Accepted Manuscript Geochemical constraints on the provenance and depositional environment of the Messinian sediments, onshore Nile Delta, Egypt: Implications for the late Miocene paleogeography of the Mediterranean Mahmoud Leila, Andrea Moscariello, Branimir Šegvić PII:
S1464-343X(18)30084-0
DOI:
10.1016/j.jafrearsci.2018.03.024
Reference:
AES 3176
To appear in:
Journal of African Earth Sciences
Received Date: 11 September 2017 Revised Date:
17 March 2018
Accepted Date: 20 March 2018
Please cite this article as: Leila, M., Moscariello, A., Šegvić, B., Geochemical constraints on the provenance and depositional environment of the Messinian sediments, onshore Nile Delta, Egypt: Implications for the late Miocene paleogeography of the Mediterranean, Journal of African Earth Sciences (2018), doi: 10.1016/j.jafrearsci.2018.03.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Geochemical constraints on the provenance and depositional environment of the Messinian sediments, onshore Nile Delta, Egypt: Implications for the Late Miocene paleogeography of the Mediterranean
Mahmoud LEILAa,b, Andrea MOSCARIELLOa, Branimir ŠEGVIĆa,c Department of Earth Sciences, University of Geneva, Geneva, Switzerland
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Department of Geology, Mansoura University, Mansoura, Egypt
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Department of Geosciences, Texas Tech University, Lubbock, Texas, USA
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Geochemical constraints on the provenance and depositional environment of
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the Messinian sediments, onshore Nile Delta, Egypt: Implications for the Late
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Miocene paleogeography of the Mediterranean
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Mahmoud LEILAa,b*, Andrea MOSCARIELLOa, Branimir ŠEGVIĆa,c
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Department of Earth Sciences, University of Geneva, Geneva, Switzerland
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b
Department of Geology, Mansoura University, Mansoura, Egypt
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c
Department of Geosciences, Texas Tech University, Lubbock, Texas, USA
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*Corresponding author:
[email protected]
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Abstract
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The Messinian sequence rocks in the Nile Delta present prolific hydrocarbon
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reservoirs and are, therefore, of great importance from the aspect of petroleum
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exploration and development strategies. Yet, little is known about their tectonic
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provenance and depositional setting. This study focuses on the geochemical signatures
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archived in the Messinian siliciclastic sediments to employ them as a powerful tool to
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elucidate the basin evolution during the Messinian salinity crisis (MSC). The pre-MSC
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Qawasim sediments are texturally and compositionally immature. They are enriched in
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lithic fragments, foraminiferal bioclasts, and rounded heavy minerals suggesting a
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significant contribution from the pre-existing Cretaceous-Eocene mixed carbonate-
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siliciclastic rocks bordering the Nile Delta. In contrast, the textural and mineralogical
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compositions as well as a range of geochemical proxies (e.g., chemical index of
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alteration and weathering CIA, CIW as well as index of chemical variability ICV and
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Zr/Sc ratio) are in favor of prolonged weathering and at least second-cycle origin of the
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MSC Abu Madi sediments. The mutually correspondent elemental ratios (e.g.,
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Al2O3/TiO2, K2O/Na2O, Zr/Hf, Rb/Sr, Cr/Zr, and Cr/Th) and uniform weathering trends
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are indicatives for a similar provenance of the pre-MSC Qawasim and MSC Abu Madi
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sediments. Rare earth element (REE) distribution reveals a significant enrichment in
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LREE, depletion in HREE, relatively high (La/Yb)N (mean> 9), low (Gd/Yb)N (mean< 2)
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and a pronounced negative Eu anomaly (mean~0.75) in the studied Messinian facies,
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characteristics of upper continental sources of mainly felsic to intermediate rock
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affiliations. Provenance proxy ratios (e.g., Al/Ti, La/Sc, Th/Sc, La/Co and Eu/Eu*) along
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with the low concentration of transition trace elements (Cr, Ni, Co, Ni) are effectively
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ruling out the contribution from mafic and ultramafic rocks.
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The investigated Messinian sedimentary facies have similar passive margin
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geotectonic setting and their source rocks were originated in a continental collision
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tectonic setting that lasted from Late Cretaceous to Oligo-Miocene time. This is
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confirmed by the Nb/Ta, Zr/Sm ratios coupled with the pronounced Nb, Ta, P, Ti
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anomalies and enrichments in Pb and U relative to primitive mantle typical of subduction
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zone environment. The petrographical and geochemical results suggest the MSC Abu
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Madi sediments to have been eroded and recycled from the older pre-MSC Qawasim
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sediments by gravity-flow processes and fluvial channels prior to redeposition as
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incised-valley-fills
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paleoenvironmental indicators such as C-value, Sr/Cu and Sr/Ba confirm arid-dry
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climatic conditions during the onset of the MSC consistent with the Mediterranean
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desiccation. These indicators also depict a transition from freshwater to relatively
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normal salinity conditions during the late stage of the MSC. Geochemical results
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presented in this study support the retrogradational depositional infill of the Messinian
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incised valleys in the Nile Delta, thus confirming an incipient rise in the Mediterranean
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Sea level prior to the major Zanclean flooding.
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Keywords:
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Geochemistry; Messinian; Nile Delta; Qawasim; Abu Madi.
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1. Introduction
the
late
stage
of
the
MSC.
