Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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. 1 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. 2 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. 3 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. 4 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. 5 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). 6 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). 7 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. 8 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. 9 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. Crosshatch 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). 10 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). 11 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 sedimentarymetasedimentary 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). 12 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. 13 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. 14 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 oilcompany 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. 15 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). 16 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. 17 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. 18 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. 19 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. 20 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. 21 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 micaschist 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. 22 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 23 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 computergenerated. 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. 24 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. 25 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 fledsparbearing 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 26 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. 27 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). 28 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). 29 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. 30 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 pullapart 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. 31 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 32