Late Miocene-early Pliocene Palaeoenvironmental Changes in the Western Rifian corridor (NE Morocco)

doi:10.21203/rs.3.rs-1602295/v1

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Late Miocene-early Pliocene Palaeoenvironmental Changes in the Western Rifian corridor (NE Morocco)

https://doi.org/10.21203/rs.3.rs-1602295/v1

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The Gharb basin constitutes the western part of one of the main corridors that connecting the Mediterranean Sea and the Atlantic Ocean during the Upper Miocene. This basin is characterized by a continuous sedimentary filling from the Upper Tortonian to the Pliocene, likely to record environmental information useful for the reconstruction and evolution of the marine environment before and during the Messinian salinity crisis.

Based on an integrated study of the planktonic and benthic foraminifera of borehole DF 101 (Gharb basin, Morocco), the objective of this work is, on the one hand, to clarify the biostratigraphic framework of the sediments studied and, on the other hand, to make a further contribution to the reconstitution and evolution of the marine environment during the Upper Neogene in the Gharb Basin.

The high-resolution biostratigraphic study of borehole DF 101 revealed the succession of eight bioevents and characterized the Upper Tortonian, Messinian and Zanclean.

The composition of benthic foraminiferal assemblages, paleoecological indices and statistical analysis show an evolution of depositional conditions in terms of bathymetry and oxygen content from the Upper Tortonian to the Pliocene.

The Upper Tortonian is marked by an association of very diversified benthic foraminifers that characterizes the upper bathyale, the estimated paleobobathymetry is over 800 m and the medium is well oxygenated. Messinian marl deposits are characterized by a decrease in bathymetry, and a reduction in the rate of oxygenation. While during the Lower Pliocene, the medium becomes relatively deeper, higher bathyal type and normal oxygenation rate.

Benthic foraminifera

Upper Miocene

Pliocene

Gharb basin

paleoenvironmental changes

Paleobathymetry

Paleoxygenation

During the Upper Neogene, the western Mediterranean experienced several geodynamic events, the main one being the opening of two Straits as a consequence of the westward displacement of the Alboran block, which came into contact with Africa and Iberia, and the continuous approach of these two plates (Rehault et al., 1984; Dewey et al. 1989; Rosenbaum et al. 2002). These are respectively the northern Betic corridor in southern Spain and the southern Rifian corridor in northern Morocco. These two corridors allowed active exchange of water masses, nutrients and different marine species between the Atlantic and the Mediterranean during the Late Miocene (Benson et al., 1991; Krijgsman et al., 1999; Flecker et al, 2015; Achalhi et al., 2016). Following the closure of these corridors, due to tectonic, glacial, and eustatic events, the Mediterranean gradually became isolated from the Atlantic Ocean and where huge amounts of evaporites were deposited during the Messinian salinity crisis (Benson et al., 1991; Krijgsman et al., 1999; Flecker et al, 2015; Achalhi et al., 2016; Suc and Frizon de Lamotte., 2019; Kontakiotis et al., 2019). The evolution of communications between the Atlantic and the Mediterranean and the chronology of the closure of the corridors is still a subject of discussion.

On the other hand, to correctly interpret the progress and evolution of the Messinian salinity crisis, it is of crucial importance that the same time interval is also studied in detail on an open marine succession. It is therefore essential to obtain high-resolution and complete stratigraphic data on the Moroccan Atlantic margin during the Upper Tortonian-Pliocene interval. Recently studies on reference sections in the Bouregreg region (northern Morocco) have improved the chronostratigraphic framework of these sediments and contributed to the understanding of the causes of the Messinian salinity crisis (Vander Lan et al, 2006; Krijgsman et al, 2004; Van der Laan et al 2012, Warny et al, 2002, 2003).

The Gharb basin, which occupies the western part of the southern Rifian corridor, is a privileged site for understanding the evolution of communications between the Mediterranean and the Atlantic before and perhaps also during the Messinian salinity crisis. Neogene sedimentary deposits accumulated along this corridor recorded the different biostratigraphic, environmental, tectonic and eustatic events that preceded the Messinian salinity crisis (Barhoun and Bachiri Taoufiq, 2008; Azdimousa et al., 2011, Flecker et al., 2015; Achalhi et al., 2016). The Gharb Basin, which forms the Atlantic part of the Rifan Corridor, is characterized by a thick and continuous sedimentary series of the Upper Miocene and Pliocene period (Flinch, 1993; Flinch et al, 1996; Zouhri et al., 2002). These sedimentary archives have recorded data that allow the evolution of sedimentary environments during the Late Neogene to be assessed. This basin has been the subject of intense oil exploration activity in recent years, with several gas exploration boreholes being drilled in addition to hydrogeological drilling. This work is focused on the micropalaeontological study of the Mio-Pliocene marine series of borehole DF 101, implanted by ONHYM in the southern part of the Gharb plain. In order to reconstruct the evolution of marine paleoenvironments in the Gharb Basin during the Upper Miocene - Pliocene and to contribute to a better understanding of the Messinian salinity crisis in the Mediterranean, we conducted a biostratigraphic and paleoenvironmental study of the Upper Neogene of borehole DF 101. In order to improve the biostratigraphic framework of the studied sediments, we reassessed the planktonic foraminifera biostratigraphy of these deposits, using the high-resolution biostratigraphy developed for the Mediterranean and adjacent Atlantic (Sierro et al., 1993; Krijgsman et al. ., 1995; Sprovieri et al., 1999; Hilgen et al., 2000a, 2000b; Krijgsman et al., 2004, Sierro et al., 2001, Lirer et al., 2019, Van der Laan et al., 2006; 2012).

The present work offers, through a high-resolution biostratigraphic analysis (based on planktonic foraminifera) and a detailed study of benthic foraminiferal assemblages, a new contribution to the reconstruction and evolution of the marine environment during the Upper Neogene in the Gharb basin. This will allow a better understanding of the evolution of communications between the Atlantic and the Mediterranean during the Late Miocene.

The Gharb Basin is located within the southern part of the Rifian chain’s structural entities (Tilloy, 1955; Suter 1980; in Litto W et al., 2001): The Outer Prerif. It constitutes the Atlantic extension of the Rifian foreland deposits. This Neogene basin is bounded to the north and northeast by hills with material belonging to the Prerifian Nappe, to the south its formations rest on the Meseta basement, and to the west by the Atlantic Ocean (Fig. 1).

