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Clastic sedimentary rocks

1. Introduction

1.1 Definitions:

Sediment is the body of loose, solid materials accumulated at or near the surface of the

Earth under low temperatures and pressures that normally characterize this environment.

The sediment is generally deposited or settled from a fluid which was in a state of

suspension or solution. But it could also include other materials not settled from

suspension such as residual deposits (laterite and bauxite for example), in situ

accumulation of organic debris (giving rise to coal deposits), and materials deposited

through glacial and aeolian agencies.

Other types of sediments might be formed at higher temperature as pyroclastic materials,

or at higher pressure as deep sea floor sediments.

All these types of loose sediments could be converted into indurate materials called

sedimentary rocks by a process termed lithification which is just part of a broader

process termed diagenesis.. Lithification may result from compaction of clay minerals

due to increasing burial depth. Also, it could be caused by cementation due to

introduction of solutions rich with dissolved elements and groups between the loose

sediment grains then precipitation of minerals in the pore spaces thus binding the grains

together. Moreover, recrystallization of original sediment, such lime mud, may give rise

to lithification.

We should distinguish between the following related terms:

1- Sedimentation is the process of sediment accumulation that is applied to settling of

solid particles from a fluid.

2- Sedimentology is the science of studying sedimentary deposits. It is a broad term

including observations gathered from the field and laboratory.

3- Sedimentary petrology deals with origin of sedimentary rocks and models of

formation of both present and equivalent ancient rocks.

4- Sedimentary petrography is the science of description of sedimentary rocks.

1.2 Occurrence of Sedimentary Rocks

Sedimentary rocks are the most abundant rocks cropping out on the Earth‘s surface. They

cover about 70% of Earth‘s surface. However, by volume, the sedimentary (and the

metasedimentary) rocks constitute only 5% of the lithosphere, whereas the igneous and

metamorphic rocks make up the remaining 95%.

Therefore, the sedimentary rocks constitute just a thin veneer on Earth‘s surface ranging

in thickness from 0 to 13 km, and averaging 2.2.km.

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Of these sedimentary rocks, three types make over 95% of all sediments, which are

mudstones or shales, sandstones and carbonates. The remaining types of sedimentary

rocks include salt deposits, chert, coal, phosphates and ironstones.

1.3 Economic Values of Sediments and Sedimentary Rocks

Most of the mineral products come from the sedimentary deposits. Just to mention few:

mineral fuels such as coal, natural gas, petroleum, and oil shale; raw materials for

ceramics and Portland cement; non-metallic deposits including sand, gravel, lime;

building stones; molding sand; mineral fertilizers such as phosphates, potash salts and

some nitrates; ore metals such as ores of iron, aluminum, copper, uranium, magnesium

and some manganese; gemstones as placer gold, tin, tungsten and platinum; and some

sands and sandstones act as reservoirs for storage of valuable fluids such as fresh water,

petroleum and natural gas, and brines for iodine and bromine.

Moreover, sands have extra uses such as in in filtration and as friction sand (on

locomotives); they are exploited for rare minerals and are elements they contain such as

gold, platinum, uranium, tins in cassiterite, tungsten in wolframite, thorium and rare earth

elements in monazite, zirconium in zircon, and titanium in rutile.

And not to forget, and perhaps most important of all, sand is what every child loves to

play in.

1.4 Classification of Sediments and Sedimentary Rocks

Since sedimentary rocks are formed through various physical, chemical and biological

processes, they can be classified into four major categories:

1- Siliciclastic sediments (also referred to as terrigenous or epiclastic deposits) are those

consisting of fragments (clasts or grains or particles) of pre-existing rocks, which have

been transported and deposited by physical processes. These rocks include:

conglomerates, breccias (rudites or rudaceous rocks), sandstones (arenites or arenaceous

rocks), and mudrocks (lutites or argillaceous rocks).

2- Sediments largely of biogenic, biochemical and organic origin are the limestones that

may be altered to dolomites; phosphate deposits; coal and oil shale; and cherts.

3- Sedimentary rocks largely of chemical origin, principally direct precipitation, are the

evaporites and ironstones.

4- Volcaniclastic deposits consisting of lava and rock fragments derived from

pencontemporaneous volcanic activity.

Each of these various sedimentary rock types can be divided further, usually on the basis

of composition. In addition, many rock types grade laterally or vertically into others

through intermediate lithologies.

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2. Siliciclastic Sediments

Siliciclastic sediments are composed mainly of grains or clasts derived from pre-existing

igneous, metamorphic or sedimentary rocks. The clastic grains are released through

mechanical and chemical weathering processes, and then transported to the depositional

site by a variety of mechanisms, including river currents, waves, tidal currents, wind,

turbidity currents, debris flows and glaciers.

Siliciclastic sedimentary rocks range from the coarse grained conglomerates, through the

sandstones to the finer grained mudstones.

2.1 Grain size of siliciclastic sediments and sedimentary rocks

Siliciclastic sediments are classified according to decreasing grain size into gravel, sand

silt and clay. Based upon Udden – Wentworth grain size scale siliciclastic sediments are

divided into four grades: clay, silt, sand, and gravel. The gravel grade is further

subdivided into four grades: granule, pebble, cobble and boulder. Each of these grades

can be further subdivided into several classes. For example the pebble grade is

subdivided into fine (f), medium (m), coarse (c), and very coarse (vc) classes. Whereas

the sand grade can be subdivided into very fine (vf), fine (f), medium (m), coarse (c), and

very coarse (vc) classes.

