Microtextures of Quartz Grain Surface from Recent ...grains from Recent sedimentary environments...
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JKAU: Mar. Sci., Vol. 23, No. 1, pp: 93-107 (2012 A.D. / 1433 A.H.)
DOI : 10.4197/Mar. 23-1.7
93
Microtextures of Quartz Grain Surface from Recent
Sedimentary Environments along Al-Khowkhah-
Al-Mokha Coastal Area, Southern Red Sea, Yemen
Saeed Omar Wasel
Marine Geology Department, Faculty of Marine Science and
Environment, Hodeidah University, Yemen.
E-mail: [email protected]
Abstract. Scanning electron microscopy (SEM) analyses of quartz
grains from Recent sedimentary environments such as beach, coastal
duns,sabkha and wadi deposits in the Al-Khowkhah and Al-Mukha
coastal area along southern Red Sea, Yemen, revealed variations in
surface textures. Results of this study reveals the existence of
distinguished surface features that reflect the effect of mechanical and
chemical actions on the quartz grains.
Quartz grains of beach, coastal dune and wadi deposits show
significant abundance of both mechanical and chemical surface
features developed during transportation of the grains in the
subaqueous and aeolian environments, respectively. In contrast,quartz
grains of sabkha are characterized by the dominance of chemical
surface features. Among these chemical surface features, those
originated from silica dissolution are considerably more numerous
than those due to silica precipitation.
Relative proportions of various surface features recorded on
quartz grains from different environments indicate a considerable
overlapping among the identified surface features in the examined
quartz grains of coastal sediments and wadi deposits.
Keywords: Microtextures; Quartz grains; Coastal sediments; Red Sea;
Yemen.
Introduction
Quartz grains microtextures have been studied by several workers.
Krinsley and his co-workers have done extensive studies on the surfaces
94 Saeed Omar Wasel
of quartz grains with the help of Scanning Electron Microscope (SEM)
of (Krinsley and Takahashi, 1962a, 1962b; Krinsley and Funnell, 1965;
Krinsley and Donahue, 1968; Krinsley and Doornkamp, 1973, and
Margolis and Krinsley, 1974).
Surface textures of quartz sand grains have been used to identify
the sources and genesis of various detrital sediments (Krinsley et al.,
1973; Ly, 1978; Frihy and Stanley, 1987, and Ambre et al., 2005).
The microtextures provide useful information regarding the
various processes acting on the grains during transportation and after
deposition (Krinsley and Funnell, 1965; Doornkamp and Krinsley, 1971;
Moral-Cardona et al., 1996, 1997; Mahaney, 1998, and Newsome and
Ladd, 1999) and the criteria for distinguishing the mechanical and
chemical features and their implications have been well established
(Krinsley and Donahue, 1968; Whalley and Krinsley, 1974; Al-Saleh
and Khalaf, 1982; Rahman and Ahmed, 1996, and AL-Hurban and
Gharib, 2004). Hence, the surface textural study on quartz grains is
considered as a powerful tool in the identification of provenance,
processes of transport and diagenetic history of the detrital sediments
(Krinsley et al., 1973; Madhavaraju and Ramasamy, 1999; Abu-Zeid et
al., 2001; Madhavaraju et al., 2004, 2006; Armstrong et al., 2005;
Kasper-Zubillaga and Faustinos-Morales, 2007, and Madhavaraju, et.
al., 2009).
The aim of this study is determine the surface feature
characteristics of quartz sand grains sampled from the recent coastal
sediments and the wadi deposits in the southern Red Sea coastal areas of
Yemen. The occurrence, proportions and overlapping of chemical and
mechanical surface features are used to interpret the environmental
conditions under which these recent sediments were formed.
Study Area
The Yemen coastline, 730 km long, is located at the southern part
of the Arabian Peninsula on the Red Sea that extends from the strait of
Bab Al-Mandab to the northern border of Yemen named Yemen Tihama
plain. The study area is bounded between Al-Khowkhah in the north and
Al-Mukha at the south and extends for about 60 km along the shoreline
Microtextures of Quartz Grain Surface from Recent Sedimentary … 95
and lies between latitude 13º 15′ and 13º 50′N and longitude 43º 14′ and
43º 20′ E (Fig. 1).
Fig. 1. Location map of the study area along Yemen coastline.
The coastal plain is characterized by relatively low-relief
topography with very gentle slope towards the sea, and occasionally
several wadis dissect the coastal area. About 75% of the Red Sea
coastline consists of loose sediments. The backshore of the coastline is
mostly covered by recent sediments of sabkhas and salt marshes. Based
on their textures these sediments are formed in a coastal environment
(beach sands, coastal dunes and coastal sabkhas) or represent wadi
deposits (Wasel, 2008).
