GEOCHEMISTRY OF THE GULF OF ADEN BEACH SANDS, AL …

 

 

Acta Geodyn. Geomater., Vol. 16, No. 1 (193), 55–69, 2019
DOI: 10.13168/AGG.2019.0005

journal homepage: https://www.irsm.cas.cz/acta

ORIGINAL PAPER

GEOCHEMISTRY OF THE GULF OF ADEN BEACH SANDS, AL-MUKALLA, YEMEN:
PROVENANCE AND TECTONIC SETTING IMPLICATIONS

Ibrahim GHANDOUR 1, 2 *, Ali BASAHAM 1), Rabea HAREDY 1), Ammar MANAA 1) ,
Khaled AL-RABAKI 3) and Khaled BAWAHIDI 3)

1) Marine Geology Department, Faculty of Marine Science, King Abdulaziz University, P.O. Box 80207, Jeddah 21589, Saudi Arabia
2) Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt
3) Department of Environmental Sciences, Faculty of Environmental Sciences and Marine Biology, Hadhramout University, Al-Mukalla,
Republic of Yemen

*Corresponding author‘s e-mail: ighandour@kau.edu.sa

ARTICLE INFO

Article history:
Received 6 August 2018
Accepted 15 January 2019
Available online 6 February 2019

ABSTRACT

Keywords:
Gulf of Aden
Al-Mukalla
Provenance
Sediment geochemistry
Weathering indices
Sediment maturity

1.

This research concerns with the mineralogical and geochemical composition of the Gulf of Aden
shallow subsurface beach sands to identify the composition and tectonic setting of source area.
The study discusses the limitation of using weathering indices and the factors hampered using
these indices. Three shallow cores (1-1.5 m long) were collected from the coastal area at the
mouth of wadis. Fuwwah (I), Ar Rukayb (II) and Shuhair (III) near Al-Mukalla, Yemen. The
sediments are dominated by well to moderately well sorted fine-grained sands. The
mineralogical composition is dominated by quartz followed by calcite with traces of plagioclase,
k-feldspars, dolomite, clay minerals and amphiboles. The chemical composition is consistent
with the mineralogy where the SiO2, CaO, Zr and Sr are the dominant oxides and trace elements,
whereas other major and trace elements are strongly depleted. The sediments are compositionally
mature and geochemically classified as quartz arenite to sublitharenite derived mainly from
recycled sedimentary rocks that is consistent with the passive margin tectonic setting. The
Chemical Index of Alteration (CIA) values suggest a low to moderate degree of weathering for
the sediments of core I, whereas they show unweathered to poorly weathered source rocks of the
sediments of cores II and III. Recycled sandstones mixed with carbonate sources and the
hydraulic sorting minimized the effective usage of weathering indices in this study.

INTRODUCTION
The mineralogical and chemical composition of
clastic sediments provides information important to
interpret the provenance, tectonic setting and the
complex interplay of weathering, recycling, hydraulic
sorting during transportation and deposition and the
post-depositional alteration (Lacassie et al., 2004;
Ohta and Arai, 2007; Armstrong-Altrin, 2009;
Hossain et al., 2010; Tao et al., 2014; Zhang, 2014;
Zaid, 2015; Tawfik et al., 2017). Relying solely on
mineralogical
investigations without geochemical
analysis is insufficient and therefore, geochemical
analysis often provides complementary data
to
mineralogical
1990;
Armstrong-Altrin, 2009). Several approaches have
been employed to extract this information ranging
from petrographic analysis, framework detrital mode
and geochemical analysis and/or their combinations.
However, the bulk sediment geochemistry is still more
suitable than any other approach or can complement
(von
and petrographical
mineralogical
Eynatten et al., 2003; Armstrong-Altrin, 2009;
Armstrong-Altrin et al., 2014; Zhang, 2014).
Unconsolidated beach sands lack diagenetic imprints
and therefore, their composition primarily reflects the

(Kroonenberg,

analysis

studies

provenance and geotectonic setting of the source area
(Carranza-Edwards et al., 2009; Armstrong-Altrin et
al., 2015; Zaid, 2015).