Table 2.1 shows the names of both the loose sediments and the lithified sedimentary rock,

and the range of each grade and its classes in millimeters and phi units (see below) based

upon Udden and Wentworth grain size scale. For example gravel is the name of the loose,

whereas conglomerate is the name of the sedimentary rock.

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Tab. 2.1: Grain size scale for sediments and sedimentary rocks, after Udden and

Wentworth.

Grain size can be expressed in millimeters or in a unit called phi unit (Φ) having the

advantage of making the statistical calculations easier. The relationship between grain

size in millimeter and grain size in Φ is:

Φ = - log2 d, where d is the grain diameter in millimeters.

Note from Tab. 2.1 that the phi scale yields both positive and negative numbers. The real

size of particles, expressed in millimeters, decreases with increasing positive phi values

and increases with increasing numerical negative values.

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2.2 Sandstone composition

The clastic texture of detrital (clastic) sedimentary rocks (conglomerates and sandstones)

consists of:

1- Framework components (clasts or grains or particles) that constitute the skeleton of the

rock. They could be potentially any mineral, bur actually only few minerals compose

these rocks according to certain factors that will be discussed below.

2- Matrix that consists of grains less than silt size (<0.63 mm) located between the clasts.

3- Cement filling remaining pore spaces between the grains and matrix.

4- Pore spaces, which are the voids left without being filled with matrix or cement.

2.2.1. Framework Components

The major minerals constituting most of the sandstones are quartz, feldspars and rock

fragments. Following is a brief description of each of them.

2.2.1.1 Quartz

Quartz (low quartz or Beta quartz) is thermodynamically stable under sedimentary

conditions, thus it is the most common detrital mineral present in all types of sandstones.

Other SiO2 polymorphs such as tridymite and cristabolite are rarely found in sandstones.

In young or recent sediments amorphous silica or opal could be present.

No sandstone could be free of quartz. Three varieties of detrital quartz are found in clastic

sedimentary rocks.

1) Non-undulose monocrystalline quartz, where each grain consists of a single crystal

that extinguishes suddenly upon rotating the polarizing microscope’s stage (having a

straight or unit extinction) (Fig. 2.1).

Fig. 2.1: Non-undulose monocrystalline quartz. Note the quartz overgrowth separated

from the detrital core by presence of the dust line, crossed polarized light.

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2) Undulose monocrystalline quartz, where each grain consists of a single crystal that

extinguishes gradually upon rotating the polarizing microscope’s stage (having wavy or

undulose extinction) (Fig. 2.2).

Fig. 2.2: Undulose monocrystalline quartz, crossed polarized light.

3) Polycrystalline quartz, where each grain consists of two or more crystals (Fig. 2.3).

The contact between adjacent crystals could be straight, sutured or irregular.

Fig. 2.3: Polycrystalline quartz, crossed polarized light.

Also, quartz grains could be characterized by presence of some inclusions in the grain,

such as needles of sillaminite, vacuoles of fluids or minute crystals of some minerals

(tourmaline, mica or rutile, for example).

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Quartz could be utilized to determine it‘s source rock. Generally it is derived from

plutonic granitoid rocks, acid gneisses and schist, and in some cases from pre-existing

sandstones. Fig. 2.4 shows the relative abundance of detrital monocrystalline and poly

crystalline quartz grains in Holocene sands derived from known plutonic and

metamorphic sources.

Fig. 2.4: Relative abundance of detrital monocrystalline and poly crystalline quartz grains

in Holocene sands derived from known plutonic and metamorphic sources.

However, there are some properties of quartz that can be employed to infer its source

rock. Quartz from volcanic rock sources is typically monocrystalline with unit extinction,

no inclusions and could reveal euhedral crystals. Quartz from hydrothermal veins could

have fluid-filled vacuoles. Polycrystalline quartz from metamorphic provenance could

posses many crystals, that are elongate, with preferred orientation and may have sutured

contacts. Obviously, quartz grains with sillaminite inclusions point to a metamorphic

origin. It was thought that the undulose extinction indicates a metamorphic origin, but

actually it is due to strain in the crystal lattice that could occur also in plutonic igneous

rocks.

Usually, the first variety of quartz is the most common one in most sandstones. This is

due to its higher stability during weathering, transportation and diagenesis than the other

two types. Therefore, recycling of quartz grains from an older sandstone leads to

enrichment with the non-undulose monocrystaline quartz. This recycled quartz could be

recognized by the presence abraded quartz overqrowth, that in some cases might be

followed by a second quartz overgrowth (Fig. 2. 5).

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Fig. 2.5: Two quartz grain exhibiting two stages of quartz overgrowth, crossed polarized

light.

2.2.1.2 Feldspars

Feldspar grains are the second common mineral in sandstones next to quartz. Although

feldspars are more abundant than quartz in granitoid and gneissos source rocks, they are

less common in sandstones than quartz. The reason for their lower concentration than

quartz is their lower chemical stability against chemical weathering, particularly

hydrolysis and leaching, and their lower resistance against mechanical abrasion according

to presence of well-developed cleavage.

Feldspar grains in sandstones could be the following types:

1- K-feldspar, either as orthoclase (Fig. 2.6), microcline, or rarely sanidine.

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Fig. 2.6: Orthoclase grain characterized by overgrowth. Note that the overgrowth lacks

the weathering or alteration products affecting the detrital core, also note the cleavage

suffering from strong alteration, crossed polarized light.

Microcline is readily identified in thin sections by the grid-iron (cross-hatch) twinning

pattern.

2- Plagioclase (Fig. 2.7). It is less common than K-feldspar according to its lower

chemical stability against weathering, and due to its less abundance in continental

basement rocks (granites and gneisses) that are the provenance of many sandstones.