Beach and nearshore sand deposits are the dominant recent
sediments in the Red Sea coast of Yemen. They are composed of poorly-
sorted mixture of sand, silt and clay (Wasel, 2008). Coastal dunes of the
tidal area cover a narrow area depending upon sand supply and climate.
This tidal sediment is composed of quartz sand occasionally mixed with
some shells and shell fragments. The isolated coastal sabkha is
widespread in the supratidal area; they vary in width and extend to about
2 km in length. They are characterized by marine transgression and
96 Saeed Omar Wasel
evaporitic condition. Therefore, sabkha sediments signify continental
and adjacent marine origin. Wadi deposits are composed mainly of sand,
gravel and mud. These deposits are almost flat and covered by scattered
vegetation (Wasel, 2008).
Methods
Ten samples have been collected from the coastal area in the
southern Red Sea of Yemen (Fig. 1). These samples were chosen to
represent various types of recent environments. Two samples from the
wadi; two samples from the coastal dune; two samples from the coastal
sabkha and four samples from the beach. The procedures used for
cleaning quartz sand grains for SEM examination are those described by
Krinsley and Doornkamp (1973). The medium grained sand fraction was
selected from each sample to study the surface textures. Monocrystalline
quartz grains were picked from each of the study samples with the use of
a binocular microscope (Delgado, 1999, and Alekseeva, 2005). Fifteen
grains were selected at random from each sample and mounted on metal
specimen stubs, were coated with a gold-palladium alloy. Grains were
visually examined using (model JEOL-JSM-5400LV) scanning electron
microscope at Assiut University of Egypt.
Results and Discussion
In general, Investigations of the surface textures of quartz grains
from the different types of recent sediments of the study area revealed the
effects of mechanical and chemical action on the studied quartz grains.
Mechanical features are mainly V-shaped and crescent-like pits, dish-
shaped depressions, upturned plates, straight and curved grooves,
striation and stepped cleavage planes.
Chemical features are manifested on quartz grain surfaces by
solution and precipitation mechanisms. These result into the formation of
triangular etching pits, irregular solution pits, crystal overgrowth and
smooth precipitation surfaces. Study of the relative frequency
distribution of various surface features recorded on the quartz grains
showed that each type of sediment is characterized by an abundance of
certain surface features, in addition to other main features, summarized
in Table 1 and their graphic representation is shown in Fig. 2.
Microtextures of Quartz Grain Surface from Recent Sedimentary … 97
Table 1. Identified microtextures, and their abundance, on quartz grains of the recent
sediments along southern Red Sea, Yemen.
Surface features
Coastal sediments Wadi
deposits Beach
sand
Coastal
dune
Sabkha
deposits
Mechanical origin
Conchoidal fractures 17 33 3 35
V-shaped pit 50 25 10 13
Crescent-like Pit 25 40 20 30
Smooth abraded surface 26 50 0 35
Dish-shaped depressions 30 55 0 51
Striations 40 52 0 15
Upturned plates 50 35 15 30
Meandering 29 0 0 25
Subangular outline 42 15 25 28
Rounded outline 10 25 15 18
Chemical origin
Triangular etching pits 52 4 74 0
Precipitation of silica 20 18 50 15
Deep grooves 40 10 55 4
Fig. 2. The frequency of various types of microtextures observed on the quartz grains from
Sedimentary Environmental of study area.CF:Conchoidal fractures,V: V-shaped
patterns, P: Pitted, SA: Smooth abraded surface, DD:Dish-shaped depressions,
S:Striations,UP:Upturned plates,M:Meandering,SO: Subangular outline, RO:
Rounded outline, TE: Triangular etching pits, PS: Precipitation of silica, DG: Deep
grooves.
0
10
20
30
40
50
60
70
80
90
100
CF V P SA DD SA UP M SO RO TE PS DG
Micrtextures
Fre
qu
ency
%
Beach sand Coastal dune Sabkha deposits Wadi deposits
98 Saeed Omar Wasel
Coastal Sediments
Most of the studied beach quartz grains are rounded to subrounded
(Fig. 3A). The surface features of these grains are mainly represented by
mechanical V- shaped impact pits, rounded and pitted grooves, surface
and edges roughness and upturned plates (Fig. 3B).