Along the young continental margins of Yemen,
the coastal area receives sediments from multiple
sources including the rift related magmatic rocks,
sedimentary successions and underlying Pan-African
basement exposed on the uplifted shoulders of the rift
(Garzanti et al., 2001). The Gulf of Aden provides
a unique site to study the provenance of rift related
sediments. East of the Gulf of Aden, the sediments are
transported to the coastal area by intermittently active
wadis, local watersheds draining into the Gulf of
Aden and by longshore transportation. They transport
detritus derived
from Mesozoic and Cenozoic
sedimentary rocks (Garzanti et al., 2001). The
provenance of the continental rift margin of the
Yemen is introduced relying solely on the detrital
mode and framework compositional trends of the
modern loose beach and bedload wadi sediments
(Garzanti et al., 2001). Until now, the geochemistry of
the Gulf of Aqaba beach sands and its provenance and
tectonic implications have not been comprehensively
investigated. The present study introduces the first
attempt to interpret the provenance and tectonic

Cite this article as: Ghandour I, Basaham A, Haredy R, Manaa A, Al-Rabaki K, Bawahidi K: Geochemistry of the Gulf of Aden Beach
Sands, Al-Mukalla, Yemen: Provenance and tectonic setting implications. Acta Geodyn. Geomater., 15, No. 1 (193),
55–69, 2019. DOI: 10.13168/AGG.2019.0005

56

I. Ghandour et al.

Fig. 1 Location map of the area of study showing the locations of collected cores.

setting of the Gulf of Aden beach sands near Al-
Mukalla (Fig. 1) using geochemical and mineralogical
proxies. The study will also discuss the source rock
weathering conditions and the circumstances under
which the use of weathering indices is inapplicable.

2. GEOLOGIC BACKGROUND

Throughout the Phanerozoic, the continental
margin of the Yemen is a passive type margin and
contains a series of NW-SE trending rift basins. These
basins are the Jurassic-Cretaceous and the Oligocene-
Pliocene rift basins related
the breakup of
Gondwana and the opening of the Red Sea and the
Gulf of Aden, respectively (Bott et al., 1992; Beydoun
et al., 1996; Bosence, 1997; D’Acremont et al., 2005;
As-Saruri et al., 2010). The Gulf of Aden, south of
Yemen is a young and narrow oceanic basin oriented
N75oE, strikes obliquely
the N25oE
(D’Acremont et al., 2005).

(50o)

to

to

in

tectonics

The Mesozoic extensional

the
Yemen resulted from the rifting between East Africa
and West India led to the development of the WNW-
ESE Say’un–Masilah and the E-W Jiza’– Qamar
basins (Fig. 2) in the center and east of the southern
Yemen (As-Saruri et al., 2010). The basins were then
differentiated structurally
into sub-basins, half-
grabens and intra-basinal horsts. The distribution and
thickness of sedimentary facies within these basins
were controlled primarily by the major paleo-highs
and arches within and between such basins. Mesozoic
basins were episodically subsided by sporadic,
localized and brief pulses of fault activity and erosion.
The pre-rift sedimentary succession within these
basins includes 1) the Paleozoic quartz rich pebbly
sandstones,
the Gondwana
soil
glaciations related conglomerates and mudrocks, 2)
the Middle to Upper Jurassic fluvial to coastal
sandstones, conglomerate and carbonaceous shales of
the Kuhlan Formation and the overlying carbonate-
dominated Amran Limestone (Simmons and Al-
Thour, 1994) and 3) the Cretaceous-Paleogene cliff
forming shelf and platform limestones (Garzanti et al.,

lateritic

and

2001). The Upper Jurassic–Lower Cretaceous syn-rift
sediments constitute the main hydrocarbon plays in
Yemen. These sediments include the organic rich
marine shales, debris flow gravel and breccias and
well-bedded
the Madbi Formation
overlain by a thick evaporite succession of the
Sab’atyan Formation (Leckie and Rumpel, 2003; King
et al., 2003; As-Saruri et al., 2010).