The Gharb basin, because of its position on the front of the Rifian orogeny, corresponds to a zone of tectonic weakness (Cirac, 1987). The deposits of this subsident depression are essentially developed on a Paleozoic substratum.

This basin has been the subject of intense oil exploration activity in recent years, several gas exploration boreholes have been conducted in addition to hydrogeological drilling by ONHYM and its partners. Therefore, most of the geological data are derived from the use of geophysical information, in particular seismic reflection and delayed logging, which have provided valuable information on the structural framework of this basin (Malod, 1982; Cirac, 1987, 1990; Flinch, 1994; Flinch et al, 1996; Litto, 2001a and b; Sefiani, 2003).

The geodynamic evolution of the Upper Neogene of the Gharb Basin, buried under the Quaternary, is subject to the influence of many factors such as eustatic variations in sea level, sedimentary inputs, subsidence and tectonic activity (ONHYM report, 1991, 1996; Flinch, 1994; Flinch et al, 1996; Zouhri et al., 2002 Sissingh, 2008).

The palaeogeographical evolution of the Gharb basin during the Upper Neogene is marked by the introduction of the Prerifian layer and variations in the sea level (Flinch, 1994; ONHYM, 1991, 2000).

In the Upper Tortonian, the distension phase, which caused a major collapse, led to an extension of the basin. The rise in sea level allowed the deposition of a thick series of marls within the subsident zones and carbonates on the edges of the basin.

In the Messinian, the facies of the platforms pass from the carbonate system to the deltaic system along the NE edge of the basin and, following the drop in sea level, the deltaic platform is in turn gradually replaced by turbiditic facies.

During the Messinian, there is the establishment of the Prerifian wrinkles, at the edge of the Meseta; uplift took place in the Tifelt area and a coastal belt was deposited along the Hecynian substratum.

In the Early Pliocene, the Prerifian wrinkles, the Lalla Zahra area and the northwestern part of the Gharb Basin are emerged. The neritic sedimentation initiated during the Messinian continues during the Pliocene and develops essentially in the southern half of the Gharb basin. The sandy sediments change to bioclastic elements and one gradually arrives at the Early Pliocene falunian facies. Following the general regression, the Gharb Basin becomes a gulf protected from oceanic agitation (Cirac, 1987).

In the Upper Pliocene, important deltaic apparatuses are built following the increase in terrigenous contributions as a consequence of the isostatic rise of the basin margins.

Stratigraphically, the Gharb basin has a chaotic filling with a marly matrix. The thickness of sedimentary deposits could reach 2000m to 4000m in the center of the basin. The elaboration of the stratigraphic framework of the Neogene formations of the Gharb basin started with the works of Feinberg (1986), Wernli (1988) and ONHYM (1991). The substratum of the Neogene series, formed of Cretaceous, Jurassic and Paleozoic, is the deepest (up to 2500 m) following the maximum advance of the Prerifian complex.

The sedimentary series of Gharb contains from bottom to top: the gray marls, faluns, sands and sandy limestones, sands and pebbles, clays with limestone lenses (Cirac, 1987).

Wernli (1988), in studying the planktonic foraminifera of the pelagic grey marls of this basin, had traced the main features of the biostratigraphic framework. Thus, on the substratum, the upper Miocene (m5 - m6) with a pelagic facies in which the pre-verified nappe is interbedded. This can reach 800 m in thickness towards the south rifts. Above this, the pelagic sedimentation continues during the m6 until the Upper Pliocene (P3) in the center of the basin.

The Miocene and Pliocene stratigraphic context of this basin is currently better known thanks to recent work in the Bouregreg valley region (Rakic El Bied and Benson, 1996; Barhoun, 2000; Vander Lan et al, 2006; Krijgsman et al, 2004; Vander laan et al 2012, warny et al., 2002, 2003, Capella et al, 2017, 2018).

The borehole DF-101 is carried out by ONHYM, it is located 18km south of the city of Sidi Slimane in the plain of Gharb. Its geographical coordinates are as follows: X: 446. 499.99, Y: 390. 699.98, Z: 107.51m. The drilling crosses a series of Plio-Quaternary, Miocene and Paleozoic sediments and stopped at the 1200 m coast (Figure.2).

Borehole DF101 is characterized by an essentially marly sedimentation. The lithological analysis of DF 101 (figure 3) allows the recognition of four different lithological sets. Thus, we distinguish from top to bottom:

Set 1 from 0 to 110m, characterized by micro-conglomerate and sandstone facies with marls containing glauconite and pyrite. It contains the following levels:
1. From 0m to 15m: heterogeneous conglomerate soils difficult to determine.
2. From 15m to 30m: pebbles and conglomerates with a more or less clayey sandy matrix, poor in fossils.
3. From 30m to 110m: grey clayey marls, sometimes silty, often fossiliferous with some shell debris, glauconous and with pyrite.
Set 2 from 110m to 950m, almost homogeneous formed by grey clayey marls passing to calcareous clays locally finely silty, with pyrite and glauconie.
Set 3 from 950m to 1129.5m, with indurated gray clayey marls, changing towards the base to slightly glauconitic marls, the last ten meters correspond to a gray silt with glauconia.
Set 4 from 1129.5m to 1200 m, formed of dark grey, grey-brown and soft quartz schist with quartz passages. This set is very azoic.

Materials

The samples studied come from the borehole DF 101 carried out by the Office National des Hydrocarbures et des Mines "ONHYM" in the framework of oil exploration. The sampling targeted the marl-clay sediments crossed by the borehole. All available samples located between 1090 m and 20 m were subjected to a detailed micropalaeontological analysis. 112 samples were processed and analyzed qualitatively and quantitatively.

Micropaleontological treatments

The samples were washed and sieved according to the method used in our laboratory. The qualitative and quantitative analysis of the microfauna is performed on residues containing at least 300 individuals of the 150 µm and 125µm fraction. In general, samples containing less than 300 foraminiferal specimens are excluded from the quantitative analysis and statistical treatments. To this end, of the 112 samples analyzed, 29 are ignored.

To appreciate the paleoenvironmental framework, different paleoecological indices are calculated; namely, the P/B+P ratio and the Shannon-Weaver diversity index H(s).

P/P+B ratio

This ratio expressed as P/ (P + B) x 100 (the percentage of planktonic foraminifera to the total foraminiferal population) is commonly used for paleobathymetric reconstructions (Gibson, 1989).