Graded arcs which are characteristic of aeolian origin were also
noticed on the surfaces of the quartz grains (Fig. 3C); they occur in
concentric series with the arcs graduated in size. Each arc series forms a
fan-shaped pattern. The graded arcs are a sub-variety of the conchoidal
breakage blocks may be due to grinding collision in a fluvial
environment. However, they are much less common than either the
conchoidal blocks or the meandering ridges formed by grinding collision
(Krinsley and Donahue, 1968, and Cater 1984).
The observed meandering ridges are suggested to be formed during
grain to grain collision in an aeolian environment (Krinsley and
Takahashi, 1962a, and Moral-Cordona et al., 1997). They were observed
in a few of the investigated quartz grains (Fig. 3D). Some quartz grains
show chemical precipitation features such as silica globules, and trapped
diatoms (Fig. 3E and 3F) which suggest that these grains were derived
from the silica saturated environments of intertidal zones (Madhavaraju
et al., 2009).
Such surface features may reflect the effects of both subaqueous
and aeolian origin. The subaqueous origin is supported by the presence
of V-shaped pits and rounded and pitted grooves (Krinsley and
Doornkamp, 1973), whereas the aeolian effects are related to the
presence of upturned plates. The presence of aeolian features in such
beach sediments may suggest that some aeolian sands were reworked by
waves and tides to form these beach sand deposits. Most of the
mechanical impact features are quite common and generally reflect
medium to high energy nearshore environments. Also the roughness of
surfaces and edges of some of the quartz grains in Fig. 2A might reflect
differential chemical weathering that may be related to differences in
chemical resistance within the grains (Krinsley and Doornkamp, 1973).
Microtextures of Quartz Grain Surface from Recent Sedimentary … 99
Fig. 3. Surface features of quartz sand grains from beach environment as observed by SEM.
A) Rounded outline and surface abrasion with some V-shaped pits. B) Upturned
plates in association with V-impact pits. C) Arcuate and straight steps in association
with silica precipitation. D) Meandering ridges, associated with scattered pits. E) Pits
grooves, arc steps and silica globules associated with diatoms. F) Numerous diatoms
are observed on the quartz grain in association with silica globules.
In the coastal dunes, quartz grains are rounded to subrounded and
mainly characterized by surface and edges roughness (Fig.4A). They are
mainly characterized by dish-shaped depressions, striations, conchoidal
fractures, upturned plates, stepped cleavage planes and V-shaped pits
(Fig. 4B). One of the most important features that characterize quartz
grains of the coastal dunes is silica precipitation (Fig. 4C). The V-shaped
pits similar to those present on the surface of quartz grains of the beach
sands are also common (Fig. 4D). This is probably a relict from some
subaqueous environments. The presence of such features also indicates
A B
C D
E F
100 Saeed Omar Wasel
the possibility of grain transportation from marine environment because
of their closeness to the seawater.
Fig. 4. Surface features of quartz sand grains from coastal dune as observed by SEM. A)
Subrounded quartz grain that shows small conchoidal fractures. B) Straight and
arcuate steps and V-shaped pits. C) Upturned plates in association with V-impact
pits, solution and precipitation. D) dish-shaped depressions and V-shaped pits.
The surface features of quartz grains from coastal sabkha deposits
are mainly characterized by chemical features rather than features of
mechanical origin. They are characterized by triangular etching pits (Fig.
5A, B and D). These textures usually indicate a high energy chemical
environment (Krinsley and Doornkamp, 1973). Deep etching grooves
(Fig. 5B) are frequently present as result of chemical processes, in
particular, silica solution. It was also noticed that the surfaces of the
quartz grains from the sabkha deposits display silica precipitation
features, for instance irregular plates and cavity filling as well as silica
plastering (Fig. 5B) where excess silica is pressed over quartz grain
surface in non-oriented pattern. This may be explained by the movement
of grains across one another under high pressure (Krinsley and
Doornkamp, 1973).
A B
C D
Microtextures of Quartz Grain Surface from Recent Sedimentary … 101
Etching and precipitation of silica on surface grain (Fig. 5C),
precipitation of silica on triangular etching pits (Fig. 5D) are also
noticed. The most diagenetic features of this environment are the
triangular etch depressions where chemical etching along planes of
weakness occur extensively. This texture usually indicates a high energy
environment. Solution (etching) and precipitation are mostly developed
in the recent nearsurface diagenetic environment. The increasing rate of
evaporation and the existence of saline brine generally increase the pH
which affects the quartz grains and influence the development of
chemical features (Udayaganesan et al., 2011). Abd-Alla (1991) found
the dominance of chemical features on the surface of quartz grains in the
Mediterranean coast of sabkha deposits. The types and frequency of
chemical features of quartz grains from sabkha deposits suggest that the
grains have been subjected to high energy chemical dissolution.