limestone of

The Cenozoic extensional tectonics in the Gulf
of Aden have led to the development of the Aden-
Abyan, Hawrah-Ahwar and Mukalla– Sayhut basins,
which run parallel to the gulf along its northern coast.
These basins are filled with thick fluvio-marine strata.
The pre-rift sediments in these Cenozoic basins
include the Jurassic (equivalent Kuhlan Formation and
the Amran group), Cretaceous (equivalent Tawilah
and Mahra groups) and Paleogene
(equivalent
Hadramawt group) sediments. The syn-rift sediments
include the shale, sandstone and evaporites of the
Ghaydah Formation, whereas the post-rift sediments
are primarily represented by the prograding carbonate-
dominated and the siliciclastic-dominated Hami and
Sarar formations, respectively (Bott et al., 1992;
Brannan et al., 1997; Garzanti et al., 2001; As-Saruri
et al., 2010). In the area of study, the surface geologic
outcrops of the study area (Fig. 3) range in age from
Pre-Cambrian
to Quaternary. The Precambrian
igneous and metamorphic rocks are unconformably
overlain by the Lower-Middle Jurassic fluvial arkosic
sandstones
the Cretaceous
continental sandstones (Tawilah Group) and the
Upper Paleocene-Lower Eocene limestones (Umm Er
Radhuma Formation) (As-Saruri et al., 2010; Al-
Wosabi and Wasel, 2011). The cross section in the
area of study shows a northward variation in the
sediments. The detritus supplied by Wadi Fuwwah are
the Cretaceous sandstones
derived mostly from
(Tawilah group), whereas the sediments supplied by
the other two wadis include detritus of the Middle
Jurassic Kuhlan Sandstone and the Lower Paleogene
limestones (Fig. 3).

(Kohlan Formation),

GEOCHEMISTRY OF THE GULF OF ADEN BEACH SANDS, AL-MUKALLA, YEMEN…
.

57

Fig. 2 Sedimentary basins and major structural highs surrounding Al-Mukalla area (after As-Saruri et al.,

2010). a-a’ is a cross section shown in Figure 3.

Fig. 3 Cross-section showing the main structural and stratigraphic framework near Al-Mukalla, Yemen

(modified after Beydoun, 1966; As-Saruri et al., 2010). The location of a-a’ line is shown in Figure 2.

The coastal plain of the Gulf of Aden is about
40 km wide and it is occupied by recent lava fields,
rocky cliffs, raised beaches which are covered in dune
sands and gravel terraces and bordered by continuous
sandy shorelines (Garzanti et al., 2001). The raised
beaches record continuous tectonic uplift during the
Pliocene-Quaternary with a limited faulting affected
the post rift section. The reactivation of old faults
changes
the detrital modes
between adjacent drainage basins (Garzanti et al.,
2001).

the composition of

The sea level at the Gulf of Aden rises between
September and May and falls during June-July to
reach
in August. The seasonal
oscillations in the mean sea level is attributed to

the minimum

astronomical effects, effects of evaporation, very low
river discharge,
to negligible precipitation and
atmospheric pressure, and steric sea-level effects. The
effects of purely astronomical conditions (long-period
tides) are not significant; they do not exceed 12 mm
(Morcos and Abdallah, 2012).

The area of study is a wild, barren region with
high temperatures in summer reaching 54 °C and
35 °C in winter with a mean annual rainfall of 50–
130 mm in coastal areas and may exceed 500 mm on
the coastal mountain belt. Monsoon winds blow in
winter westward and
in summer northeastward,
bringing violent storms up to 100 km/h with sporadic
rains (Beydoun, 1964). Numerous occasionally active
wadis run southeastward across major extensional

58

I. Ghandour et al.

structures drain the adjoining mountainous and hilly
hinterland and discharging freshwater and sediments
into the Gulf of Aden mainly during heavy rains as
flash floods. Rare perennial wadis such as Hajar and
tracts of Wadi Hadhramaut occur (Greenwood and
Bleackley, 1967).