Fig. 2.7: Plagioclase grain characterized by multiple twining, crossed polarized light.

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However, plagioclase is more common in sandstones derived from uplifted oceanic and

island-arc terranes, which are generally less important source areas.

Feldspar grains can be easily distinguished from quartz grains in thin sections. Cross-

hatch twining of microcline, and sometimes Carlsbad twining of orthoclase are diagnostic

features not present in quartz. Cleavage also characterizes feldspar grains, particularly

when it is associated with chemical alteration products (clay minerals and sericite) along

the cleavage planes (Fig. 2.6). On the other hand, quartz shows no cleavage. Also

chemical weathering of feldspar grains imparts a turbid color or a cloudy or dusty

appearance, whereas quartz grains are usually clear lacking this appearance.

Feldspar grains are derived from the same crystalline rocks as quartz. These chiefly are

granites and gneisses, where potash feldspar dominates over sodic plagioclase. Texture in

feldspar crystals may contain clues to their origin. Various types of zoning are frequently

seen indicating volcanic origin. Pyroclastic feldspars tend to be anhedral, which

frequently are broken. Perthites are the result of slow cooling and so are more typical of

plutonic source rocks.

The majority of feldspar grains in sedimentary rocks are of first cycle origin. According

to their mechanical indurability during transportation, and chemical instability, they are

destroyed through recycling. Favorable conditions for existence of feldspar in sediments

are arid climate, since humid climate promotes their chemical weathering, and high rate

of erosion associated with the high relief in tectonic active areas that enables feldspar

grains to escape even intensive chemical weathering in humid regions.

2.2.1.3 Rock Fragments

Rock (or lithic) fragments are more abundant in conglomerates than in sandstones. This is

according to the following reason. If source rocks are coarse crystalline granite or gneiss,

rock fragment grains of sand size will be composed of just one crystal of an individual

mineral. Therefore, only fine-grained or fine crystalline volcanic, metamorphic and

sedimentary rocks can supply sandstone with rock fragments. Among those are:

1) Fine-grained sedimentary rocks as mudstone, shale or some times siltstone, limestone,

and siliceous sedimentary rocks as chert (Fig. 2.8).

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Fig. 2.8: Chert rock fragments, crossed polarized light.

2) Fine-grained metamorphic rocks as slate, phyllite, pelite and mica schist.

3) Volcanic rocks as felsic rhyolite, intermediate andesite or mafic basalt.

Rock fragments are very useful in studies of provenance of sandstone. But it is important

to study rock fragments of similar size, since their percentage increases with increasing

grain size. Therefore, the “Gazzi-Dickinson” method is applied, where sand-sized

crystals and grains within a larger rock fragment are assigned to the category of the

crystal or grain, rather than to the rock fragment class.

Many rock fragments are unstable (labile grains) such as mudstone or slate that my

become indistinguishable from the primary mud matrix by diagenitic compaction, or

could be altered or replaced by chlorite or zeolite.

Rock fragments in sandstones give very specific information on the provenance of a

deposit if they can be tied down to a particular source formation.

Rock fragments generally are derived more from supracrustal rocks undergoing uplift and

erosion. Mountain belts and volcanic areas supply large quantities, whereas

continental/granitic basement does not.

The types of lithic grain do relate to plate-tectonic setting of the provenance terrane and

adjoining sedimentary basin as will be discussed later on.

Calculating the percentage of sedimentary rock fragments (Ls) and low-grade (Lm1, slate

+ quartzite) and high grade (Lm2, phyllite + schist + quartz/mica/albite aggregates)

metamorphic grains up through a succession may show trends related to source-area

uplift (Fig. 2.9).

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Fig. 2.9: The trend in lithic grains (Ls, sedimentary; Lm1, low grade metamorphic; Lm2,

medium grade metamorphic) in sandstones derived from the unroofing of a sedimentary-

metasedimentary complex of an arc-continent collision belt).

2.2.1.4 Other Components

2.2.1.4.1 Detrital micas present in sandstone include biotite, muscovite and rarely

chlorite.

According to their sheet silicate structure, they occur in sandstone in form of flakes

arranged parallel to bedding planes. They are derived from many granitoid plutonic

rocks, and metamorphic schists and phyllites. According to chemical instability of biotite

against chemical weathering, muscovite is more common in sandstones. If biotite is

present, it is mainly altered into iron oxide, chlorite or clay minerals (illite or kaolinite).

2.2.1.4.2 Heavy minerals are accessory minerals present in sandstone with a

concentration, usually less than 1%. They have a specific gravity greater than 2.9,

whereas quartz and feldspars have specific gravity around 2.6.

Heavy minerals are obtained from sandstone by separation from the light minerals (quartz

and feldspars) employing a heavy liquid such tetrambromoethane or bromoform having a

specific gravity of 2.9. Heavy minerals could be transparent (might be considered

translucent) or opaque. The transparent heavy minerals can be identified using the

polarized transmitting microscope after mounting on a glass slide by a resin with a known

refractive index and covering with a glass cover. The opaque heavy minerals, on the other

hand, can be identified using reflecting light microscope after mounting on a glass slide

and polishing the outer slide surface without covering by a glass cover.

Transparent heavy minerals are mainly silicates, whereas the opaque ones are oxide. The

latter are less significant than the former in provenance determinations.

Transparent heavy minerals could be ultrastable, stable, metastable (or moderately

stable), unstable or very unstable according to their resistance against chemical and

physical weathering (Tab. 2.2).