Shannon-Weaver diversity

Species diversity in benthic foraminiferal assemblages is derived from the Shannon-Weaver function (Shannon & Weaver, 1949). This index is widely used to characterize the diversity of species in a population.

Isc= - ( qi / Q x Ln qi / Q)

qi = the number of individuals counted for each species

Q = the total number of individuals for each level.

Statistical analysis

The statistical data of the benthic microfauna are analyzed with the SPSS software. The data processing consists first of establishing the Pearson correlation matrix for the twenty-six most abundant species. Then a cluster analysis in Q mode of unweighted pairs using the cluster method. These are used to produce a hierarchical dendrogram classification for the twenty-six most abundant species. From this classification, different types of associations were identified.

Oxygen concentration

To calculate the oxygenation index, we applied the oxygen transfer method of Kouwenhoven and Van der Zwaan (2006) to the benthic microfauna of borehole DF101 using the following formula:

(Oxygen content µmol / l) = 7.9602 + 5.95 (% oxyphilic taxa)

A group of oxyphilic species is selected for the reconstruction of oxygen levels in the bottom waters (Table.1). Generally, we used the same species used by Kouwenhoven (2000).

Agglutinated are omitted due to the obvious fragility and low conservation potential of a number of them. Depending on oxygen levels, an environment can be oxic (> 2 ml / l;> 89.3 µmol / l), dysoxic (2-0.2 ml / l; 89.3-8.9 µmol / l), suboxic (0.2-almost 0 ml / l; 8.9-almost 0 µmol / l), or anoxic (0 ml / l; 0 µmol / l) (Tyson and Pearson, 1991).

The group of oxyphilic species is established from the data of Kouwenhoven (2000) (Table.1).

Paleobathymetry

The paleo depth is estimated from the following formula:

Paleodepth (m):= e^{3,58718 + (0,03534 *% P)}

With % P = 100 * (number of planktonic foraminifera / total number of foraminifera - number of environmental stress taxa) (Van der Zwaan et al., 1990; Van Hinsbergen et al., 2005).

Environmental stress markers include the following genera: Bulimina (except costulate forms), Cancris, Chilostomella, Globobulimina, Uvigerina and Valvulineria (Van Hinsbergen et al., 2005). Results are compared to the depth distribution of several species.

Microhabitat

Five microhabitats were distinguished (Table 2):

Epifauna;
Shallow epifauna-endofauna;
Shallow endofauna;
Intermediate endofauna;
Deep endofauna ;

The preferred microhabitat of each species was determined according to Lutze et al (1989), Schmiedl et al (2000).

Introduction

The biostratigraphic analysis is based on the qualitative and semi-quantitative study of planktonic foraminiferal assemblages. Species identification is carried out based on specialized references (Feinberg, 1986; wernli, 1988, Iaccarino, 1985, Barhoun, 1999, 2000, Iaccarino et al., 2007; Sierro et al., 1987; 1993, 2001).

In order to accurately date the studied sediments, we adopted the high-resolution, astronomically calibrated biostratigraphy recently used in the Mediterranean and adjacent Atlantic domain (Hilgen et al. (1995) krijgsman et al. (1994, 1995, 1999), Sierro (1985) and Sierro et al. (1993), Sierro et al., 2001; Lourens et al, 2004; Hilgen et al, 2012 in Gradstein et al. 2012, Lirer et al., 2019).

The biostratigraphic study is based mainly on the vertical distribution of the marker species (Globorotalia menardii, Globorotalia miotumida, Globorotalia margaritae, Globoturborotalita nepenthes,....), the change of coiling of Globorotalia and Neogloboquadrina as well as the presence and absence of carinated and non-carinated Globorotalia

Biostratigraphic analysis and dating test of borehole DF 101

Planktonic foraminifera are well represented and diversified. The preservation quality of the planktonic microorganisms is good and favors a precise biostratigraphic study. The evolution of the planktonic foraminiferal assemblages and the semi-quantitative analysis of the planktonic foraminiferal species and especially the markers, along the series, allows highlighting a succession of biostratigraphic events (Table. 3), from the bottom to the top we distinguish:

From 1090 m to 1010 m The planktonic foraminifera assemblage is dominated by globular forms such as: Globigerinoides, Globigerina, Globoturborotalita, Neogloboquadrina... compared to the carinated forms of Globorotalia, represented only by the species Globorotalia menardii. From 1090 m, we noted the presence of the sinisterly coiled Globorotalia menardii, form 4, which disappears around 1050 m. This event (LCO G. menardii 4), of age 7.51 Ma, has been observed in the adjacent Atlantic by Sierro, 1985; Sierro et al, 1993; Hilgen et al., 2000 and Lourens et al., 2004 and in the Mediterranean basin by (Krijgsman et al., 1995; Krijgsman et al. 1999; Hilgen et al., 1995; Lourens et al., 2004; Lirer et al., 2019).

The first regular occurrence of dexterously coiled Globorotalia menardii, form 5, was observed at 1040 m. The G. menardii 5 FCO event has been recorded at 7.35 Ma in the Atlantic and Mediterranean domain (Sierro, 1985; Sierro et al., 1993; Krijgsman et al., 1995, 1999; Hilgen et al., 1995, 2000; Lourens et al., 2004, Lirer et al., 2019).

On the other hand, we noted that the Globorotalia scitula group is very poorly represented in these sediments.

The first common occurrence of the Globorotalia miotumida group (FCO G. miotumida) is indicated at 1010 m in this borehole. This event, identified by Lourens et al, (2004); Hilgen et al, (2000a,200b); Lirer et al, (2019) Ma, is dated at 7.24Ma. It allows us to locate the Tortonian-Messinian boundary (Hilgen et al., (2000a,2000b).

From the succession of these three events, the sediments between 1120 m and 1010 m are assigned to the Upper Tortonian (between 7.72 Ma and 7.24 Ma), while the Tortonian-Messinian boundary is located at the depth of 1010 m.

From 1010m to 490m, planktonic stands are also dominated by globular forms such as: Globigerinoides, Globigerina, Globoturborotalita, Neogloboquadrina, the Sphaeroidinellopsis... compared to the forms of Globorotalia carinae represented mainly by Globorotalia miotumidaand Globorotalia margaritae. The last common occurrence of Globorotalia miotumida (LCO G. miotumida) was spotted at 720m.This event, observed in the Mediterranean domain and the adjacent Atlantic, at an age of 6.52 Ma (Sierro et al., 1993; Lourens et al., 2004; Sierro et al., 2001; Lirer et al., 2019).

