Lecture 6 Sedimentary Petrology.pdf

Sedimentary Petrology and Sedimentology Sedimentary Rocks What are Sedimentary Rocks? • Sedimentary rocks are those rocks which form at or near the earth's surface primarily through: o Deposition of weathered material by water, wind, or ice (detrital, clastic, terrigenous) o Direct inorganic che...

Sedimentary Petrology and Sedimentology Sedimentary Rocks What are Sedimentary Rocks? • Sedimentary rocks are those rocks which form at or near the earth's surface primarily through: o Deposition of weathered material by water, wind, or ice (detrital, clastic, terrigenous) o Direct inorganic chemical precipitation from water o Precipitation by organic processes ⮚ Sedimentary rock were formed at low temperature and pressure compared to igneous and metamorphic rocks. 3 What are Sedimentary Petrology? • It is a branch of study concerned especially with the composition, characteristics, and origins of sedimentary rock. • Focus on the physical, chemical, and biological characteristics of the principal kinds of sedimentary rocks. • Concerned also with the relationship of these priorities to depositional conditions and provenance. 4 FORMATION OF SEDIMENTARY ROCKS 1. Weathering 2. Erosion 3. Transport 4. Deposition 5. Diagenesis Weathering Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near the Earth’s surface Weathering o Causes the physical and chemical breakdown of source rocks. Leading to concentration of resistant particulate residues (mainly silicate mineral and rock fragments) and formation of secondary mineral such as clay minerals and iron oxides. o Soluble constituents such as Ca, K, Na, Mg and silica are released in solution. o Two types • Physical / Mechanical weathering • Chemical weathering Weathering o Physical weathering ⮚These are processes that break the solid rock into pieces and may separate the different minerals without involving any chemical reactions. The most important agents in this process are as follows. 1. 2. 3. 4. 5. 6. 7. Freeze-thaw action Temperature change Salt dome intrusion Root wedging Wetting and drying Organic activity Unloading Mechanical Weathering The physical breaking up of rock into smaller pieces → leads to an increase in surface area requires the application of some physical force or stress to be applied to the rock no accompanying changes to the composition of rocks Prevails in: cold climates, high altitudes, dry regions Mechanical Weathering 1. Frost wedging – repeated cycles of freezing and thawing; the expansion force of water as it freezes is sufficient to split any mineral or rock. Mechanical Weathering 2. Heating and cooling - Differences in temperature in a rock give rise to differential expansion (heating) and contraction (cooling). 3. Wetting and drying - The disruption of soil results in the swelling and contracting of soil particles. Mechanical Weathering 5. Organisms- Action of organisms, including animals and plants, reduces the size of rocks and minerals. 6. Unloading - the removal of thick layers of sediments overlying deeply buried rocks by erosion or uplift. What produced these features? Wetting and drying What produced these features? Mechanical weathering by biological activity What produced these features? Unloading Weathering o Chemical weathering ⮚These processes involve changes to the minerals that make up a rock. 1. Solution 2. Hydrolysis 3. Oxidation Weathering • Factors controlling weathering o Source Composition o Climate o Topographic Relief Chemical Weathering 1) Dissolution - the dissolving of a solid in a liquid Ca 2+ + SO4 + 2H2O CaSO4.H2O 2) Hydration - combination of a solid mineral or element with water. 2K+ + Al2Si2O5(OH)4 + 4SiO2 2KAlSi3O8 + H2O + 2H+ 3) Oxidation and Reduction - used in mineral weathering, is both the chemical combination of oxygen with a compound and the change in oxidation number of some chemical element (Reduction is the chemical process in which electrons are gained.) 4) Ion-exchange - involves the transfer of charged atoms (ions) of calcium, magnesium, sodium, and potassium between waters rich in one of the ions and a mineral rich in another (Most effective in clays.) Examples Weathering Goldich Stability Series Erosion • The removal of material by mobile agents such as water, wind, ice or man o o o o o o Landslides or mass movements Mass wasting Tectonics Scouring Sand blasting Wave actions Transportation • Transport media o o o o Air Water Ice Gravity • Mode of transport o Rolling: the clasts move by rolling along at the bottom of the air or water flow without losing contact with the bed surface. o Saltation: the particles move in a series of jumps, periodically leaving the bed surface, and carried short distances within the body of the fluid before returning to the bed again. o Suspension: turbulence within the flow produces sufficient upward motion to keep particles in the moving fluid more-orless continually. Particles being carried by rolling and saltation are referred to as bedload, and the material in suspension is called the suspended load. Transportation • Laminar flows - all molecules within the fluid move parallel to each other in the direction of transport: in a heterogeneous fluid almost no mixing occurs during laminar flow. • turbulent flows - molecules in the fluid move in all directions but with a net movement in the transport direction: heterogeneous fluids are thoroughly mixed in turbulent flows. Transportation • Flows can be assigned a parameter called a Reynolds number(Re), named after Osborne Reynolds who documented the distinction between laminar and turbulent motion in the late 19th century. This is a dimensionless quantity that indicates the extent to which a flow is laminar or turbulent. The Reynolds number is obtained by relating the following factors: the velocity of flow (y), the ratio between the density of the fluid and viscosity of the fluid (n– the fluid kinematic viscosity) and a ‘characteristic