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Tab. 2.2: Heavy minerals arranged according to decreasing stability.

The major transparent heavy minerals are zircon (Fig. 2.10), tourmaline (Fig. 2.11) and

rutile (Fig. 2.12). Theses three minerals are present in every sandstone, since they are the

most stable, or ultrastable minerals against chemical weathering and mechanical abrasion.

Fig. 2.10: Zircon grain characterized by very high relief, high order interference colors,

and presence of inclusions. Also note that the borders of the grain are heavily abraded.

Cross polarized light.

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Fig. 2.11: Tourmaline grain characterized by strong relief, and high order interference

colors and strong pleochroism, plane polarized light.

Fig. 2.12: Rutile grain characterized by deep red color, very strong relief and interference

colors masked by the original color of the mineral, plane polarized light.

Among the stable heavy minerals are apatite, garnet, monazite, and staurolite. The

moderately stable heavy minerals include: epidote, kyanite, sillimanite, sphene, and

zoisite. The unstable heavy minerals are pyroxene (augite, diopside, hypersthene),

hornblende and biotite, whereas olivine is the very unstable one.

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Uses of heavy minerals

Heavy minerals are very useful in determination of the provenance, climate-dependent

weathering, distance of transportation, depositional environment, burial depth during

diagenesis.

Besides these uses that will be discussed below, heavy minerals can be used in oil-

company research laboratories because useful information can be obtained from small

samples, such as those brought to surface during drilling of exploratory borings. They can

be used in matching sands from one hole to another, even if the provenance is not known.

In stratigraphic correlation, heavy minerals are of great use, because theoretically, each

stratigraphic unit differs in some degree from other in character and abundance of its

suites of heavy minerals. This is the basis of petrographic correlation, where peculiar

varieties and changing proportions of the constituent heavy minerals with time can be

employed.

Such differences are secured by progressive denudation of a varied terrane. Each new

rock mass unroofed contributes new species or varieties to the accumulating sediments op

changes the proportions of species already present. Correlation is complicated by

reworking of older sediments so that the new deposits have many species in common

with the sediment from which it was derived.

It is to be emphasized that, heavy minerals cannot be used as time-stratigraphic markers

in the sense that a certain association or suite of heavy minerals points to a specific

geologic time. However, heavy minerals can be used in facies correlation indicating

sedimentary dispersal from particular source areas which are undergoing tectonic

evolution.

Thus one can map the progress of an orogenic episode in a source area which led to the

gradual unroofing from sedimentary to a metamorphic to an igneous terrane by noting the

change in heavy mineral suites going upwards in the sandstone succession derived from

that source. The stratigraphy of the heavy minerals zones of the sandstone succession will

be the reverse of the sequence in the source area.

Heavy minerals and provenance

The term provenance come s from the French “Provenir” meaning to originate or to come

forth, thus it encompasses all the factors relating to the production or birth of the

sediment. Most often it refers to the source rocks from which the materials were derived.

Each type of source rock tends to yield a distinctive suite of minerals which, therefore,

constitutes a guide to the character of that rock. But composition of sediments is not

determined solely by the nature of the source rock, it is also a function of other factors

that will be discussed later on. However, certain detrital mineral association could be

indicative of a major class of source rocks as can be seen in Tabs. 2.3, 2.4.

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Tab. 2.3: characteristic heavy minerals of different source rocks.

Tab. 2.4: characteristic heavy minerals of different source rocks.

Moreover, certain varieties of a specific heavy mineral having a characteristic color or

form or inclusions could be very helpful in source determination. For example, purple

zircon (variety hyacinth) is derived from ancient Precambrian rocks. It results from long

periods of radioactive bombardment with alpha particles.

Stability of heavy minerals

The stability of a mineral is its resistance to alteration. The chemical stability is the

resistance of the mineral to solution and decomposition, whereas the mechanical stability

is its resistance to abrasion.

When minerals are subjected to an environment different from that under which they

were formed, they are now unstable and could be dissolved or decomposed at the

aqueous environment at or near Earth‘s surface (in soil or in sedimentary envelope).

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Corrosion and etching are indicative of instability, whereas overgrowths are indicative of

stability.

The selective solution of heavy minerals in the soil profile during weathering,

transportation, at depositional site and during diagenesis could affect the presence of

heavy minerals released from the source rock. Thus heavy minerals should have a certain

resistance, both chemical and mechanical, in order to survive in the sediment pile and

later on during burial.

Many attempts have been made to determine the relative stability of minerals in soils and

sediments. Goldich (1938) arranged the minerals in “mineral stability series” which is

identical to Bowen Reaction Series (Tab. 2.5). Also, Tab. 2.5 shows the stability of most

heavy minerals in soil profile, weathering site and during diagenesis (intrastratal

solution).

Tab. 2.5: Stability of heavy minerals in soil profile, weathering site, and during

diagenesis (intrastratal solution).

The other aspect of stability is the mineral stability during transit, where the sand is

subjected to modification and fractionation through its journey from source rock to

depositional basin. Therefore, it is expected that the processes operative during transport

which are responsible for rounding the debris transported, would also modify the

composition by selective abrasion and sorting according to specific gravity. Such changes

in the sand size cause softer and more cleavable species to be destroyed by abrasion, with

the complementary enrichment in harder and more durable components.

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Fries (1931) determined experimentally the durability (abrasion resistance or resistance

against mechanical weathering) of a considerable number of minerals giving hematite

(the least durable) a value of 100 (Tab. 2.6).

Tab. 2.6: Abrasion resistance of heavy minerals arranged in increasing order of

resistance.