The evolution of the percentage of senestial and dexterous forms of Neogloboquadrina acostaensis revealed a change of coiling from senestial to dexterous (S/D N.acostaensis) at 630m. From work in other regions in the Mediterranean and Atlantic realms, this event has been recorded at 6.35- 6.37 Ma (Sierro et al., 1993; Sierro et al., 2001; Lourens et al., 2004, Lirer et al., 2019).

The first occurrence of Globorotalia margaritaea was observed at 520 m in DF 101. Indeed, the first occurrence of G. margaritae is indicated at the end of the Miocene in the Atlantic domain (Benson and Rakic-El Bied., 1996; Krijgsman et al., 2004) mentions that the FO G. margaritae is recorded in the Messsinian in the Ain El Beida section (Rabat region, Morocco) with an astronomical age of 6,314 - 6,311 Ma. While in the Mediterranean the first common occurrence of Globorotalia margaritae (FCO Globorotalia margaritae) is indicated at 5.08 Ma (Lourens et al., 2004; Lirer et al., 2019).The Miocene-Pliocene boundary in the Mediterranean domain corresponds to the recovery of marine conditions after the Messinian salinity crisis (Hsu¨ et al., 1973; Cita., 1975; Hilgen and Langereis., 1993; Suc et al., 1997; Iaccarino et al., 1999, 2007). It is formally defined at the base of the deep marine Trubi marls that overlie the upper evaporites and the Messinian Arenazzolo Formation with Eraclea Minoa (Van Couvering et al., 2000). Biostratigraphically, it is marked by the acme of Sphaeroidinellopsis followed by the first common occurrence of Globorotalia margaritae. However, these biosignatures have been shown to be of purely local significance (Benson and Rakic el-Bied., 1996). Because the first occurrence of G. margaritae is recorded in the Messinian in the Atlantic domain (Benson and Rakic-El Bied., 1996; Krijgsman et al., 2004). On the other hand, some authors propose that the bioevent corresponding to two sinister-coiled N. acostaensis influxes could be used at the global scale to identify the Miocene-Pliocene boundary (Di Stefano et al., 1996; Van Der Laan et al., 2006). Recently Lirer et al, (2019), mentions the presence of 2 influxes of Neogloboquadrina acostaensis S in the Rossello Composite reference section (Eraclea Minoa, Sicily), the first one at 5.32 (Sgarrella et al, (1997); Iaccarino and Bossio., (1999) And the second one at 5.29 (Sgarrella et al, (1997); Iaccarino and Bossio., (1999)).

On the other hand, Suc et al. (1997) suggested that the TG5 "interglacial" or high level peak may correlate with the base of the Zankean. Recently, Van Der Laan et al, (2006) stipulate that the Miocene-Pliocene boundary identified in the Loulja region does not coincide with the TG5 marine isotope stage but in fact corresponds to an additional (weak) cycle controlled by the obliquity between TG7 and TG5.

In the borehole studied, Neogloboquadrina acostaensis is mainly dexterly coiled between 630m and 500m. At the 490m level we observed a slight abundance of Neogloboquadrina acostaensis sinister. This biosignal likely corresponds to the bioevent used to identify the Miocene-Pliocene boundary (Di Stefano et al., 1996; Van Der Laan et al., 2006) and likely coincides with the first Inﬂux Neogloboquadrina acostaensis Senestre of age 5.32 Ma identified in the Mediterranean domain (Lirer et al., 2019). Sphaeroidinellopsis are present in all samples and we did not notice an acme at this level. This lack of acme of Sphaeroidinellopsis was also observed by Van Der Laan et al. in the El Oulja section (2006). On the other hand, Globorotalia margaritae present since 520 m, becomes abundant at this depth (490 m).

Therefore, the Miocene-Pliocene boundary is probably located at depth 490 m in the studied borehole. The age of this boundary has been astronomically dated at 5.333Ma (Van Couvering et al., 2000 and at 5.332 Ma Lourens et al., (2004).

Based on the succession of these events, the sediments between 1010 m and 490 m are assigned to the Messinian, from 7.24 to 5.33 Ma.

From 490 m to 20m, the planktonic foraminiferal assemblages are composed mainly of Globigerinoides, Globigerina, Neogloboquadrina, Sphaeroidinellopsis and Globorotalia margaritae. This level is characterized by the presence of G. margaritae up to the top of the borehole and the rarity or absence of Globorotalia puncticulata. Indeed, the top Globorotalia margaritae was recorded in the Zankee at 3.85-3.81 Ma (Lourens et al., 2004).

On the other hand, the last common occurrence of Globoturborotalita nepenthes (LCO G. nepenthes) was observed at 130m. This event, identified by Lourens et al. (2004) and Lirer et al., 2019 Ma, is dated at 4.37 Ma.

The succession of biostratigraphic events distinguished between 490m and 20m, allows us to attribute these sediments to the Zanclean, from 5.32 Ma to 3.85 - 3.81 Ma.

Conclusion

Qualitative and semi-quantitative analysis of the planktonic foraminiferal assemblages revealed the succession of eight biostratigraphic events that allowed the recognition of the Upper Tortonian, Messinian and Zanclean (Early Pliocene). The sedimentary series studied is located between 7.72 Ma and 3.8 Ma.

The sediments located between 1120m and 1010m are attributed to the Upper Tortonian (7.72 Ma and 7.24 Ma), while the Tortonian-Messinian boundary is located at the depth of 1010m. Whereas the sedimentary deposits located between 1010m and 490 m are of Messinian age, from 7.24 to 5.33 Ma.

The upper part of the borehole, from 490m to 20m, is dated to the Zanclean (Early Pliocene), from 5.32 Ma to 3.85 - 3.81 Ma. The Miocene-Pliocene boundary is probably located at 490m.