The
geochemical
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The composition of siliciclastic sediments supplied to a sedimentary basin depends
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on the nature of their parent rocks, climatic and physiographic circumstances which
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significantly control the intensity of the source area weathering. Therefore, the chemistry
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and mineralogy of clastic sedimentary rocks are readily used to infer on sediment Page 2
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provenance, weathering rates, hydraulic sorting and abrasion effects (Nesbitt et al.,
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1980; Dickinson, 1985). However, diagenesis may exert major modifications in the
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mineralogical composition of the sediments obliterating the original signal characteristic
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for the sediment source area (Taylor and McLennan, 1985; McLennan et al., 1993; Pe-
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Piper et al., 2008). Therefore, only the integration between the petrographical
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observations and geochemical composition of clastic sediments could provide
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comprehensive information about the pre-, syn- and post-depositional processes and,
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hence, the evolution of the sedimentary basin (Dickinson, 1985; McLennan et al., 1993;
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Pe-Piper et al., 2008).
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In the onshore part of the Nile Delta including the study area (Fig. 1), the Messinian
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sequence hosts the main hydrocarbon producing reservoirs (Qawasim and Abu Madi
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formations) (Dolson et al., 2001, Niazi and Dahi, 2004; Leila et al., 2016). The dramatic
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sea-level fall and rise occurred during and after the Messinian salinity crisis (MSC) were
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resulted in incision of deep valleys (e.g. Eonile (Nile predecessor) canyon, Fig. 1) that
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were infilled by the MSC Abu Madi reservoir sediments and buried under a thick
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sequence of Plio- Pleistocene sediments (Dolson et al., 2001). Furthermore, the pre-
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MSC sediments of the Qawasim Formation have recently been assessed as very
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prospective hydrocarbon reservoirs (Leila et al., 2016; Leila and Moscariello, 2017).
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However, the published data concerning the provenance of both the pre-MSC and MSC
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Messinian sediments are very meagre (El-Sisi et al., 1996) and only rely on scarce
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petrographical observations. Therefore, many uncertainties and debates concerning
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their provenance and depositional evolution still exist. Depending solely on the modal
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composition which is a function of weathering intensity and post-depositional alterations
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to infer the sedimentary rock provenance would be unreliable. As a consequence, this
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study is the very first to tackle these issues using comprehensive integration between
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geochemical and petrographical approaches as powerful means for deciphering the
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provenance and constraining the tectonic and depositional settings of the Messinian
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sediments. Identifying the geochemical fingerprints archived in the Messinian
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siliciclastics is supposed to successfully delineate the sedimentation history as well as
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the paleogeographic and paleoenvironmental conditions needed to understand the Nile
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Delta basin evolution during the MSC.
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2. Geologic setting The Nile Delta is located in the slightly deformed North African margin between three
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main tectonic boundaries: the African-Anatolian plate, the Syrian arc system and the
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Red Sea Rift (Said, 1990; Sarhan and Hemdan, 1994). A faulted flexure zone (Hinge
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zone) is the most prominent structural feature in the Nile Delta. It dates back to the
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Jurassic crustal breakup of the southern Neotethys, extends E-W across the middle
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delta area, and strongly controlled its sedimentary patterns (Ross and Uchupi, 1977;
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Kamel et al., 1998; Sarhan and Hemdan, 1994; Abdel Aal et al., 1994). The Hinge Zone
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subdivides the Nile Delta basin into two main structural sedimentary provinces: the
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South Nile Delta block and the North Nile Delta basin (Kamel et al., 1998) (Fig. 1). In
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contrast to the North Nile Delta basin which is located in the steeply faulted continental
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shelf, the South Delta block lies on the unstable shelf area (Kerdany and Cherif, 1990;
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Meshref, 1990). Therefore, the Hinge Zone represents also a prominent boundary
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between the South Delta Cretaceous-Eocene platform and the Tertiary thick basinal
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facies of the North Nile Delta basin (Kamel et al., 1998). The Hinge Zone also controlled
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the tectonic evolution of the entire Nile Delta province, as continuous subsidence along
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it in the North Nile Delta basin triggered a massive and rapid thickening of the Tertiary
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sediments basinwards (Sestini, 1995). During Upper Cretaceous and Neogene times,
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Temsah and Rosetta fault trends had been activated and subdivided the North Nile
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Delta basin into three sub-basins: eastern, western and central (Fig. 1) (Kerdany and
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Cherif, 1990; Meshref, 1990; Kamel et al., 1998). In the central sub-basin -where the
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studied fields (West Dikirnis (WD) and West Al-Khilala (WAK)) are located - the Tertiary
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sediments thicken and host the most potential hydrocarbon source rocks and reservoirs
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(EGPC, 1994).
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As revealed from the stratigraphy of the study area (Fig. 2), the Upper Jurassic
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shallow marine carbonates are the oldest sedimentary rocks penetrated by drilling
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(Abdel Aal et al., 1994). During Early Cretaceous, shallow-marine facies covered the Page 4
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entire Nile Delta (Guiraud and Bosworth, 1999). The depositional environment has been
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changed from open marine to alternating marine and continental during the early Late
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Cretaceous prior to the restoration of open marine conditions at the end of the
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Cretaceous (Said, 1990; Guiraud and Bosworth, 1999). The subsequent erosion and
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non-deposition to the Syrian Arc folding had resulted in a very thin, rarely penetrated
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Upper Cretaceous-Eocene strata (Harms and Wray, 1990). The Oligocene sediments
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are represented by alternating coarse-grained terrigenous fluviatile facies and open
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marine mudstones (Harms and Wray, 1990; Said, 1990).
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interval, two main unconformities are recorded in the Miocene and Pliocene sequences
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of the Nile Delta similar to the other places in the circum-Mediterranean region (Rizzini
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et al., 1978; Barber, 1981; Said, 1990). These unconformities document several major
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changes in the Nile Delta paleobathymetry and sedimentation rates (Harms and Wray,
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1990) (Fig. 2). The first unconformity separates the Middle Miocene from the Upper
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Miocene strata, whereas the second separates the Upper Miocene from the Lower
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Pliocene sediments. The latter represents the major MSC desiccation event which
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marks the onset of widespread fluvial incisions in the Nile Delta ended by prominent
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fluvio-marine infill (Said, 1990). Marine transgression occurred during Early Pliocene
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(Zanclean) covered the entire Nile Delta basin with marine facies (Ross and Uchupi,
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1977; Said, 1990). The marine conditions continued until Late Pliocene which
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documents a period of major climatic changes accompanied by a significant shift of the
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depositional system from fluviomarine to fluvial (Said, 1990).
