Microtextures of Quartz Grain Surface from Recent ...grains from Recent sedimentary environments...

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.


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).


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


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


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


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


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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