated an increase of illite from bottom (Burdigalian-Langhian) to top (Messinian) and conversely a decrease of chlorite leading to its disappearance in the Messinian unit.These sediments originated mainly from a mixed source and were influenced by physical erosion and chemical weathering processes.

Figure 1 .
Figure 1.Geographic and geological sketch.a) Map of the Chelif Basin through the Miocene with location of the studied section (modified from Rouchy et al., 2007).b) Geological and tectonic map of the lower Chelif Basin.(A) Amarna section, (O) Ouillis section, (T) Tiraouet section (after Meghraoui, 1986).

Figure 3 .
Figure 3. a) Stratigraphic columns of Ouillis section, b) Member A, c) Member B.

Figure 4 .
Figure 4. a) Stratigraphic columns of Amarna section, b) Blue Marls and MGC (Marls including Gypsy-Calcareous) limit of Amarna section.
(average: 0.90) suggests the dominance or equilibrium of clay minerals formed under physical erosion.Conversely, Member B shows greater values (1.00-1.67;average: 1.25), indicating the presence of clay minerals derived from chemical weathering.Kaolinite/smectite values are greater than in Member A, indicating warm and humid climatic conditions.

Figure 7 .
Figure 7. Ternary diagrams of the major clay mineral groups illite+chlorite, kaolinite, and smectite.(a) The principal forcing processes on clay mineral formation are also indicated (after Liu et al., 2012).(b) The provenance sources and alteration processes are also indicated (after Hu et al., 2014).