Diversity and taphonomie

The analysis of the benthic foraminiferal assemblages of the samples studied, 112 samples, allowed the identification of 134 species classified in 58 genera. The most abundant species are : Valvulineria bradyana ,Uvigerina semiornata,Uvigerina peregrina, Uvigerina hispida, Sphaeroidina bulloides, Siphonina reticulata, Sigmoilopsis schlumbergeri, Pullenia bulloides , Planulina ariminensis, Oridorsalis umbonatus, Nonion boueanum, Melonis soldanii, Lenticulina inornata, Lenticulina calcar, Cibicidoides dutemplei, Cibicidoides dertonensis, Gyroidinoides altiformis, Gyroidina neosoldanii, Globocassidulina subglobosa, Globobulimina affinis, Cibicidoides pachyderma, Cibicidoides floridanus, Bulimina striata , Anomalinoides ornatus, Textularia sagitula, Martinotiella communis ,Ammonia beccarii.

Benthic and planktonic foraminiferal tests are well preserved and well represented in almost all samples.

The Shannon-Weaver diversity index (H) of benthic species in borehole DF101 is nearly constant and shows relatively uniform values. It shows a marked fluctuation at the Mio-Piocene boundary. A decrease is recorded towards the top of the borehole at depths between 80 m and 20 m. 20m, 30m, 40m, 50m, 60m, and 80m.

The equitability index shows moderate values in the range of 0.18 to 0.64, with a maximum value recorded at the 720 m level.

Vertical distribution of abundant species

The three major groups of benthic foraminifera present in borehole DF 101 are: agglutinated (18 species), porcelain (19 species) and hyaline (21 species). The distribution of abundant taxa in borehole DF101 is shown in Figure 6. The most abundant taxa, with a percentage greater than 10%, are represented by the following species: Ammonia beccarii, Cibicidoides pachyderma, Melonis soldanii, Planulina ariminensis, Pullenia bulloides, Uvigerina peregrina, Uvigerina semiornata, Valvulineria bradyana, Textularia sagitula.

While the secondary taxa, whose percentage varies between 4% and 10%, are: Uvigerina hispida, Lenticulina calcar, Lenticulina inornata, Cibicidoides dutemplei, Cibicidoides dertonensis, Gyroidinoides altiformis, Gyroidina neosoldanii, Globocassidulina subglobosa, Globobulimina affinis, Bulimina striata, Martinotiella communis, Siphonina reticulata, Oridorsalis umbonatus, Anomalinoides ornatus, Sphaeroidina bulloides, Sigmoilopsis schlumbergeri.

Ammonia beccarii, Nonion boueanum are major species widespread at the top of the borehole with high percentages. The abundance of Ammonia beccarii can reach 64.86% at the 20m sample, while Nonion boueanum is represented with a percentage of 27.91% in the 90m depth.

Cibicididae (Cibicidoides pachyderma and Cibicidoides floridanus, Cibicidoides dutemplei), are abundant in most of the samples with percentages between 0.35-26.96%, 0.39-13.85% and 0% -10.20% respectively.

The species Melonis soldanii, is weakly represented at the base of the borehole, then becomes relatively abundant throughout the borehole, with a maximum value of 14.42% at the 400m level (Figure.6).

Uvigerinidae (Uvigerina peregrina and Uvigerina semiornata) are represented along the borehole with variations in the range of 0.31%-29.96 % and 0.31 %-96.25 %. These species are known to have an opportunistic lifestyle and the ability to tolerate reduced dissolved oxygen levels (Jorissen et al., 1992; Kaiho, 1994).

As for Valvulineria bradyana, it is characterized by an increasing percentage from 730m to the top of the borehole.

The species Planulina ariminensis is represented by different percentages and a remarkable dominance throughout the borehole with a sudden disappearance recorded towards the top of the borehole from the depth of 100m.

Results of the statistical analysis

Principal component analysis applied to the twenty-six most abundant species allows the determination of six characteristic benthic foraminiferal assemblages (Figure.8, 9).

Assemblage (FA-1): composed mainly by the following species: Oridorsalis umbonatus, Lenticulina inornata, Anomalinoides ornatus, Uvigerina hispida, Gyroidina neosoldanii, Sphaeroidina bulloides, Globocassidulina subglobosa, Siphonina reticulata. This assemblage consists of a quasi-homogenous microfauna, represented by shallow epifauna-endofauna species and shallow endofauna species.

Assemblage (FA-2): the main species that characterize this assemblage are: Cibicidoides dertonensis, Textularia sagittula, Globobulimina affinis, Cibicidoides floridanus, Martinotiella communis, Pullenia bulloides, Cibicidoides dutemplei. The microhabitat of this assemblage has a homogeneous character like the first one but with a dominance of species with shallow epifauna-endofauna microhabitat.

Assemblage (FA-3): represented mainly by: Sigmoilopsis schlumbergeri, Gyroidina altiformis and Bulimina striata. This last species (Bulimina striata) reveals a remarkable dominance that can reach very important percentages of 29.88% at the depth of 640m. The preferred microhabitat of this assemblage is shallow endofauna.

Assemblage (FA-4): consists of the following forms: Valvulineria bradyana, Planulina ariminensis, Melonis soldanii. This assemblage is characterized by a mixed microhabitat of shallow epifaunal species (Planulina ariminensis) that coexist with deep endofaunal species (Melonis soldanii). The overall percentages of these species generally range from 4.31% to 15.18%.

Assemblage (FA-5): this assemblage includes the taxa Nonion boueanum and Lenticulina calcar. It characterizes a microhabitat Endofauna to intermediate

Assemblage (FA-6): consists of Uvigerina peregrina, Cibicidoides pachyderma, Uvigerina semiornata, Ammonia beccarii. This assemblage dominates in the summit part of borehole Df101 with very important global percentages from 10.39% to 69.25%. The microhabitat that characterizes this assemblage is shallow epifauna-endofauna.

Microhabitat

In the borehole studied, the majority of the benthic foraminiferal species observed were still alive today, either at the surface or within the sediments of the marine environment.

In the Tortonian, the preferred microhabitat in the first ten samples (1090 to 1010 m) is endofaunal-epifaunal-shallow (Fig. 11) with percentages ranging from 45.58% to 26.72%. The highest percentage is observed at the 1060m depth.

In the Messinian, the two shallow endofaunal and shallow endofaunal-epifaunal microhabitats are indicated by significant percentages. Deep endofaunal species are also present during this period with the recording of percentages of 5.42% ,12.96%, 13.04%,8.06% respectively at the depths of 960m and 720m,710m, 650m and 570m.