It is clear that durability and Moh‘s scale of hardness are correlative (at least for minerals

less in hardness than quartz).

Following deposition, sediments are subjected to artesian flow, and leaching, or late in

post depositional history (during burial), the heavy mineral could be dissolved, what is

called intrastratal solution or dissolution. The teeth, hacksaw, or cockscomb character of

some heavy minerals indicate a post depositional origin.

A strong evidence for intrastratal solution is preservation of unstable heavy minerals in

layers or concretions cemented by early carbonates, whereas the non-cemented layers are

or away from the concretions are devoid of these unstable heavy minerals.

Heavy minerals zones

That beds of differing age, even in the same district, having different assemblages of

heavy minerals is a common observation that could be attributed to unroofing of the

source rocks.

Such heavy mineral zones in Tertiary and Mesozoic sections reveal three points:

1- Number of heavy mineral species increases from older to younger beds.

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2- The order of appearance of mineral species is remarkably similar (even in widely

separated and unrelated basins, Fig. 2.13).

Fig. 2.13: Heavy mineral zones. Solid lines, present in more than one-half of samples;

dashed lines, present in fewer than one-half of the samples. Left, are samples from

Atlantic Coastal Plain, Maryland; right, are samples from Egyptian sediments.

3- The order of appearance is the reverse order of stability of heavy minerals in question.

As can be seen from Tab. 2.7 hornblende is the most typical in the highest zone.

Tab. 2.7: Order of appearance of index species in heavy mineral zones

The lowest zone is restricted to tourmaline, zircon and rutile (in some cases staurolite and

garnet). The intermediate zone is restricted to kyanite, epidote, and titanite.

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As a rule, minerals of the lowest zone are also present in the higher zone, so that the

latters have enlarged or enriched suite.

These observations can be explained as a result of progressive denudation and unroofing

of new sources (Fig. 2. 14 left).

Fig. 2.14: The three hypotheses of heavy-mineral zonation.

As erosion proceeds, deeper levels of the crust would become contributors to the basin of

sedimentation. Because minerals in rocks of the deeper provenance level are, on the

average, least stable, there might be a normal order of succession that correlates with

stability order. This is the view of Krynine (1942) and Van Andel (1959).

A second hypothesis assumes correlation between mineral sequence and progressive

uplift of the source area associated with intensive chemical weathering (Fig. 2.14 center).

Under this thesis, the terrane of varied lithology would be near base level at the initial

stage and would be progressively elevated with consequent increase in gradient and

accelerated erosion.

During initial stages (where a low rate of erosion prevails)only the most stable species

escape destruction in the soil profile; in the final stages (where a high rate of erosion

prevails) even the least stable minerals would appear in the sediment, provided same

weathering intensity.

A third hypothesis supposes that all sediments deposited had about the same mineral suite

at the time of deposition but that, because of intrastratal solution, deeper and older beds

have lost all unstable species (Fig. 2.14 right). The probability of survival is a function of

depth of burial and of time. The deeper the burial and/or the older rocks, the less probable

the presence of a given species. Preservation of unstable species in early carbonate

cemented layers or concretions, as well as in moderately permeable shale, is a further

support of this thesis.

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2.2.1.4.3 Other detrital components

In some cases, carbonate fragments can be found in sandstones, including shell and fossil

fragments, ooids, peloids, and intraclasts. Glauconite and phosphatic grains could occur

also in some sandstones.

2.2.2. Detrital matrix

Between the framework components of sandstone and conglomerate occur finer grained

detrital minerals constituting the matrix. The grain size of the matrix minerals is usually

considered the clay size in sandstones (<20 micrometers, although some sedimentologists

consider it to be less than 4 microns), and the silt size in conglomerates (less than 63

microns). Detrital matrix could consist of the same minerals constituting the framework

components, but generally clay minerals are the dominant constituents of the matrix.

This interstitial detrital matrix should be distinguished from other non-primary types of

matrix, including:

1- Protomatrix that is the trapped detrital clay minerals.

2- Orthomatrix which is the recrystallized material into matrix

3- Epimatrix that is the diagenitic product of the alteration of sand-sized grains

4- Pseudomatrix which is the deformed and squashed lithic fragments.

2.2.3. Cements

Pore spaces left between framework grains and interstitial matrix could be filled during

diagenesis by chemically precipitated minerals in form of cements or authigenic

(neoformed) minerals. Details will be given in the section of diagenesis below.

2.3 Factors influencing composition of framework components

Composition of detrital framework grains in sandstones, as well as conglomerates,

depends on the following factors:

1- Provenance or the source rock that provided the mineral grains through mechanical

weathering. For example, a granitic source rock may supply quartz, K-feldspars,

plagioclase, biotite, muscovite, but not calcite or sillaminite. Whereas, limestone source

rock supplies calcitic rock fragments not detrital quartz.

2- Tectonic setting which determines the type of relief dominating the provenance. High

tectonic activity, such orogenic movements are responsible on a high relief, whereas a

low tectonic activity causes a low rate of epeirogenic uplift that in turn leads to a low

relief.

3- Climate and consequently the type of weathering in the source area, where a humid hot

climate promotes chemical weathering processes. On the other hand, a dry, arid to semi

arid climate whether it is cold or even hot, hinders leaching and other chemical

weathering processes, and facilitates the physical weathering disintegration.

4- Type and distance of transportation. Rivers or streams, glaciers, wind, tidal currents,

all have roles on dissolution or preservation of detrital minerals during transport. Long

distance of transport influences the degree of mechanical abrasion of the transported

detrital minerals.