3. MATERIALS AND METHODS

The database of the present study includes three
shallow sediment cores (1–1.5 m long) collected from
the Gulf of Aden sandy beach at the mouth of 3
wadis; Fuwwah (core I), Ar Rukayb (II) and Shuhair
(III) near Al-Mukalla, Yemen (Fig. 1). All cores were
sub-sampled at intervals of 5–10 cm apart. Laboratory
techniques
size,
mineralogical and geochemical analyses.

sediment

included

grain

The grain size analysis was performed on 69
samples (20, 22 and 27 samples from cores I, II and
III, respectively) using the traditional mechanical
sieving technique. The sediments were dried at 105 °C
for a night. About 50 g of dry and homogeneous
sediments were sieved for 20 minutes at one phi size
interval using standard ASTM sieve set ranging from
2 to 0.063 mm. The size fraction retained in each
sieve was carefully weighed and
the weight
percentage and cumulative weight percentage were
computed. Mean size (Mz) and inclusive standard
deviation (σI) were determined using the method of
Folk and Ward (1957).

The mineralogical composition of 40 samples
(13, 13 and 14 samples from cores I, II and III,
respectively) was determined using X-ray powder
diffraction (XRD) (SHIMAZU) with Ni-filtered Cu
Kα radiation at 15 kV to 40 mA at the XRD
laboratory, Faculty of Marine Science, King
Abdulaziz University. The minerals were identified
using the peak heights of basal reflections (Hardy and
Tucker, 1988). The relative abundance of minerals is
determined semi-quantitatively using the peak heights
of basal reflections for the mineral. The identified
minerals are grouped into abundant (A>40 %),
moderate (M = 10 – 40 %) and trace (T<10 %). The bulk sediment geochemical composition of 40 samples was determined by conventional XRF technique at the department of Geosciences, Osaka City University, Japan. The analysis conditions were 50 kV and 50 mA accelerating voltage and tube current, respectively using a RIGAKU RIX 2100 X- ray fluorescence spectrometer (XRF), equipped with Rh/W dual-anode X-ray tube. Fused glass discs were prepared by mixing 1.8 g of powdered sample (dried at 110o C for 4 hours), 3.6 g of spectroflux (Li2B4O7 20 %, LiBO2 80 %, dried at 450 °C for 4 hours), 0.54 g of oxidant LiNO3 and traces of LiI. The mixture is then fused at 800 °C for 120 s and 1200 °C for 200 s (Tawfik et al., 2017). The accuracy of the analysis was estimated to be ±2-3 % for major elements and ±10-15 % for trace elements. The total iron is introduced as Fe2O3t. Loss on ignition (LOI) was determined by heating the dried samples for 2 h at 1000 °C (Tawfik et al., 2017). The results of geochemical analysis were employed to determine the provenance, tectonic setting and paleoweathering indices. Chemical weathering was evaluated using the chemical index of alteration (CIA). The CIA (Nesbitt and Young, 1982) is determined using the equation: CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] x 100, where Al2O3, CaO*, Na2O and K2O are molecular concentrations, with CaO* representing Ca in silicate minerals only. To calculate the CaO* in silicate minerals, the formula: CaO** = mol CaO (10/ 3 x mol P2O5) is used. If the CaO** is < Na2O, then CaO* = CaO**, otherwise the CaO* = Na2O (McLennan, 1993). The CIA values <50 suggest unweathered fresh rocks and minerals, values from 50 to 60 indicate low chemical weathering, values between 60 and 80 suggest moderate chemical weathering and values >80
indicate intensive chemical weathering (Nesbitt and
Young, 1982; Fedo et al., 1995).