In the Early Pliocene the shallow endofaunal and shallow endofaunal-epifaunal microhabitats continue their dominance (Fig. 11) accompanied by species with epifaunal preference with percentages between 1.35% and 11.03%

Paleobathymetry

The equation obtained by Van Der Zwaan et al (1990), and the corrections applied by Van Hinsbergen et al (2005), allows the reconstruction of the Paleobathymetry of the water slice in the studied sedimentary series. In addition to the information deduced from the P/P+B ratio and the composition of the benthic foraminiferal assemblages.

The Upper Tortonian corresponds to the lower part of the borehole from 1090m to 1010m. This period is characterized by the recording of a maximum depth of about 871.10m at the 1090m level. Bathymetry values in this interval are generally between 871.10m and 622.41m. The ratio (P/P+B) indicates values that oscillate between 90% and 80% (figure 10). It provides information on an open marine environment.

The Messinian is characterized by a Paleobathymetry higher than 300 m in the sediments located between 1010 m and 910 m. and the P/P+B ratio oscillates between 87% and 63%. The microfauna of the interval 910m to 730m is not representative to estimate the Paleobathymetry.

From 730 m to 490 m, we observed a decrease in bathymetry with paleoprofunds lower than 300 m, as shown. The P/P+B ratio indicates values between 53% and 36% which indicates a shallow environment. During this interval, it is noted that the water slice is semi stable with the recording of fluctuations in Paleobathymetry at the depths (550m, 710m) with respectively paleoprofunds of the order of 249.46 m, 280.30m. The P/P+B ratio varies between 53%; 46% during this period.

The Zanclean begins, from 490m to 400m, with a stable shallow marine environment with a paleobathymetry between 143.99m and 379.79m. The P/P+B ratio varies between 65% and 49%.

From 390m to 300m, a very remarkable variation in water depth is observed, the marine environment becomes deeper, with an estimated paleobathymetry varying between 203.20m and 578.88m. The P/P+B ratio varies from 76% to 46%.

The sediments from the top of the borehole located between 280 m and 20 m provide information on a paleoenvironment of less than 200 m depth, with a P/P+B ratio showing values below 60%.

Oxygenation

The percentages of oxygen content obtained from the application of the transfer function (Kouwenhoven et al., 2006) calculated on the sediment-water interface, indicate variations between the oxic and dysoxic domain (figure. 12).

In the Upper Tortonian, calculated values are generally in the oxic range (Figure 12). Sedimentary deposits at the base of the borehole, from 1090 m to 1010m, show values greater than 100 µmol/l with 134.19 µmol/l recorded at the1030m depth.

In the Messinian, oxygenation values vary between the oxic and dysoxic domains throughout the Messinian interval with the oxygen record dropping to values below 100 µmol/l. A peak is recorded at the 960 m level with a value of 240.88 µmol/l.

In the Early Pliocene, from 490m to 280m, the benthic foraminiferal microfauna reveals a paleoenvironment with oxygen concentrations that vary between 146.45 µmol/l and 68.99 µmol/l. The sediments located towards the top of the borehole show a decrease in oxygenation percentages to the lowest value of 11.48 µmol/l at the 40m level.

Reconstruction of the Tortonian to Zanclean paleobathymetry

Analysis of the distribution of benthic foraminiferal assemblages and paleoecological indices (Van Der Zwaan et al 1990) (P/P+P ratio, Shannon-weaver diversity index, statistical analysis, oxygen transfer function), allow us to reconstruct a paleobathymetric evolution from the Upper Tortonian to the Zanclean (figure.5).

Upper Tortonian (7.72 Ma - 7.24 Ma)

In borehole DF101, the Upper Tortonian is located between 1090 m and 1010 m depth, the benthic foraminifera are well diversified (more than 40 species), the Shannon-Weaver diversity index has high values between 3.01 and 3.22. The benthic foraminiferal assemblage deduced from the statistical analysis includes the following species: Sphaeroidina bulloides, Siphonina reticulata, Oridorsalis umbonatus, Globocassidulina subglobosa, Anomalinoides ornatus, Gyroidina neosoldanii, Uvigerina hispida. In the literature (Blanc vernet 1967, Mathieu 1986, van Morkhoven et al., 1986, Van de Poel, 1992), these species live in great depths (bathyal to abyssal domain). From these data, the estimated paleo-depth for these sedimentary deposits is of the order of 871.10m (Middle bathyal domain) with a maximum value recorded at depth 1090m.

These results are in agreement with the values of the P/P+B ratio, which shows the same trend with the recording of values between 80% and 90% (Figure. 10).

Previous work on benthic foraminifera during the Upper Tortonian at the Oued Akrech section (Barbieri and Ori (2000); Warny et al 2000, 2003) also suggests a deep marine environment.

At the Tortonian-Messinian boundary 7.24 Ma, the continuity of marine conditions observed in the Upper Tortonian is observed at a depth of 1010 m. From 980 m, just above the boundary, a decrease in water depth begins.

In addition, work on dinokysts (Warny et al 2003) shows the transition from open marine to external neritic conditions, while studies by (Hodell et al., 1994 and Zhan and Scott, 1996) report an increase in δ18O at the Tortonian-Messinian boundary in the North Atlantic, possibly related to a decrease in temperature and an increase in ice volume.

Messinian

Analysis of benthic foraminiferal assemblages from borehole DF 101 reveals significant changes in paleoenvironments during the Messinian. This is related to climatic changes and the tectonic and paleogeographic context that characterize the western Rifian corridor during this period (Flinch, 1994; Flinch et al, 1996; Krijsman et al, 1999; Hodell et al, 2001; Zouhri et al, 2002; Warny et al, 2003; Krijgsman et al, 2004; Sissingh, 2008; Iribarren et al, 2009).

Between 7.24 Ma and 6.52 Ma ( Early-Middle Messinian): after the Tortonian-Messinian boundary (from 980m to 720m), the distribution of benthic foraminifera shows that the Gharb Basin is located at a depth that varies between 729.75m and 347.69 m (middle to upper bathyal). At this interval the P/B+P ratio presents values between 55% and 85%, and the paleobathymetry indicates the beginning of a progressive decrease of the bathymetry. This is confirmed by a reduction in the percentages of the assemblage composed of Globocassidulina subglobosa, Siphonina reticulata and Oridorsalis umbonatus that colonize the bathyal and abyssal zones (Berggren and Haq, 1976; Berggren et al., 1976; Hayward et al., 2003, Pérez-Asensio et al. (2012)) until their near total disappearance from 730m.