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5- Depositional environment. Preservation of unstable or slightly stable detrital minerals

in fluvial environments, braided or meandering has a less chance compared with their

preservation in the milder marine environment.

6- Diagenesis. Diagenesis includes all the physical and chemical processes that affected

the sediments since the beginning of sedimentation until the on set of low grade

metamorphism. In particular, the most effective diagenitic process that influences heavy

minerals is intrastratal solution, where specific types of heavy minerals could be partly or

even completely dissolved by action of pore fluids present at great burial depth.

The following is an example of how the above factors could influence the composition of

two sandstone formations in Jordan. The Saleb Sandstone Formation is of early Cambrian

age overlain conformably by Cambrian Umm Ishrin Sandstone Formation. The Saleb

Sandstone Formation consists of mono crystalline quartz, K-feldspar, apatite and trace

amounts of plagioclase, biotite, muscovite, zircon, tourmaline and rutile. This mineral

assemblage points to granitic and granitoidal source rocks. In addition, the high content

of polycrystalline quartz and the presence of muscovite and biotite and undulose

(strained) monocrystalline quartz point to metamorphic source rocks, probably mica-

schist and metasediments. All these source rocks crop out in the crystalline basement of

Wadi Araba and South Jordan as part of the Arabian-Nubian Shield. The

sedimentological investigation of this conglomerate to coarse sandstone formation

indicates an alluvial fan to braided river depositional environment with a northward

dispersal direction, thus proving the south-located Arabian-Nubian Shield provenance.

The high content of apatite, illitic matrix, as well as feldspars is due to rapid

sedimentation and very short distance of transport between the source rocks and the

depositional environment. The conglomerates indicate a high rate of erosion that is

usually associated with a strong relief. This relief was very likely the result of rapid uplift

and intense faulting of the source area during the molasses phase of the Pan African

Orogeny. Concerning the role of climate, it is agreed that the Cambrian Period over the

globe was warm, and the same should be in the source area, and probably humid. Even in

this humid climate, the unstable feldspar and apatite were not destroyed, due to rapid

deposition in the adjacent depositional environment. Feldspars and apatite were preserved

during diagenesis from intrastratal solution by the preservation role of the illitic matrix.

The Cambrian Umm Ishrin Sandstone Formation is a fluvial one consisting of mature

quartz arenite. The unstable feldspars are totally absent and the heavy mineral suite is

restricted only to the ultrastable zircon, tourmaline and rutile. This mature sandstone does

not display any petrographic indication of a second-cycle origin. Such mature sandstone

of first-cycle origin is rarely described in literature. The high content of the three varieties

of quartz and the ultrastable heavies indicate a plutonic/metamorphic provenance. There

is no reason to consider a different source rock than that for the underlying formation.

The absence of feldspars, micas and apatite can be explained in the following way.

Through the Middle-Late Cambrian the Middle East area underwent neither orogenic nor

epeirogenic movement. The source area was tectonically stable, which lead to a low relief

and a retarded rate of erosion.

23

Under the warm humid climate prevailing, as stated above, chemical weathering was

intensive under the following conditions: 1) a low relief typical of a tectonically stable

source area; 2) a retarded rate of erosion; 3) a relatively long distance of transportation by

the low-braided rivers; 4) a slow rate of deposition in the fluvial depositional

environment associated with a low rate of subsidence; and 5) slight acidic conditions

prevailing in the weathering site, the transport way, and the depositional environment

(indicated from the presence of kaolinite). Such vigorous chemical weathering conditions

are sufficient to dissolve all feldspars and all unstable heavy minerals. If a slight amount

of unstable light and heavy minerals succeeded to reach the depositional site, the

intrastratal solution action of hot pore water present at a burial depth of around 2000 m

was enough to dissolve them completely, and even to corrode tourmaline within the

ultrastable heavy fraction.

This interpretation of the first cycle mature sandstone of the Umm Ishrine formation of

Cambrian age in Jordan can be applied on the overlying Lower Ordovician fluvial Disi

Formation and the marginal marine Umm Saham Formation.

The overlying Middle Ordovician marine Hiswa and Upper Ordovician Mudawwara

Formation records the appearance of K-feldspar within the light mineral suite and garnet

and staurolite within the heavy mineral fraction. These immature sandstones are

interpreted to be derived from the same provenance which is the Arabian-Nubian Shield.

The tectonic setting was stable where no orogenic movement is recorded, so that the rate

of erosion was low, but the feldspar clasts and the garnet and staurolite grains could

escape weathering at the provenance because the clime was cold, where the area was

subjected to the Upper Ordovician event that affected Arabia as well as North Africa. The

marine conditions were friendly to the unstable light and heavy minerals rendering them

to survive in the depositional basin. The same is applied on the moderate burial depth,

and consequently, the intrasratal solution was not pronounced leaving these labile light

minerals and unstable heavy mineral intact giving rise to the immature sandstone of the

Hiswa and Mudawwara Formations.

2.4 Compositional maturity

Compositional maturity of sandstone refers to the content of chemically stable light and

heavy minerals. Therefore, compositional supermature sandstone consists entirely of

quartz, and the three ultarstable heavy minerals: zircon, tourmaline and rutile.

Compositional mature sandstone consists mainly of quartz, slight amounts of feldspar or

rock fragments, and may contain one of the metastabe heavy minerals such as apatite or

garnet, besides the three ultrastable heavies. Compositionally immature sandstone

consists of quartz and considerable amounts of feldspars and/or labile rock fragments,

besides some of unstable heavy minerals such as hornblende or even pyroxene.