4. RESULTS
4.1. GRAIN SIZE AND MINERALOGICAL

COMPOSITION
Grain-size data show that most of the samples
are dominantly fine and rarely very fine and medium
sands with mean size values range from 2.08 to 2.44
(average 2.29ɸ), 1.65 to 3.03 (average, 2.43ɸ) and
from 2.1 to 3ɸ (average, 2.49) in the sediments of
cores I, II and III, respectively (Table 1 and Fig. 4).
The sediments in the three cores display a slight
lateral variation in the degree of sorting. The inclusive
graphic standard deviation values range from 0.49 to
0.6 ɸ (well sorted to moderately well sorted), 0.6 to
1 ɸ (moderately well sorted to moderately sorted) and
0.31 to 0.73 ɸ (very well sorted to moderately sorted)
in the sediments of cores I, II and II, respectively
(Table 1 and Fig. 4).

is

technique

The mineralogical composition of sediments
determined by XRD
relatively
homogeneous with very negligible variation among
the three cores (Table 2). The mineral composition is
overwhelmingly dominated by quartz followed by
calcite with moderate relative abundance. In addition,
traces of dolomite, plagioclase, K-feldspars, clay
minerals and local occurrence of aragonite and
amphiboles are recognized (Table 2). Though of low
relative abundance, the sediments of core I show
traces of clay minerals.

4.2. CHEMICAL COMPOSITION
4.2.1. MAJOR OXIDES

Ranges and mean values of major oxides and
trace elements concentrations are listed in Table 3 and
shown in Figures 5 and 6. The concentration of SiO2
varied in the sediments of core I from 61.83 to 79.51
(avg. 72.38 %), from 40.52 to 86.18 (avg. 64.82 %) in
core II and 52.58 to 76.90 (avg. 66.67%) in the
sediments of cores III. CaO is the second abundant
oxide showing average concentrations 12.73, 18.97
and 16.11 % in the sediments of cores I, II and III,
respectively. The other oxides; Al2O3, Fe2O3, MgO,
K2O, Na2O, TiO2, P2O5 and MnO surprisingly display
low concentrations. The average concentrations of
MgO, Al2O3 and Fe2O3 in the sediments of core I are

GEOCHEMISTRY OF THE GULF OF ADEN BEACH SANDS, AL-MUKALLA, YEMEN…
.

59

Table 1 Results of grain size analysis of the Gulf of Aden shallow subsurface beach sands.

Sample

Core I

Core II

Core III

Mean (ɸ)

σi (ɸ)

Mean (ɸ)

σi (ɸ)

Mean (ɸ)

σi (ɸ)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Min
Max
Average

2.135
2.098
2.148
2.080
2.156
2.146
2.418
2.440
2.392
2.387
2.424
2.419
2.375
2.336
2.103
2.368
2.092
2.407
2.397
2.381

2.080
2.440
2.290

0.508
0.507
0.510
0.511
0.512
0.513
0.495
0.485
0.506
0.505
0.491
0.495
0.514
0.513
0.518
0.510
0.605
0.504
0.509
0.518

0.490
0.600
0.510

1.74
1.65
1.77
2.46
2.44
2.45
2.07
2.43
2.41
2.68
2.95
3.03
2.71
2.39
2.48
2.33
2.41
2.93
2.55
2.98
2.40
2.12

1.65
3.03
2.43

0.85
0.94
0.95
0.63
0.63
0.76
1.00
0.77
0.63
0.76
0.61
0.66
0.76
0.61
0.61
0.61
0.64
0.70
0.71
0.63
0.72
0.64

0.61
1.00
0.72

2.347
2.422
2.415
2.384
2.370
2.103
2.148
2.317
2.332
2.411
2.474
2.511
2.442
2.449
2.485
2.720
2.768
2.765
3.005
2.921
2.980
2.976
2.425
2.103
2.124
2.120
2.137
2.100
3.000
2.490

0.511
0.497
0.502
0.508
0.509
0.639
0.726
0.634
0.608
0.510
0.476
0.455
0.488
0.488
0.471
0.400
0.306
0.305
0.480
0.564
0.592
0.605
0.666
0.509
0.520
0.517
0.521
0.310
0.730
0.520

1.31, 1.25 and 1.02, respectively, whereas their
average concentrations in the sediments of cores II are
0.9, 0.58 and 0.41, respectively. In the sediments of
core III, the average concentrations of MgO, Al2O3
and Fe2O3 are 1.15, 0.56 and 0.79, respectively
(Table 3).