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3. Materials and methods
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3. 1. Sedimentary facies analysis
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Above this stratigraphic
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A total legnth of approxiomately 255 m of conventional cores from five wells (WD-2,-
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1,-9 and WAK-2,-5; Figs. 1, 2) was used in this study. The cores were examined using
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hand lens and binocular microscope for identification of different lithologies and
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sedimentary structures required for interpretation of the different sedimentary facies and
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depositional regimes.
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3.2. Petrography 40 thin sections were prepared from the conventional core plugs and examined by
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standard optical microscopy in order to investigate their textural and mineralogical
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compositions. The thin sections undergone vacuum impregnation with blue resin for
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porosity determination.
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potassium ferricyanide solution which imparts blue colour with iron rich carbonates. For
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the recognition of alkali feldspars, the thin sections were stained with a solution of
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sodium cobaltnitrate. Moreover, cathodoluminescence microscopy (CL) was conducted
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on 15 carbon-coated polished thin sections at the University of Geneva, Switzerland,
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using a ERI-MRTech-optical cathodoluminescence microscope equipped with cold
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cathode and mounted on an Olympus BX41 petrological microscope. The beam
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conditions were 18 kV at 120–200 µA with about 1 cm unfocused beam. The residual
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pressure in the observation chamber is approximately 80 mTorr.
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They were stained with a mixture of Alizarin Red-S and
Automated mineral and textural characteristics were analyzed using a FEI
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QEMSCAN® Quanta 650F facility at the Department of Earth Sciences (University of
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Geneva, Switzerland) on 12 carbon-coated rock chips molded into epoxy and minimally
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polished in order to avoid clay fraction loss. QEMSCAN® technology essentially relies
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on automated scanning electron microscopy coupled with the energy dispersive X-ray
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spectra unit (EDS). Identification of the different mineral phases relies on a combination
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of back-scattered electron (BSE) brightness values, EDS low-counts and X-ray count
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rate thus providing the elemental composition (Gottlieb et al., 2000). Individual X-ray
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spectra are then compared against a library of known spectra allowing a proper mineral
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identification.
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3. 3. Major and trace element geochemistry
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Bulk-rock powders for chemical analyses were prepared from 40 representative
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samples (Fig. 2). The samples were analyzed for major elements by ICP-ES and for all
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trace elements by ICP-MS at the commercial ACME Laboratories in Vancouver,
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Canada (Tables 1-3). Lithium metaborate fusion was utilized for the dissolution of major Page 6
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elements and triple acid digestion for the dissolution of trace elements in the studied
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samples. Standard reference material STD SO-18 provided by ACME was used to
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determine the accuracy of the analyses and selective repeated measurements were
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used to acquire the precision. The loss on ignition (LOI) represents the difference in the
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sample weight after ignition at 1000 °C for approximately one hour. Major element and
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trace element concentrations were measured with an accuracy not exceeding 1% and
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5%, respectively.
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The elemental compositions retrieved from the lithogeochemical analyses were used
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in the calculation of several geochemical proxies such as chemical index of alteration
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CIA= ( (Al2O3/(Al2O3+CaO*+ Na2O+ K2O)) x 100 (Nesbit and Young, 1982), Chemical
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index of weathering CIW= (Al2O3/(Al2O3+Na2O+ CaO*)) x100 (Harnois, 1988),
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plagioclase index of alteration PIA= (Al2O3- K2O)/ ((Al2O3- K2O)+ CaO*+ Na2O)) x 100
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(Fedo et al., 1995). All the major oxides are expressed in molar proportion and CaO* is
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the content of CaO incorporated in silicate fraction (McLennan et al., 1993). For the
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index of chemical variability ICV= (Fe2O3+ MgO+ CaO+ MnO+ Na2O+ K2O+ TiO2)/
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Al2O3 the oxides are expressed in Wt% (Cox et al., 1995). Moreover, the
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paleoenvironmental index C-value = Ʃ (Fe+ Mn+ Cr+ Ni+ V+ Co)/ Ʃ (Ca+ Mg+ Sr+ Ba+
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K+ Na) was also calculated using the elemental compositions in Wt% (Zhao et al., 2007;
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Cao et al., 2012).
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4. Results
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4. 1. Seismic stratigraphy and sedimentary facies
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Using seismic reflection profiles acquired within the study area, we were able to
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distinguish between the pre-MSC and MSC facies based on their acoustic
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characteristics, geometry and stratal terminations. The pre-MSC facies are erosionally
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truncated by a prominent high-amplitude, highly-undulated seismic reflector displaying
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significant erosional features and V-shaped channels (Fig. 3). This reflector is topped by
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transparent seismic facies which are typically corresponding to the Pliocene open
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marine mudstones.