Wadi Deposits
Quartz grains from wadi deposits are commonly characterized by
mechanical features and minor chemical features. Mechanical surface
features of quartz grains are mainly represented by conchoidal fractures,
upturned plates, mechanical V- pits and pitted grooves (Fig. 6A-D).
Upturned plates (Fig. 6C), which are usually developed by mechanical
action, have been enhanced by silica precipitation on these grains tends
to give them a dull surface, characteristic of desert dune sand grains
(Kuenen and Perdok, 1962).
From the preceding discussion it is clear that the coastal and wadi
sediments are characterized by the prevalence of mechanical features
rather than chemical one. The Mechanical features may reflect the effect
of both subaqueous and aeolian origins. The subaqueous origin is
supported by the presence of V- shaped pits, conchoidal fractures, pitted
grooves and straight or pitted grooves (Krinsley and Doornkamp, 1973),
whereas the aeolian effects are related to the presence of upturned plates,
meandering ridges and graded arcs. The presence of aeolian features in
such beach sediments which are probably relict from aeolian
environments may suggest that some aeolian sands were reworked by
waves and tidal currents to form these beach sand deposits.
Alternatively, some of such beach sands were subjected to transportation
102 Saeed Omar Wasel
by winds following their deposition. On the other hand, chemical
features are intensely imprinted on some of our studied sand grains
showing two clear phases of both dissolution and precipitation features;
comprising irregular solution pits, etched cavities and smoothed
surfaces. The dissolution and precipitation features observed in the
studied quartz grains have been considered by several authors to be
related to diagenetic processes, e.g. (Krinsley and Doornkamp, 1973).
Fig. 5. Surface features of quartz sand grains from sabkha deposit as observed by SEM. A)
Medium relief with chemically weathered surfaces. B) Upturned scattered silica
plastering associated with triangular solution pits and irregular plates. C)
Precipitation of silica on grain surface. D) Precipitation of silica on triangular etched
pits.
In hot and arid conditions which prevail along Red Sea,
evaporation of pore water in these sediments during diurnal tidal cycles
may have led to high concentration of dissolved salts and rising the pH
values. Under such conditions, silica dissolution altered some of the
quartz surfaces, that it may be redeposited when suitable
microenvironmental conditions are available (Krinsley and Doornkamp,
1973).
C
D
B
C
A
Microtextures of Quartz Grain Surface from Recent Sedimentary … 103
Fig. 6. Surface features of quartz sand grains from wadi deposit as observed by SEM. A)
dish-shaped depressions associated with conchoidal fractures and deep pits modified
by silica solution. B) Upturned plates in association with V-impact pits. C) Upturned
plates and meandering ridges. D) Straight steps, associated with scattered pits.
Conclusions
Examination of quartz grain surfaces from four recent
environments along the coastal area in the southern Red Sea of Yemen
reveals the occurrence of mechanical and chemical surface features.
From the relative frequency distribution of the various surface features
on the quartz sand grains from the sediments studies, it is noticed that
mechanical features are prevalent in wadi deposits, coastal dune sands,
and beach and nearshore sands. Chemical features are mostly confined to
coastal sabkha deposits. Mechanically formed grooves are predominant
feature followed by impact V-marks and conchoidal fractures. Rounding
of the grains and smoothening of the edges indicate high energy zone.
Etch V- shaped and solution pits are dominant features followed by
precipitation of chemical processes. Evaporation and exposure of quartz
grains in the dry intervals increase the pH which subsequently led to the
C
A
D
B
104 Saeed Omar Wasel
etching process. In addition, the presence of diatoms on the quartz grains
of the beach supports the process of chemical precipitation in a high-
silica environment.
Mechanical and chemical surface features of quartz grains from the
study recent sediments along the coastal area in the southern Red Sea of
Yemen and associated environments show overlap similarity as would
be expected in fluvial- coastal marine system where sediment is readily
recycled between markedly different environments.
Relative frequency distribution study highlighting the proportions
of mechanical and chemical features, however, shows that there are
sufficient differences in the assemblages of textures to distinguish the
aeolian-dominated environment (wadi deposits and coastal dune sands)
from the subaqueous environment beach sands and nearshore sands from
the chemical active- dominated ones coastal sabkha deposits.
Acknowledgements
The author expresses his deep appreciation to Prof. Omran E.
Frihy, Coastal Research Institute, Alexandria, Egypt for his critical
review and constructive comments of this manuscript. Thanks also to Dr.
Mahmoud Essa, Geology Dept., Assiut University, Egypt for his valuable
assistance with the SEM.
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