Comparing to the UCC values (Rudnick and
Gao, 2003), the sediments of the three cores are
strongly depleted in TiO2, Al2O3, Fe2O3, MnO, MgO,
Na2O, K2O and P2O5, whereas SiO2 show values
within the range of the UCC values. CaO is highly
enriched comparing to the UCC (Fig. 7).

4.2.2.

TRACE ELEMENTS

The average concentrations of trace elements in
the
to average upper
three cores normalized
continental crust (UCC) (Taylor and McLennan,
1985) are shown in Figures 6 and 7. In comparison
with UCC, sands from the three cores are highly
depleted in trace elements except for Zr and Sr.
However, the sediments of core I and III are highly
enriched in Zr and the sediments of core II are highly
enriched in Sr and slightly enriched in Zr with respect
to the UCC (Fig. 7). The transitional trace elements
like V, Cr, Co, Ni, Zn, Cu and Rb did not show much
variation between the sediments in the three cores
(Fig. 6). The concentrations of Sr varied between 205-
495 µg/g (avg. 302 µg/g), 225 and 820 µg/g (avg. 485
µg/g) and 125 and 297 µg/g (avg. 200 µg/g) in the
sediments of cores I-III, respectively (Table 3).

4.2.3.

PROVENANCE, RECYCLING AND TECTONIC
SETTING

The sediments are classified geochemically
based on the biplot of log (SiO2/Al2O3) vs. log
(Fe2O3/K2O) (Herron, 1988). This classification shows
that the majority of samples are classified as quartz
arenite, few samples from the sediments of core I are
plotted in the field of Fe-sands and a single sample
from core II is classified as sublitharenite (Fig. 8).

Discriminant

function diagram

(Roser and
Korsch, 1988) indicated that the beach sediments of
the Gulf of Aden were derived mainly from a recycled
sedimentary source rocks (Fig. 9A). The sediments of
core I cluster in the field of recycled sedimentary
source rock at a position lower than the samples of
cores II and III, suggesting possibly different
sedimentary source rock composition. The sediments
of core I were derived mainly from a sand-dominated
source with possible slight mixing with other
carbonate sediments. The clustering of samples of the
cores II and III in a higher position suggests a mixing
of carbonate and siliciclastic source rocks, with
a higher siliciclastics contribution than carbonates in
the sediments of core III. The ternary plot (Fig. 9B)
portraying weight percentages SiO2, Na2O+K2O and
TiO2+MgO+Fe2O3 is employed herein to identify the
effect of recycling (Kroonenberg, 1990). Generally,
the samples are clustered at high to moderate values of
SiO2 suggesting the major influence of recycling that
led to increasing residual enrichment of quartz

60

I. Ghandour et al.

Fig. 4 Vertical variation of the grain size parameters mean size (Mz) and standard deviation (σI) in the Gulf of Aden shallow subsurface beach sands for core I (a),

core II (b) and core III (c)

GEOCHEMISTRY OF THE GULF OF ADEN BEACH SANDS, AL-MUKALLA, YEMEN…
.

Table 2 The relative abundance of the different minerals in the shallow subsurface beach sands of the Gulf of

61

Aden that recognized by XRD analysis.
Qz = quartz, Plag= plagioclase, K-fels= K-feldspars, hornb= hornblende, calc= calcite, dolm= dolomite, arag=
aragonite, A=abundant, M=moderate and T= traces

Core

I

E
R
O
C

I
I

E
R
O
C

I
I
I

E
R
O
C

Sample
1
3
5
7
9
10
11
12
13
15
17
19
20
1
2
4
6
9
10
12
14
16
18
20
21
22
1
3
5
7
9
11
13
15
17
19
21
23
25
27