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organization of this seismic reflector virtually refer to the Messinian erosion surface
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(MES) as described by Lofi et al. (2011a, b). Internally, the pre-MSC facies consist of
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wavy, high-amplitude, high-frequency and fairly continuous reflectors. Two acoustically
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different units were differentiated within the pre-MSC facies: the first unit thickens
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southwards and consists of high-amplitude, wavy and sub-parallel reflectors, whereas
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the second unit thickens northwards and is represented by semi-transparent, fairly-
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chaotic and low-amplitude reflectors downlaping the basal boundary (Fig. 3). Herein, the
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MES is a diachronous surface passes northwards into another prominent erosion
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surface. The latter consists of high-amplitude, undulated and continuous reflector
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forming an erosively-based U-shaped canyon (Eonile canyon). This canyon is infilled
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by the MSC Abu Madi facies (Fig. 3), and therefore it corresponds to the bottom erosion
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surface (BES) as described by Lofi et al. (2011a, b). Furthermore, the MSC infill facies
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are topped by high-amplitude and continuous erosional seismic unconformity separating
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them from the transparent Pliocene mudstones (Fig. 3). This surface corresponds to the
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top erosion surface (TES) which marks the end of the MSC and the onset of Zanclean
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flooding in the entire Mediterranean basin (Lofi et al., 2011a, b). Internally, the Eonile
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canyon is infilled by three distinctive Abu Madi MSC units. The lower unit paves the
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canyon floor and consists of chaotic, transparent and low-amplitude reflections that
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reach a maximum thickness of ~150 ms TWT (millisecond two-way time). The middle
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unit thickens southwards and consists of high-amplitude, continuous and parallel
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reflectors (Fig. 3). The upper unit pinches out northwards, and is represented by low-
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frequency, low-amplitude and semi-transparent seismic reflections.
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The pre-MSC and MSC sequences were penetrated and cored in the study area and
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based on the digital core images we were able to characterize and interpret their
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different sedimentary facies. The pre-MSC Qawasim Formation has been deposited in a
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delta dominated by coarse-grained sediments (Leila and Moscariello, 2016). Based on
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the sedimentological
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recognized in the Qawasim Formation. These are the distributary channel fill, proximal
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delta front and distal delta front facies. The first two facies associations represent the
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Qawasim proximal deltaic facies (first pre-MSC seismic unit) that are composed mainly
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characteristics, three main lithofacies associations
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of coarse-grained, pebbly massive and cross-bedded sandstones (Fig. 4, core 1). The
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pebbly and cross-bedded sandstones are common in the distributary channels, whereas
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the proximal delta front deposits are dominated by massive sandstones with rare
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horizontal faint laminations. The coarse-grained and clean (low-clay content) sandstone
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composition of these facies explain the high seismic amplitude of the pre-MSC first unit
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discussed above. The crossbedding sets are mainly dipping NE with mainly uniform dip
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magnitude and direction. The proximal deltaic facies grades laterally northward into fine-
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grained facies containing planar crossbedded sandstones, ripple- and cross-laminated
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sandstones (Fig. 4, core 2). These fine-grained sediments correspond to the northward
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thickens, fairly transparent and low amplitude reflectors of the second pre-MSC unit.
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This transition in sedimentary facies suggests a northward progradation of the pre-MSC
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deltaic system. The distal deltaic (distal delta front) sediments are also enriched in
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mudstone/siltstone interbeds and mud drapes reflecting the abrupt decrease in the
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depositional energy accompanied by a relatively lower discharge rate compared with
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the proximal deltaic sediments. Moreover, the distal sediments are tidally influenced as
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evidenced from the common mud drapes on current ripples and ripple foresets (Fig. 4,
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core 2).
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Consistent with the abovementioned seismic stratigraphy, the MSC Abu Madi infill
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facies are subdivided into three main sedimentary units. Deformed sandy-muddy
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heterolithic deposits are paving the canyon floor (Fig. 4, core 3). Being enriched in
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pervasive sandstone injectites and chaotic muddy breccia, these sediments retained the
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signs of the mass transport deposits (MTD). They were build up during the downslope
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collapse of the sediments along the margins of the canyon by mass transport processes
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and most likely generated by subaerial gravity flows. This unit grades up-sequence into
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coarse-grained, massive and crossbedded sandstones (Fig. 4, core 4). The massive
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appearance and crossbedding sedimentary structures suggest high energy depositional
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conditions accompanied by high discharge rate mostly similar to sheet-floods (Miall,
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1978). The crossbedding foresets are dipping to the NW indicating a uniform
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paleocurrent-flow direction. These sandstones represent the prograding fluvial channel-
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fill deposits that are thickening landward. They are followed by fine-grained ripple
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laminated sandstones interbedded with mudstones/siltstones (Fig. 4, core 5). Analogue
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to Qawasim distal deltaic facies, these fine-grained sediments are highly enriched in
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mud drapes, current ripples and tidal rhythmites typifying their deposition in tidal and
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coastal marine environment (Leila and Moscariello, 2016). Moreover, the sandstones
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are highly calcareous at the base and argillaceous toward the top. These characteristics
263
reflect the retrogradation of the system from continental fluvial to marginal marine tidal-
264
dominated environment corresponding to the backstepping infill of the canyon.
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Based on the sedimentary facies analysis, the pre-MSC Qawasim sediments were
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deposited in an overall fluvio-deltaic environment with deltaic progradation laterally
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basinward.
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Formation display a complex interplay of geologic events ranging from the continental
269
mass-flows, fluvial to marginal marine tidal processes. Therefore, a comprehensive
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correlation between the preserved geochemical signatures in the pre-MSC and MSC
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sedimentary facies is crucial for understanding their depositional setting, and hence the
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Nile Delta basin evolution. Moreover, it may provide some new scenarios for the MSC
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in the Nile Delta and the range of marginal basins along the Mediterranean.
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4. 2. Petrology of the Messinian sedimentary facies
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On the other hand, the MSC incised-valley infill facies of Abu Madi
The pre-MSC Qawasim sediments are dominated by sub-rounded, poorly sorted
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detrital grains composed mainly of quartz and lithic fragments (Fig. 5), reflecting their
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texturally and compositionally immature to sub-mature characteristics. The MSC Abu
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Madi sediments are moderately to moderately-well sorted, and consist mainly of sub-
279
rounded to rounded detrital quartz grains (Fig. 6). Therefore, the Abu Madi facies is
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comparatively more mature. Nonetheless, Qawasim detritus are coarser in grain size.