Compositionally supermature and mature sandstones may result from multiple cycling of

sediments, or in certain cases, they represent first-cycle sediments that underwent

intensive chemical weathering under humid climate in a tectonically stable provenance,

and went a long distance of transportation, and probably were deposited in an energetic

24

environment leading to a strong reworking. For example, the Cambrian, fluvial Umm

Ishrine Sandstone of Jordan described above is first-cycle deposit.

2.5 Sandstone Classification

Modal composition of sandstone can be obtained by making 300-500 counts of the

framework grains, matrix and cement using an automated point counter. Or of rapid

estimation of percentages of framework components or matrix percentages charts can be

employed (Fig. 2.15).

Fig. 2.15: Percentage estimation comparison charts, conventional and computer-

generated.

The percentage of the framework grains is recalculated to constitute 100%. The

percentage of the matrix is employed to distinguish between arenites and wackies; if the

matrix attains less than 15%, the sandstone is considered arenite, whereas if the matrix

exceeds 15% the sandstone is considered a wacky.

Then the percentage of the three major components of sandstone, quartz, feldspars, and

rock fragments is recalculated to attain 100%. The result is plotted on the triangular

diagram shown in Fig. 2.16 to determine the type of the sandstone.

25

Quartz arenites contains not more than 5% of either feldspars or rock fragments. It is also

called orthoquartzite. Arkosic arenites contain 25% or more feldspars which should

exceed rock fragments. Arkoses belong to this clan.

Lithic arenites contain 25% or more rock fragments but less feldspar. Transitional classes

as subarkose and sublitharenite may be recognized. Rock fragments in litharenites are

mainly politic in nature( shale, siltstone, slate, phyllite and mica schist).

Fig. 2.16: Classification of sandstone.

The term wacke does not involve the mechanism of transport or origin of the matrix. In

this sense, wackes differ from greywackes which are deposited by density currents (or

turbidity currents), one that flowed, impelled by gravity, downhill along sea bottom, and

are tough, well-indurated rocks, characterized by a dark “chloritic paste” matrix. Many

greywackes exhibit graded bedding, convolute and small-scale current laminations, and

various sole markings such as flute, groove and load casts. Therefore, wacke is just a

synonym for muddy or clayey sandstone.

For example, a sandstone has the following modal composition: 7% matrix, 10% iron

oxide cement, 60% quartz, 15% feldspar, 8% rock fragments. The three framework

components attain the following percentages: quartz 72%, feldspar 18%, rock fragments

10%. Therefore, the sandstone falls in the suabarkose field. Since the matrix is less than

15%, the sandstone is arenite, accordingly, the type of sandstone is subarkosic arenite. To

take into consideration the iron oxide cement, the prefix ferruginous or hematitic (if the

iron oxide is proved to be hematite) is added, so that the term becomes hematitic

subarkosic arenite.

26

Another classification of sandstone has been proposed by Amireh (1987) to suit for the

Cambrian-Lower Cretaceous clastic sequence of Jordan that is generally poor in feldspars

and rock fragments (Fig. 2.17). He considered the sum of both labile components, the

feldspars and rock fragments to attain 25%. Thus arkosic arenite may consist of at least

12.5% of feldspars, and lithic arenite may consist of only 12.5% lithics.

Fig. 2.17: Amireh‘s (1987) classification of sandstone, considering both of the labile

components the feldspars and the rock fragments to attain 25%. Thus arkosic arenite may

consist of at least 12.5% of feldspars, and lithic arenite may consist of only 12.5% lithics.

The above classification of sandstone has genetic implications. The ratio Q/(F + Rx) is a

rough measure of compositional maturity. The ratio measures the progress toward the

ultimate end-type that is pure quartz sand.

The ratio F/Rx reflects provenance and distinguishes between a deep-seated provenance

and a supracrustal provenance. Supracrustal rocks, whether igneous, metamorphic, or

sedimentary, are apt to be fine grained and hence yield sand-sized particles. Coarse

crystalline plutonic rocks yield only mineral grains in the sand range. Most are fledspar-

bearing and yield only feldspar.

The (Q + F + Rx)/ matrix (grain/matrix) ratio is less easy to interpret. Sediments with an

overwhelming matrix are likely the products of a quasi-liquid or mass flow of a mud-sand

mixture; normal dilute suspensions deposit matrix-free sands. It was therefore once

considered an index of fluidity. But if some matrix is post-depositional product (perhaps

27

diagenetic), the ratio has a different significance- a measure, perhaps, of the dgredation

framework elements.

It should be stated that there are special types of sandstone that include: calclithite which

is a clastic rock composed of sand-sized limestone or dolomite fragments, to be

distinguished from clacarenite which is a carbonate sand produced by biochemical or

chemical precipitation.

Calcarenaceous sandstone is applied to carbonate detritus mingled in all proportions with

quartz and other clastic grains.

Chert arenite and volcanic arenite are applied to clastic sandstone composed of chert and

volcanic rock (derived from disintegration of extrusive or flow rocks), respectively.

2.6 Conglomerate classification

Coarse clastic rocks can be classified in several ways. According to roundness of grains

they can be subdivided into conglomerates having subrounded to well rounded clasts, and

breccia consisting of angular grains. Based on origin, the term extraformational

conglomerate is used to indicate those composed of clasts derived from source rocks

away from depositional site, whereas, intraformational conglomerate or breccia is applied

on those composed of clasts derived from within the basin of deposition.

On base of composition, conglomerate could be either monomictic, consisting of just a

single type of clasts (Fig. 2.18), or polymictic, consisting of two or more clast types (Fig.