Between 6.52 Ma and 6.35 Ma (Middle Messinian), from 720m to 630m, the estimated depth varies between 280.30m to 199.84m, and the P/P+B ratio oscillates between 0.52and 0.40. The Q-mode PCA reveals that the assemblage composed of: Cibicidoides floridanus, Textularia sagittula, Globobulimina affinis, Martinotiella communis, Pullenia bulloides, Cibicidoides dutemplei, Cibicidoides dertonensis predominate with high percentages (Figure.8,9). The taxa Cibicidoides floridanus and Cibicidoides dutemplei characterize the lower neritic zones (Barberi et al,2000, Van Morkhoven et al. (1986). Whereas Textularia sagitula has the same bathymetric preference, with maximum abundance generally between 50 m and 200 m (Blanc Vernet et al., 1983, Pujos-lamay, 1973). All these indices testify to a decrease in paleobathymetry, thus from 720m, the environment evolves from upper bathyal to lower neritic. This sea level decrease is well marked from~6.52 Ma. Barberi et al., 2000 also mentioned a sea level decrease during the Middle Messinian. This paleobathymetric variation could be a consequence of regional tectonic uplift (Warny et al, 2003, Krijsman et al; Capella et al 2018).

Between 6.35Ma and 5.33 Ma: from 630m to 490m, the paleobathymetric decrease is well accentuated, (less than 250 m). The P/P+B ratio is from 53% to 36% (0.53-0.36) indicating a shallow environment. This decrease coincides with the period of glacial intensification (6.26Ma to 5.5Ma) reported by (Hodell et al., 2001) from the δ18O isotope record from ODP in the North Atlantic and shows that δ18O values remained elevated during Chron C3An until Chron C3r (Hodell et al., 2001, Hodell et al., 1996; Braberi et al.,2000 ). In this interval the benthic foraminiferal microfauna presents the abundance of taxa that characterize the neritic zone (VanMorkhoven et al. (1986), Murray (2006), Blanc vernet et al., 1983): Cibicidoides pachyderma, Ammonia beccarii, Uvigerina peregrina in addition to species that mark the ecological stress (Kouwenhoven et al., 2006; Van der Zwaan et al., 1990; Van Hinsbergen et al.,2005; Corbí, 2010). (Figure 6).

The δ18O records from Ain el Beida (Krigjsman et al.,2004) and Salé (Hodell et al., 1994), ODP Site 846 (Shackleton et al. 1995a, Shackleton et al. 1995b) show that the highest δ18O values are recorded in the Middle Messinian from Chron C3r onwards (Barberi et al.,2000 ), the most pronounced glacial stages coinciding with these values are TG22, TG20, (Krijgsman et al.,2004).

The ages of these stages are 5.79 Ma and 5.75 Ma (Krijgsman et al,2004), well after the onset of the MSC which occurred at 5.96 ± 0.02 Ma (Krijgsman et al, (1999 ,2004); Van den Berg et al, 2018).

In borehole DF 101, these glacial stages are probably recorded around depths of 520 m and 510 m. In these levels we note an abundance of Ammonia beccarii with percentages >20%, Textularia sagittula and Nonion boueanum, Cibicidoides pachyderma (figure.6). These taxa are typically characteristic of the outer neritic zones (Blanc vernet et al.,1983, Mathieu 1986). The estimated paleo-depth varies between 151.70m and 142.52m. This sea level lowering in the western Rifian corridor, observed in borehole DF101 and also in the Bouregreg composite section (Barberi and Ori,2000) in the Messinian is likely to post-date the onset of the MSC (Warny et al.,2003 ; Krijgsman et al., 2004; Vander Lan et al., 2012) Therefore, sea level declines associated with these glacial stages could not be the primary cause of the onset of the MSC (Krijgsman et al.,2004; Van der Laan et al., 2012).

Miocene-Pliocene boundary 5.33 Ma:

Around the Mio-Piocene boundary (i.e. from 500 m to 490 m), we notice that the sea level drop continues (of the order of 143.99m), the P/P+B ratio also shows a value of 0.39, with the abundance of the species Cibicidoides floridanus. In addition the diversity index shows a value < 2 at DF101 490m. Warny et al (2003) detected a strong continental influence around 5.4Ma marked by the high concentration of Pollen grains. From sample 480m, just above the Mio-Piocene boundary, we note the beginning of a slight rise in bathymetry of the order of 217.00m with an abundance of the assemblage Valvulineria bradyana, Melonis soldanii, Planulina ariminensis, this last species characterizes the lower neritic and upper bathyal conditions, these results are in agreement with Barberi et al. At the Bourgreg composite section, the Mio-Piocene boundary is marked by a decrease in oxygen isotope values δ 18O (Barberi et al.,2000) following a glacio-eustatic rise. Similarly, the work of Van der laan et al. 2006 in the Oulja-A section showed that a minimum of δ 18O is recorded around 5.315 Ma. The P /P+B ratio and the benthic foraminiferal assemblage show that this passage is similar to that illustrated in borehole DF 101.

Zanclean from 5.33 Ma to 3.8 Ma

From 450 m to 280 m, the vertical distribution of benthic foraminifera is marked by the continuous dominance of the Planulina ariminensis assemblage (Figure 6). The diversity index records values >3 (figure 5) which means that from the Mio-Pliocene boundary the diversity increases. The equation of Vander Zwaan (1990) reveals a variation of the paleo-depth that varies from 100.27m- 578.33m, as well as the P/P+B ratio varies between 0.31 and 0.75 shows the transition to the upper bathyal environment following the sea level glacio-eustatic rise (deglaciation) from 5.5Ma Vandelaan et al,2006, 2012). This eustatic rise played only a minor role in the basal flooding of the Pliocene Mediterranean. The most important factor is probably tectonics (Vandelaan et al, 2006). In the Mediterranean basin this level marks the beginning of the installation of normal marine conditions opened following the Messinian salinity crisis (Hsu¨ et al., 1973; Cita, 1975; Hilgen and Langereis, 1993; Iaccarino et al., 1999, 2007) marked by a high sea level probably an intensification of maritime communication via the Strait of Gibraltar (Warny et al 2002).

From 280m to the top of the borehole, there is a strong decrease in depth to 50m with a dominance of taxa that characterize the outer shelf Ammonia beccarii, Nonion boueanum (Mathieu,1986) with abundances >20%. The continuous diversity index recorded high values from the Mio-Piocene boundary to 140m. From this level, the diversity progressively decreases as well as the paleoprofund >100m.