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Quartz is the most abundant detrital framework both in the Qawasim and Abu Madi
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sediments (Fig. 7). It decreases up-sequence in the Qawasim Formation from the
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proximal to distal deltaic facies, and in Abu Madi Formation from the fluvial to the tidal
284
and coastal marine sediments (Fig. 7E, F). Both monocrystalline and polycrystalline
285
quartz grains are present (Fig. 5A, E). However, they all display dull blue luminescence
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colors (Figs. 5H, 6H). Notably, the fractured quartz grains are more common in Abu
287
Madi than in Qawasim sediments (Fig. 6A, G). Lithic fragments are ubiquitous component in the Qawasim and Abu Madi facies.
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However, they are more common and are recorded almost in all the samples of the
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Qawasim Formation. Lithic fragments are mainly sedimentary in origin consisting mainly
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of limestone (Figs. 5G, H and 6A), sandstone (Fig. 5E), claystone (Fig. 7C) and
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phosphate fragments (collophane) (Figs. 5C, 6B). The limestone fragments are partly
293
(Fig. 6A, 5H) to completely dolomitized (Fig. 6C), likely during burial. Traces of igneous
294
lithics are present and mainly represented by feldspar-rich granitic fragments (Fig. 7B).
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Feldspars include both plagioclase (Fig. 5B) and K-feldspars (Fig. 5E). Their
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luminescence colors range from pale green for the sodium-rich plagioclase (Fig. 5H) to
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bright and dark blue for K-feldspars (Fig. 6H). K-feldspars are more common than
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plagioclase and both display different rates of alteration, ranging from fresh (Fig. 5F) to
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partly and completely altered (Fig. 5C, E and 6H). According to the percent components
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of quartz-feldspars-lithic fragments (QFL) (Pettijohn et al. 1987), the Qawasim facies
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range from sublithic arenite (Q80F7L13) proximal to subarkose arenite (Q89F7L4) distal
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deltaic facies. Similarly, Abu Madi facies range from quartz wacke MTD to sublithic
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arenite fluvial (Q81F8L11) and quartz arenite and quartz wacke tidal and coastal marine
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facies (Q92F5L3).
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Bioclasts are represented by foraminifera which are more common in the Qawasim
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than in Abu Madi facies (Fig. 5D). Foraminifera are represented mainly by reworked
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Cretaceous-Eocene species (EREX, 2007; 2008), that are relatively better preserved in
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the Qawasim than in Abu Madi sediments (Figs. 5H, 6H). Biotite (Fig. 5B, C) and
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muscovite (Fig. 6D, E) comprise the recorded mica minerals. Biotite is preferentially
310
present along the laminae and bedding planes forming pseudo-matrix (Fig. 5F),
311
whereas muscovite plates display bended structures between the rigid detrital grains
312
(Fig. 6D).
313
However, they are more common in the Qawasim than in Abu Madi sediments. Well-
314
rounded tourmaline (Fig. 5A, E) and irregular garnet (Fig. 5B) are the most abundant
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Heavy minerals are volumetrically rare in the studied Messinian facies.
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heavy minerals.
Glauconite is very common component in the studied Messinian
316
facies. It is much more abundant in the Qawasim distal deltaic front (Fig. 5F) and Abu
317
Madi tidal and coastal marine than in the other Messinian facies (Fig. 6F). However,
318
glauconite grains are coarser and well-rounded in the Qawasim proximal deltaic and
319
Abu Madi fluvial facies (Figs. 5C, 6C and D), where they are present in different maturity
320
colours ranging from low-mature nascent pale green to evolved mature brownish green
321
revealing a dominantly detrital extraformational origin (Triplehorn, 1965; Amorosi, 1995;
322
Kelly and Web, 1999). Detrital clays are also very common and forming the matrix
323
component of the Qawasim distal deltaic and Abu Madi MTD and tidal and coastal
324
marine facies (Fig. 5F, 6E and G).
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Carbonate cementation- mainly calcite and dolomite- is the most abundant
326
authigenic component (Figs. 5G, 6F). Calcite cements are more common in the
327
Qawasim distal deltaic (Fig. 5G) and Abu Madi tidal and coastal marine facies (Figs. 6F,
328
6D). Siderite is only recorded in the Abu Madi MTD replacing the argillaceous matrix
329
components (Fig. 6G). Authigenic clays are also present and mainly are represented by
330
kaolinite in the Qawasim (Fig. 5B, F) and chlorite in the Abu Madi sediments (Figs. 6C,
331
7D). Unlike quartz, clay content increases up-sequence in the Qawasim Formation from
332
the proximal to distal deltaic facies and in Abu Madi Formation from the fluvial to tidal
333
and coastal marine sediments (Fig. 7E, F). Furthermore, sub-equal rare amounts of
334
authigenic euhedral cubic pyrite crystals are recorded in the Qawasim and Abu Madi
335
sediments (Figs. 5G, 6C). Petrographical observations and sedimentary facies
336
mineralogical composition suggest that sediment provenance did not significantly
337
change during the deposition of the pre-MSC and MSC facies, nor did average
338
composition vary among the sedimentary facies during burial.
339
4. 3. Geochemistry
340
4. 3. 1. Major elements
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The studied Messinian sedimentary facies display wide variations in their major
342
elements concentrations (Table 1), except for SiO2 which decreases up-sequence in Page 12
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the Qawasim and Abu Madi formations consistent with petrographic observations (Fig.
344
7E, F). All the other major elements are increasing up-sequence from the proximal
345
deltaic to distal deltaic facies and from the fluvial to the tidal and coastal marine
346
sediments (Table 1). When comparing with the Upper Continental Crust (UCC) (Fig.