2.19).

Fig. 2.18: Monomictic Umm Ghaddah Conglomerate of Jordan of late Ediacaran-Early

Cambrian age consisting totally of rhyolitic rock fragments.

28

Fig. 2.19: Polymictic Sarmuj Conglomerate of Jordan of Late Precambrian. Note the

presence of granitic (pink colored), rhyolitic (brownish red colored), and few basaltic

(black colored) well-rounded rock fragments.

According to sediment fabric, conglomerates could be either orthoconglomerate that is

clast-supported, or paraconglomerate (or diamictite), matrix-supported.

2.7 Sandstone composition, provenance and tectonic setting

The detrital composition of sandstone may be related to the tectonic setting of its

provenance region.

The detrital modes of both modern and ancient sands can be used in this aspect, which

often can be supplemented with chemical analysis of grains, including age-dating of

zircon and rock fragments.

In simple quartz-feldspar-lithic plot of modern deep sands, Yerino & Maynard (1984)

showed that the five tectonic settings could be distinguished, but with much overlap (Fig.

2.20).

29

Fig. 2.20: Composition of modern deep sea sands from trailing-edge (TE, also called

passive margin), strike-slip (SS), continental-margin arc (CA), back-arc to island arc

(BA) and fore-arc to island-arc (FA) tectonic settings (after Yerino & Maynard, 1984).

In the work of Dickinson (1985) on ancient sands, four major provenance terranes were

distinguished: stable craton, basement uplift, magmatic arc, and recycled orogen. Stable

cratons and basement uplifts form the continental blocks, i.e. tectonically consolidated

regions of amalgamated ancient orogenic belts, which have been eroded to deep levels.

Magmatic arcs include the continental and island arcs associated with subduction, and

these are areas of volcanics, plutonic rocks, and metamorphosed sediments.

Recycled orogens are uplifted and deformed supracrustal rocks, which form mountain

belts, and they mostly consist of sediments, but include volcanics and metasediments.

Detritus from the various provenance terranes generally has a particular composition and

the debris is deposited in associated sedimentary basins, which occur in a limited number

of plate-tectonic settings (Tab. 2.8).

30

Tab. 2.8: The major provenance terranes, their tectonic setting and typical sand

composition (after Dickinson, 1985).

From a modal analysis of a sandstone, the percentages of various combinations of grains

are plotted on triangular diagrams, and these are used to differentiate the different

provenance terranes (Fig. 2.21).

Fig. 2.21: Triangular diagrams showing average compositions of sand derived from

different provenance terranes (after Dickinson, 1985).

The categories of grain determined (Qt, Qm, Qp; F, Fp, Fk; L, Lv, Ls, Lt) are shown at

Tab. 2.9.

31

Tab. 2.9: Classification of sand-grain type.

A triangular plot of Qt-F-L takes all the quartz grains together (Qm + Qp) and so places

emphasis on the maturity of sediment.

Plots of Qm-F-Lt include Qp with the lithic grains and so give weight to the source rock.

Plots of Qp-Lv-Ls consider just the rock fragments, and those of Qm-Fp-Fk involve only

the single mineral grains.

Care must be exercised where there is more than 10% pseudomatrix in the sandstone. The

use of these diagrams allows sandstones from the four major terranes to be discriminated

(Fig. 2.21).

Stable cratons of low relief generally produce quartzose sands from the granite-gneiss

basement and recycling of earlier sedimentary strata. They are deposited on the cratons or

transported to passive continental margins.

Basement uplifts are areas of high relief along rifts and strike-slip zones, and the

dominantly quartzo-feldspathic, lithic-poor sands are deposited in extensional and pull-

apart basins.

Magmatic arcs produce sands with high contents of volcanic rock fragments, and as they

are dissected down to their plutonic roots, quartzo-feldspathic debris is generated. A

volcanic to plutonic trend may thus result. The sands are deposited in forearc and interarc

basins. The volcanic grains commonly will have andesitic compositions usually they are

microlitic. After diagenesis greywacke-type sandstones may be formed.

32

Detritus derived from recycling of orogonic belts is very varied in composition, reflecting

the different types of orogen (broadly as continent-continent or continent-ocean

collision).

Sediments from a recycled orogen may fill adjacent foreland basins and remnant oceanic

basins or be transported in major river systems to more distant basins in unrelated

tectonic setting.

Lithic fragments dominate in many recycled-orogen sandstones, and in those derived

from continental collision mountain belts (such as the Alps and the Himalayas), quartz

plus sedimentary rock fragments dominate, and then the metamorphosed equivalents of

the latter as deeper levels of the orogen are uplifted.

These sands thus trend to be more quartz-lithic, with few feldspar and volcanic grains (a

high Ls/Lv ratio).

Detritus from an uplifted subduction complex in a continent-ocean orogen, by way of

contrast, will have a high igneous rock fragment content, as well as fine-grained

sedimentary rock fragments such as chert. Feldspars will be more abundant too.

Studies of sandstone petrofacies within a basin can be used to unravel the geologic

history of the provenance terrane. Examples have already been given where uplift in a

source area reveals deeper levels to erosion, so that the composition of the detritus

gradually changes.

One example (Cretaceous sandstone filling fore-arc basin in the Great Valley of

California, derived from uplift of the magmatic arc of the Sierra Nevada) showed that the

sandstones are more quartzo-feldspathic and less lithic upwards, and potash feldspar

increases relative to plagioclase, as volcanics in the arc were eroded and then more

plutonic rocks were exposed.

2.8 Sandstone diagenesis