Oxygen content of the seabed

The benthic foraminiferal assemblages identified in borehole DF101 provide useful information for the paleoenvironmental reconstruction of the Upper Miocene-Pliocene sedimentary series of the Gharb basin. In general, 6 benthic foraminiferal assemblages are identified that provide important indications on the supply of organic matter and dissolved oxygen content in seawater. These two bioindicators are the factors that control the vertical distribution of benthic foraminifera. Their effects on benthic assemblages cannot be analyzed separately (Pérez-Asensio 2012).

Upper Tortonian (7.72 Ma - 7.24 Ma)

During the Upper Tortonian (1090 m to 990 m) the sediments were deposited in a deep environment, this interval is characterized by the FA-1 assemblage (Fig. 8) which is typical of lower bathyal environments. The transfer function of Kouwenhoven et al, (2006) allowed us to deduce that the oxygenation rate during the same period was >100 µmol/l O2, indicating that this is a moderately oxygenated paleoenvironment. The distribution of benthic foraminiferal assemblages is dominated by taxa with a shallow epifaunal-endofaunal microhabitat (Siphonina reticulata, Oridorsalis umbonatus, Sphaeroidina bulloides) with the presence of species that have a shallow endofaunal lifestyle (Bolivina spathulata). At the Oued Akrech section, benthic oxygen isotopes show no change, suggesting that the deeper waters are not influenced by changes in climate and precipitation (Kouwenhoven et al, 2006). At the Tortonian-Messinian boundary 7.24 Ma there is a slight decrease in oxygenation of about 132.36 µmol/l O2.

Messinian between 7.24Ma and 5.3 Ma:

The Messinian is characterized by a middle bathyal paleoenvironment with an increase in oxygen content >150 µmol/lO2. This increase coincides with the abundance of oxyphilic taxa (Table 1): Cibicidoides floridanus, Globocassidulina subglobosa (Kouwenhoven et al 2006). This period presents a mixed microhabitat, as epifauna-endofauna species (Cibicidoides floridanus) coexist with shallow endofauna taxa (Pullenia bulloides) and deep endofauna taxa (Globobulimina affinis) (Table 2). This assemblage explains the variation in calculated oxygen content observed in this interval.

At samples between 910 m and 500 m, there is a strong decrease in oxygenation >90 µmol/lO2 associated with increases in bathymetry (Fig. 12). This could be explained by the abundance of Globobulimina affinis, which is considered to be closely related to areas of zero oxygen concentration (Eberwein, A. et al 2006). The same ecological preference is noted in Bulimina striata (Kouwenhoven et al,2000) which also dominates in these samples. This is supported by the work of Kouwenhoven et al. (2006) who demonstrate that after 7.2 Ma, the deepest sites became increasingly populated by taxa tolerant of ecological stress.

Zanclean (5.33Ma - 3.8Ma)

During the Zanclean the benthic foraminiferal assemblage is composed mainly of Nonion boueanum and Uvigerina peregrina associated with Uvigerina peregrina. The latter species, considered opportunistic, is present in environments with high nutrient flux and moderate oxygen depletion (Schmiedl et al., 1997, 2000; 2010; Fontanier et al., 2002; Koho et al., 2008). Its presence with Uvigerina semiornata influences the calculation of the oxygenation rate.

The Nonion boueanum assemblage alternates with the Uvigerina peregrina assemblage. The species Uvigerina peregrina, which is considered opportunistic, is present in environments with high nutrient flux and moderate oxygen depletion (Schmiedl et al., 1997, 2000; 2010; Fontanier et al., 2002; Koho et al., 2008). Its presence with Uvigerina semiornata affects the oxygenation rate calculation.

In general, the oxygen content during the Zanclean is higher than 150 µmol/lO2 until the 210 m level. From this depth, a period of dysoxia (values below 100 µmol/lO2) is observed until the top of the borehole.

A detailed paleoenvironmental analysis of the Neogene deposits of the Gharb Basin is carried out in support of the microfaunal data set, and paleoecological indices (P/P+B ratio, diversity indices, statistical analysis, oxygen transfer function) of benthic foraminifera belonging to borehole DF 101. The latter allowed us to trace the evolution of the sedimentary environments of the southern Rifian corridor during the Late Tortonian to Zanclean.

The Upper Tortonian is marked by a very diverse benthic foraminiferal assemblage with a dominance of shallow epifauna-endofauna and shallow endofauna taxa. This Upper Tortonian assemblage characterizes the upper bathyal zones (Siphonina reticulata). Paleobathymetry and paleoecological indices show that the environment is deep (WD >800 m). The sea level decrease started from the Tortono-Messinian limit detected from the P/P+B ratio and microfaunal data.

The Messinian marl deposits are characterized by 3 paleobiological assemblages (FA-2 to FA-4) inferred from statistical analysis. These assemblages reflect shallower paleoenvironments (Cibicidoides pachyderma, Cibicidoides floridanus) with variations throughout the Messinian. The TG 20 and TG 22 glacial stages of age 5.79 Ma and 5.75 Ma probably caused sea level decreases of about 107m. This transition to shallower areas is often accompanied by a reduction in oxygenation rates.

From the Mio-Piocene boundary onwards there is a sea level increase. The Early Pliocene record shows significant changes in paleobathymetry as a result of climate change. The first interval shows depths >100m and a dominance of Planulina ariminensis and Valvulineria bradyana species. This increase may be due to the glacio-eustatic sea level rise (deglaciation) from 5.5Ma.

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Tables 1-3 are in the supplementary files section.

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Table 1: Species introduced in the transfer function allowing the calculation of bottom water oxygenation.
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Table 2: The preferred microhabitat of benthic foraminiferal species belonging or drilling DF 101(Pérez-Asensio et al., 2012).
tablchr.jpg
Table 3: Late Neogene events observed by Hilgen et al. (1995) in the southern Rifian corridor (Morocco). Abbreviations: D, dexter; S, sinister.

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Late Miocene-early Pliocene Palaeoenvironmental Changes in the Western Rifian corridor (NE Morocco)

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Abstract

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Introduction

Geographic And Geological Setting

Description Of Borehole Df-101

Materials And Methods

Biostratigraphic Framework Of Borehole Df101

Results

Discussion

Conclusion

References

Tables

Supplementary Files

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