347
8A) and Post Archean Australian Shale (PAAS) (Fig. 8B) (Taylor and McLennan,
348
1985), the Qawasim proximal deltaic facies and Abu Madi fluvial sediments have
349
similar SiO2 contents, whereas the other Messinian facies are slightly depleted. On the
350
other hand, the Qawasim proximal deltaic and Abu Madi fluvial facies are depleted in
351
all the other elements, whereas the other facies display slight enrichments in some
352
elements such as CaO, Fe2O3 and Al2O3. All the studied Messinian samples are highly
353
depleted in Na2O (Fig. 8A, B). Despite these variations, the major elements display a
354
very uniform geochemical behavior in all the Messinian sedimentary facies exhibiting
355
very similar elemental ratios. The Al2O3/TiO2 ratio shows a narrow range and average
356
values vary from 13.97 in the Qawasim proximal deltaic to 14.02 in the Qawasim distal
357
deltaic facies. In the MSC Abu Madi sediments, similar values are recorded in Abu
358
Madi fluvial (mean= 13.71) and tidal and coastal marine sediments (mean= 13.48),
359
whereas the MTD facies has comparatively higher values (mean= 15.69) (Table 1).
360
Similarly, K2O/Na2O ratio is also very similar in the Qawasim proximal and distal deltaic
361
facies (mean= 4.39 and 4.29, respectively).
362
values range from 5.25 and 5.53 in the fluvial and tidal and coastal marine facies,
363
respectively; whereas the highest value of 7.69 is recorded in the Abu Madi MTD. The
364
only wide variation in the elemental ratios is observed for the SiO2/Al2O3 ratio (Table
365
1). It is very high in the coarse-grained Qawasim proximal deltaic facies and Abu Madi
366
fluvial sediments (mean= 24.9 and 17.43, respectively) and relatively lower in the other
367
Messinian facies (mean< 5) (Table 1). Furthermore, the K2O/Al2O3 ratio show a very
368
narrow range, yet increases up-sequence from the Qawasim proximal (mean = 0.25) to
369
the distal deltaic facies (mean= 0.20). In the Abu Madi facies, the values of K2O/Al2O3
370
are comparatively lower than those of the Qawasim facies (Table 1). Its average
371
values range from a minimum of 0.18 in the Abu Madi tidal and coastal marine
372
sediments to a maximum value of 0.21 in the Abu Madi MTD (Table 1).
In Abu Madi, the K2O/Na2O average
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All the elements have strong negative correlations with SiO2 and are positively
374
correlated together. Furthermore, the most prominent negative linear correlation is
375
obtained between SiO2 and Al2O3 (Fig. 9), suggesting that quartz is the bulk source of
376
silica in all the studied Messinian facies. Linear positive correlation between Fe2O3
377
and Al2O3 (Fig. 9) and their elevated values in the Abu Madi than Qawasim sediments
378
are consistent with the relative enrichment of chlorite in Abu Madi sediments. Similar
379
positive correlations are obtained between K2O and Al2O3 (R2>0.85), Na2O and Al2O3
380
(R2>0.56); indicating that their host minerals are mainly clays and feldspars. Moreover,
381
the prominent linear correlation between TiO2 and Al2O3 (R2>0.89) (Fig. 9), either
382
reveals the occurrence of Ti4+ within the lattice of the phyllosilicates or their detrital
383
host mineral was derived from the same source (Ross and Bustin, 2009).
384
4. 3. 2. Trace elements
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Except for Mo which decreases up-sequence in the Qawasim and Abu Madi
386
sedimentary facies all the other trace elements are progressively increasing up-
387
sequence in the pre-MSC Qawasim Formation from the proximal deltaic to the distal
388
deltaic facies and also in the Abu Madi Formation from the fluvial to tidal and coastal
389
marine sediments (Table 2). Moreover, not taking into account Mo which is weakly
390
correlated positively with SiO2 and negatively with Al2O3, all the other elements display
391
negative correlations with SiO2 and strong positive correlations with the other major
392
elements. The Qawasim proximal deltaic facies and Abu Madi fluvial sediments are
393
depleted in large ion lithophile elements (LILE), whereas the other facies display
394
similar values compared to UCC (Fig. 10A) and PAAS (Fig. 10B). All the studied
395
Messinian facies are highly depleted in Rb and Sr. The LILE group displays a strong
396
positive correlation with Al2O3; with R2 > 0.8 for Rb and Sr. These are also positively
397
correlated with K2O and Na2O, consistent with their association with feldspars and,
398
therefore they should be very sensitive to chemical weathering. All the Messinian
399
sedimentary facies have a uniform Rb/Sr ratio with average values range from 0.15 to
400
0.18 in the pre-MSC Qawasim and from 0.12 to 0.17 in the MSC Abu Madi facies
401
(Table 2).
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In regard to the high field strength elements (HFSE), the Messinian sedimentary
403
facies are depleted in Th and slightly depleted in U (Fig. 10A, B). The Qawasim
404
proximal deltaic facies and Abu Madi fluvial sediments are slightly depleted in Y, Zr, Nb
405
and Hf. In contrast, the other Messinian facies have similar values of those elements
406
relative to PAAS and are slightly enriched in Nb compared with UCC (Fig.10, B).
407
Among all the major elements, the HFSE show the strongest correlation with TiO2 and
408
Al2O3 (R2 > 0.7 and 0.63, respectively). The Zr/Hf ratio has a very narrow range; its
409
average value ranges from 39.54 and 40.89 in the Qawasim and from 38.75 and 39.19
410
in the Abu Madi facies (Table 2). This ratio is similar to the values recorded in zircon
411
crystals (Zr/Hf= 39; Murali et al. 1983), suggesting that their values are strongly
412
controlled by zircon content.