Qz
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A

Plag
T
T
T
T
T
T
T
T
T
T

T
T

T

T

K-fels

T

T
T
T
T

T
T

T

T

T

T
T
T
T
T
T
T

T

clays
T

T
T
T
T
T
T

T
T
T
T

hornb
T

T

T

T

calc
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M

dolm
T
T
T
T
M
T
T
T
T
T

T
T
T
T
T
T
T
T
T
T
T
T
T
M
T
T
T
T
T
T
T
T
T
T
T
T
T
T
T

arag

T

T

T

T

T

to

the other

two cores. The

(Fig. 9B). Though the sediments of core II have the
lowest average concentrations of SiO2, the samples of
core II are clustered near the SiO2/20 apex comparing
to the samples of cores I and III. The samples of core
II show very low concentrations of TiO2+MgO+Fe2O3
comparing
linear
correlation observed between K2O vs. Rb and Ba
(Fig. 10) indicates the effect of multiple cycling (Tao
et al., 2014). Both Rb and Ba have low solubility and
remain in the residual constituents and K are easily
moved during the recycling and chemical weathering.
The tectonic setting of the Gulf of Aden beach
sediments is determined using the major element
based diagram (Active Passive Margin Discriminant)
(Verma and
using APMDISC; online software
Armstrong-Altrin, 2016). The diagram showed that all
the samples of the three cores without exception
plotted in the field of passive margin setting (Fig. 11).
The compositionally mature Gulf of Aden beach
sands are well correlated with the passive margin

tectonic setting. The SiO2/Al2O3 ratios vary in the

sediments of core I from 35 to 105 (avg. 62) and
varied in the sediments of cores II and III from 36 to
188 (avg. 132) and 86 to 179 (avg. 125), respectively.
These extremely high ratios are consistent with
continentally derived recycled sedimentary source
consistent with the passive margin setting.

4.2.4.

PALEOWEATHERING

The estimated average values of CIA (Table 3)
show wide ranges varying in the sediments of core I
from 50 to 67 (average, 62), core II from 23 to 68
(average, 40) and core III from 25 to 54 (average, 34)
suggesting lateral variations in the degree of chemical
weathering. The CIA values of core I suggest low to
moderate degree of chemical weathering, whereas the
III are generally
sediments of cores
unweathered
to poorly weathered with a bulk
composition similar to their source material. The
variations in the CIA values among cores are possibly
the source rock
attributed
composition and the relatively higher Al2O3 content

the variation

II and

in

to

62

I. Ghandour et al.

Fig. 5 Box chart showing the minimum, maximum and average concentrations of major oxides (wt%) for the

Gulf of Aden shallow subsurface beach sands.

and clay minerals in the sediments of core I than the
sediments of the other two cores. The source of the
sediments in core I possibly has older weathered
components.

5. DISCUSSION

The Gulf of Aden shallow subsurface beach
sediments at the mouths of wadis Fuwwah, Ar
Rukayb and Shuhair near Al-Mukalla, Yemen are
dominantly moderately to well-sorted fine-grained
sands, classified geochemically in general as quartz
arenite. Texturally, the sediments show a relatively
similar average mean size. However, they show
slightly different degree of sorting with the sediments
of core I are generally well sorted relative to the
sediments of the other two cores. This is possibly
to different grain composition. The
attributed
relatively homogeneous quartz rich sand grains are
more sorted than a mixture of carbonate and quartz
grains. Though they have same size, both grain types
behave
and
during
differently
sediments are
deposition. Mineralogically,

transportation
the

overwhelmingly dominated by quartz followed by
calcite. In contrast to the sediments of core II and III,
the sediments of core I contain traces of clay minerals.
The depletion of trace elements in the sediments of
cores II and III comparing to the sediments of core I
could be attributed to the scarcity and absence of clay
minerals. Trace elements preferentially concentrated
in clay minerals during hydraulic sorting. Hydraulic
sorting is an important process that controls textural
and compositional maturity of sediments (Singh,
2009; Wu et
sorting
preferentially enriches specific grain size fraction and
minerals and therefore, controlling the chemical
composition of bulk sediments. The chemical
composition shows enrichment of SiO2, CaO, Zr and
Sr with a noticeable depletion of other major and trace
elements. The relatively high content of Ca and Sr is
attributed to the mixing with sand sized carbonate
grains. The geochemical information held in the
sediments characterizes a sedimentary source rock
that is compatible with the passive margin tectonic
setting. The higher average concentrations of SiO2 and

al., 2013). Hydraulic