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The Qawasim proximal deltaic and Abu Madi fluvial facies are depleted in the
414
transition trace elements (Sc, V, Co, Ni and Cu) compared with UCC and PAAS (Fig.
415
10A, B), whereas the other facies have similar and/or slightly depleted values. Except
416
Th/Sc ratio, the average values of La/Th, Co/Th and La/Sc ratios are higher than those
417
of PAAS and UCC. The prominent positive and negative correlations between the
418
transition metals and major elements are obtained with Al2O3 (R2> 0.9) and SiO2 (R2> -
419
0.9).
420
4. 3. 3. Rare earth elements
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Concentrations of the rare earth elements (REE) in the studied Messinian sediments
422
are listed in (Table 3). The Qawasim proximal deltaic facies and Abu Madi fluvial
423
sediments are depleted in REE relative to UCC, whereas the other fine-grained facies
424
are comparatively more enriched (Fig. 11A). Chondrite-normalized REE patterns for the
425
studied Messinian facies show smooth, parallel REE trends with concentrations ranging
426
from 10 to 200x chondrite (Sun and McDonough, 1989). All the Messinian facies display
427
an overall fractionated concave-up patterns enriched in light REE (LREE) with respect
428
to heavy REE (HREE) (Table 3) which display flat and/or slightly depleted patterns (Fig.
429
11A). The ƩLREE/ƩHREE ratio is very high in the studied Messinian facies, and its
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average values vary from 11.58 in the Qawasim sediments to a maximum value of
431
12.03 in the Abu Madi MTD. The Messinian sediments have high La/Sm ratios with
432
average values ranging from 5.18 and 5.54; thus reflecting their uniform LREE contents.
433
In other words, the (Gd/Yb)N is relatively low in all the studied facies (mean -0.9) and positive correlations with
443
Al2O3 (R2> 0.85). The LREE and HREE show a magmatic fractionation pattern and the
444
pronounced negative Eu anomaly accompanied with enrichment of LREE relative to
445
HREE are typically inherited from their source rocks. Spider diagram of the REE with
446
some trace and major elements normalized over the primitive mantle values (Hofmann,
447
1988) (Fig. 11B) illustrates almost 100x enrichments in the mobile elements (U, K, Pb,
448
Rb and Ba), and significant depletions in the immobile elements (Ta, Nb, Y and P).
449
5. Discussion
450
5. 1. Post-depositional alterations
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The post-depositional alterations and diagenetic mineral phases result from pore-
452
water exchange with meteoric water and seawater during early diagenesis or deep
453
fluids during mesodiagenesis may induce major influences which completely obliterate
454
the variations due to provenance (Garzanti and Andó, 2007; Pe-Piper et al., 2008).
455
Petrographic observations revealed that the studied Messinian sedimentary facies were
456
subjected to eogenetic carbonate cementation particularly in the Qawasim distal deltaic
457
and Abu Madi tidal and coastal marine facies (Figs. 5G, 6F). This is fairly consistent
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with their relative enrichment in CaO and MgO compared with the other facies (Table 1).
459
The uniform elemental ratios obtained for the Messinian sedimentary facies indicate a
460
very mild impact of these post-depositional alterations. Plots of CaO/Al2O3, Fe2O3/Al2O3
461
and P2O5/Al2O3 (Fig. 12), show that the Messinian facies are clustered together away
462
from the field of severe diagenesis confirming the mild influence of the diagenetic
463
alterations on the geochemical proxies (Pe-Piper et al., 2008). However, some samples
464
from Abu Madi fluvial and MTD facies are plotted close to the field of severe diagenesis
465
due to their relative enrichment in chlorite (Fig. 6C) and siderite (Fig. 6G) formed during
466
burial diagenesis (Fig. 12). Overall, the diagenetic alterations do not actively obliterate
467
the geochemical proxies within the studied Messinian facies, thus confirming their
468
usefulness for deciphering the sediments tectonic provenance and assessing the
469
paleoweathering conditions prevailed in the provenance terrain.
470
5. 2. Paleoenvironmental indicators
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The climatic changes which control the intensity of chemical weathering and
472
terrigenous sediment surges to the basin induce major impacts on the geochemical
473
characteristics of the clastic materials (Weltje et al., 1998; Nesbitt et al., 1980; Tanaka
474
et al., 2007). In the Mediterranean marginal basins (e.g. Nile Delta), several cycles of
475
climatic fluctuations from humid episodes prior to the MSC to arid during the onset of
476
the MSC are recorded and impacted the terrigenous materials flux into the basin and
477
hence controlled both the syn- and post-depositional processes (Vidal et al., 2002;
478
Griffin, 2002).
479
climatic conditions and are used for monitoring the paleoclimatic changes. Hence, Zhao
480
et al. (2007) and Cao et al. (2012) based on the hypothesis of enrichment of Fe, Mn, Cr,
481
V, Ni, Co relative to Ca, Mg, Sr, Ba, K, Na under humid climatic conditions, applied the
482
paleoclimatic index (C-value). The C-value differs significantly in the Messinian
483
sediments (Table 1), recording the highest average value of 0.92 in the Qawasim distal
484
deltaic facies. In Abu Madi facies, the C-value increases up-sequence from the MTD to
485
the tidal and coastal marine sediments. These values confirm the fluctuating climatic
486
conditions from warm and humid during the Early Messinian to arid during the onset of
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Some elemental concentrations in the sediments are sensitive to
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the MSC prior to restoration of the humid conditions by the end of the MSC (Fig. 13).
488
The Sr/Cu ratio is also sensitive to climatic changes and increases in sediments under
489
warm and humid conditions (Lerman, 1978; Cao et al., 2015; Moradi et al., 2017). The
490
elevated Sr/Cu values in the Qawasim sediments (mean= 8.30 and 9.35) compared with
491
those of Abu Madi (mean70 reflect prolonged weathering accompanied
532
by removal of the mobile cations (Na+, Ca2+ and K+) relative to the immobile ones (Al3+,
533
Ti4+). In the pre-MSC Qawasim facies, the average CIA values range from 66.93 to
534
71.96 reflecting a moderately weathered source. In contrast, the average values are
535
comparatively higher in the MSC Abu Madi facies; and range from 74.69 to 75.90,
536
confirming more intense weathering. Similarly, PIA values are higher in the Abu Madi
537
(mean= 57. 11 to 60.90) than in the Qawasim sediments (mean= 48.83 to 56. 30)
538
(Table 1).
539
which often causes a wrong estimation for the weathering intensity (Harnois, 1988).
540
Nonetheless, CIW values are also higher in the Abu Madi facies compared to those of
541
Qawasim (Table 1), thus confirming the prolonged weathering conditions for the MSC
542
Abu Madi sediments. The intense weathering fingerprints in Abu Madi facies despite the
543
arid climatic conditions prevailed during the MSC together with their more mineralogical
544
and compositional maturity likely point to their derivation from the pre-MSC Qawasim
545
facies.
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Unlike CIA, CIW is not influenced by post-depositional K-metasomatism
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The element mobility during chemical weathering is also constrained using the (A-
547
CN-K) ternary plot (Nesbitt and Young, 1984) (Fig. 14A). As depicted from this plot, the
548
qualitatively estimated weathering intensity and CIA values of the Abu Madi facies are
549
ranging from 60 to 78 higher than those of the Qawasim facies which are ranging from
550
55 to 70. The A-CN-K plot also extrapolates the probable source composition based on
551
the weathering profiles of the weathered clastic materials. The Messinian facies display
552
a linear trend parallel to the A-CN boundary intersecting the plagioclase-feldspar joint
553
near the granodiorite initial composition, thus suggesting a plausible source rock (Fig.
554
14A). Similarly, the Messinian facies display a trend parallel to the A-CN boundary in
555
the A-CNK-FM ternary plot (Nesbitt and Young, 1989) (Fig. 14B), and the samples are
556
clustered close to the granodioritic composition consistent with the results obtained from
557
the A-CN-K plot. The linear weathering trends in the A-CN-K and A-CNK-FM plots
558
suggest steady-state weathering conditions for the Messinian facies (Nesbitt et al.,
559
1997; Nesbitt and young, 2004). Moreover, the similar weathering trends of the pre-
560
MSC and MSC facies suggest their derivation from the same source or at least the MSC
561
facies were derived from the pre-MSC strata. The Th/U ratio is a good indicator for
562
weathering intensity due to preferential loss of U during weathering and oxidation. The
563
mild impact of diagenesis revealed for the studied Messinian facies (Fig. 12),
564
demonstrates the usefulness of this ratio as a paleoweathering proxy. The ratio greater
565
than 4 reveals an intense weathering (McLennan et al., 1995).
566
facies has a narrow range of values varying from 3 to 4 (Table 2), thus suggesting an
567
intermediate to intense weathering, which confirms steady state weathering conditions
568
consistent with the uniform weathering trends inferred from the A-CN-K and A-CNK-FM
569
plots.
570
5. 4. Sedimentary sorting and recycling
Studied Messinian
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571
Qualitative petrography demonstrates the quartzose lithic composition of the studied
572
Messinian sedimentary facies. Abu Madi facies are on the other hand texturally and
573
compositionally more mature than those of Qawasim. ICV (see section 3.3) is frequently
574
used to evaluate the mineralogical maturity and degree of sedimentary recycling (Cox et Page 20
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al., 1995). Generally, rock forming minerals have ICV values greater than 1, whereas
576
the alteration byproducts have values less than 1. The average ICV values range from
577
1.23 to 1.61 in the Qawasim and from 1.5 to 2.05 in Abu Madi facies (Table 1),
578
corroborating the more abundance of alteration byproducts within the Qawasim. Those
579
relatively higher ICV values suggest more impact of sedimentary recycling on the Abu
580
Madi sediments (Cox et al., 1995; Nesbitt et al., 1997). Furthermore, mechanical sorting
581
during sediment transportation and deposition may affect the distribution of geochemical
582
provenance proxies and alter the paleoweathering trends (e.g. Garcia et al., 1991;
583
Bauluz et al., 2000). The process resulted in fractionating clay minerals (Al2O3) from
584
feldspars and quartz (SiO2) and also TiO2 in clay minerals and Ti-oxides from Zr mostly
585
in Zircon (Garcia et al., 1991; Mongelli et al., 2006). Therefore, the ternary plot of Al-Ti-
586
Zr would be very useful in illustration the presence of such fractionations related to
587
mechanical sorting (Garcia et al., 1991). The studied Messinian sedimentary facies
588
display mixing trend parallel to the Al2O3-Zr boundary and characterized by a
589
progressive decrease in Al2O3/Zr ratio (Fig. 15A), likely related to recycling effect and
590
Zircon addition. Increasingly, high Zr/Hf ratios (Table 2) practically identical to those
591
reported from the zircon crystals are consistent with the enrichment of zircon during
592
sedimentary recycling and mechanical sorting (McLennan et al., 1993). In addition,
593
some REE such as Sm and Ce exhibit very similar geochemical characteristics and,
594
therefore, their behavior in the studied clastics should monitor the influence of
595
sedimentary recycling (Sun et al., 2008, Ma et al., 2017). Ce and Sm display with each
596
other relatively more prominent linear correlation in the Qawasim (R2>0.98) than in Abu
597
Madi facies (R2