length’ (l – the diameter of a pipe or depth of flow in an open channel). The equation to define the Reynolds number is: l=n Fluid flow in pipes and channels is found to be laminar when the Reynolds value is low (<500) and turbulent at higher values (>2000). Sediment origin and classification Sediment origin and classification • Volcaniclastic sediments - These are the products of volcanic eruptions or the result of the breakdown of volcanic rocks. • Terrigenous clastic material – This is material that is made up of particles or clasts derived from preexisting rocks. The clasts are principally detritus eroded from bedrock and are commonly made up largely of silicate minerals: the terms detrital sediments and siliciclastic sediments are also used for this material. Clasts range in size from clay particles measured in microns, to boulders metres across. Sandstones and conglomerates make up 20–25% of the sedimentary rocks in the stratigraphic record and mudrocks are 60% of the total. Sediment origin and classification • Carbonates - By definition, a limestone is any sedimentary rock containing over 50% calcium carbonate (CaCO3). In the natural environment a principal source of calcium carbonate is from the hard parts of organisms, mainly invertebrates such as molluscs. Limestones constitute 10–15% of the sedimentary rocks in the stratigraphic record. • Evaporites - These are deposits formed by the precipitation of salts out of water due to evaporation • Other sediments and sedimentary rocks - are sedimentary ironstone, phosphate sediments, organic deposits (coals and oil shales) and cherts (siliceous sedimentary rocks). These are volumetrically less common than the above, making up about 5% of the stratigraphic record, but some are of considerable economic importance Terrigenous clastic sediments and sedimentary rocks The Wentworth Scale Phi (φ) = -log2(Diameter in mm) Best way to remember: 1mm: φ = 0 φ increases as diameter decreases Every factor of 2 change in diameter = one step in φ Siliciclastic Rocks: Texture • Descriptive Textural Classification o Grain Size • Uden-Wentworth grain size scale • Phi = -log2 (grain diameter in mm) • naturally occurring groups o Gravel ~ rock fragments o Sand ~ individual mineral grains (particulate residues) o Mud ~ particulate residues +/chemical weathering products o Clay ~ chemical weathering products (clay minerals, etc.) 31 Classification based on grain size A simple classification of terrigenous clastic rocks and sediment is based on the predominant grain size of the material: Grain Size1 (mm) Sediment name Rock Name Adjectives >2 Gravel Rudite Cobble, pebble, well sorted, etc. 0.0625-2 Sand Arenite Coarse, medium, well sorted, etc. < 0.0625 Mud Mudstone or Lutite Silt or clay Sediment Sizes and Clastic Rock Types Rock Type Sediment Grain Size Shale Clay less than 0.001 mm Siltstone Silt .001-0.1 mm Sandstone Sand .01-1 mm Conglomerate Gravel 1mm + Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks. Terrigenous clastic sediments and sedimentary rocks CONGLOMERATE Source: https://www.sandatlas.org/conglomerate/ Terrigenous clastic sediments and sedimentary rocks • Conglomerate o Consolidated grains of gravel sized particles o If the particles were 64-256mm in diameter, the term used is Cobble Conglomerate o If the clasts were angular, the term used is Breccia • Sedimentary Breccia • Tectonic Breccia o Breccio-conglomerate is used if the clasts are both rounded and angular. Terrigenous clastic sediments and sedimentary rocks • Conglomerate o Till (tillite) - Debris deposited directly by melting ice in a glacier o Tilloid – a non-glacial till-like deposit (olistostrome or grainflows) Terrigenous clastic sediments and sedimentary rocks • Conglomerate o intraformational conglomerate composed of clasts of the same material as the matrix and is formed as a result of reworking of lithified sediment soon after deposition. o Have an interior (intrabasinal) source: that is; they are eroded from the same sedimentary rock unit they are a part, rather than being derived from rocks located outside the depositional basin. Consequently intraformational conglomerates and breccias have framework grains identical in composition to those in the matrix. Source: geolocation.ws/v/W/File:Mud%20Chips%20mcr1.jpg /-/en Terrigenous clastic sediments and sedimentary rocks • Conglomerate o Extraformational Conglomerate - A conglomerate in which clasts are exotic (i.e., derived from outside the depositional basin). Clasts are normally very well rounded and well sorted. Clasts derived from a distant source o Detritus weathered from external sources is carried away and deposited elsewhere. As a result framework clasts differ markedly in composition from matrix.Framework matrix is exotic; that is, not derived by the erosion and redeposition of matrix material. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Orthoconglomerate (clast-supported conglomerate or “true conglomerate) - A conglomerate in which all clasts are in contact with other clasts (i.e., the clasts support each other). Such conglomerates may have no matrix between clasts (open framework) or spaces between clasts may be filled by a matrix of finer sediment (closed framework). o Matrix (sand or finer) is less than 15%. o Types: • Open framework • Close framework Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Clast-supported framework is typical of gravels deposited from water flows in which gravel-size sediment predominates. Open framework suggests an efficient sorting mechanism that caused selective removal of finer grained sediment. Closed framework suggests that the transporting agent was less able to selectively remove the finer fractions or was varying in competence, depositing the framework-filling sediment well after the gravel-size sediment had been deposited. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture • • They are matrix-poor (80% or more framework grains) and have an intact, stable, grain-supported fabric. They are transported and deposited on a grain-by grain basis by fluids, specifically water or air. Oligomict or petromict conglomerate are further divided into these on the basis of framework grain composition. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture • In oligomict (orthoquartzose) conglomerates (or breccias), more than 90% of the framework clasts consist of fragments of only a few varieties of resistant rocks and minerals as metaquartzite, vein quartz, and cherts. • In petromict (polymict) clasts of many different composition of metastable and unstable rocks are abundant; for example, basalt, slate, and limestone. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture • • Oligomict orthoconglomerates imply wholesale decomposition and disintegration of immense volumes of rocks, reflecting climate and topography that promote chemical decomposition and physical disintegration of all but the most resistant components. Typically stream channels deposits and bars deposits, or near shore marine settings. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture • Petromic are much more abundant than oligomict orthoconglomerates and are mainly alluvium eroded from high-relief areas. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Paraconglomerate (matrix-supported conglomerate) A conglomerate in which most clasts are not in contact; i.e., the matrix supports the clasts. o Matrix of sand or finer is at least 15% to 50%. More than 50% means it is already sandstone or mudstone with gravel particle scattered. o Typical of the deposits of debris flows or water flows in which gravel size clasts were not abundant in comparison to the finer grain sizes. Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Paraconglomerates and parabreccias are further divided on the basis of their inferred origin as well as the size and internal organization of their matrix. o Is the matrix sand or mud? o Is the matrix internally laminated or chaotic? o Is the framework graded? imbricated, sorted, and vertically o Is the deposit sheetlike or lenticular? o With what other types of sediment is the deposit associated? Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Paraconglomerates containing a matrix of delicately laminated mudrocks in which coarser framework grains float are called laminated pebbly (or cobbly, or bouldery) mudrock. o Dropstone- ice rafting Ice-rafted dropstone, Ghaub Fm., Namibia Terrigenous clastic sediments and sedimentary rocks • Conglomerate: Defined on the basis of texture o Paraconglomerates in which the matrix is disorganized and non-laminated are either tillite (only if glacial origin can be inferred) or tilloid (deposited by mass movement). Terrigenous clastic sediments and sedimentary rocks • Conglomerate o Diamictite - A rudite composed of poorly sorted, mud to gravel-sized sediment, commonly with angular clasts. Commonly refers to sediment deposited from glaciers or sediment gravity flows, particularly debris flows. Terrigenous clastic sediments and sedimentary rocks • Conglomerate o Resistant lithologies - those which are less susceptible to physical and chemical breakdown, have a higher chance of being preserved as a clast in a conglomerate. o Factors controlling the resistance of a rock type include the minerals present and the ease with which they are chemically or physically broken down in the environment. Terrigenous clastic sediments and sedimentary rocks • Conglomerate Terrigenous clastic sediments and sedimentary rocks SANDSTONE Source: https://en.wikipedia.org/wiki/Sandstone Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o The abundance of a particular mineral in a sedimentary rock is dependent on its availability, mechanical and chemical stability. o Climate and Relief in the source area is important. o With low relief and humid climate, quartz will likely be the only grains which survive. In regions of very intense weathering, quartz will go too and laterites and bauxites, residual deposits then form. o However, in an area of high relief, some unstable grains will always be liberated for erosion and later deposition, even if weathering is extreme. o The mechanical stability of grains depend on hardness and cleavage. Unstable grains are called labile grains. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Sand grains are formed by the breakdown of pre existing rocks by weathering and erosion, and from material that forms within the depositional environment. o The breakdown products fall into two categories: • detrital mineral grains, eroded from pre-existing rocks, • and sand-sized pieces of rock, or lithic fragments. • Grains that form within the depositional environment are principally biogenic in origin, that is, they are pieces of plant or animal, but there are some which are formed by chemical reactions. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Sand - particles from 0.0625 mm to 2mm o Sandstone - defined as a sedimentary rock with grains of these sizes. This size range is divided into five intervals: very fine, fine, medium, coarse and very coarse. o Major Components • Detrital Minerals • Lithic Fragments • Biogenic particles • Authigenic materials – (Minerals that grow as crystals) • Matrix – Fine-grained materials Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Rock Fragments ⮚(a) fine-grained sedimentary and metasedimentary rocks (b) siliceous sedimentary rocks and (c) igneous, in particular volcanic rocks. Plutonic rocks tend to break into individual grains. ⮚Rock fragments are very useful in studies of the provenance of a sandstone, but intrabasinal lithics, which are commonly of mud and carbonate, are usually excluded. ⮚The types of lithic grains can be related to the plate tectonic setting of the provenance terrane and adjoining sedimentary basin. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Quartz ⮚the most common mineral in sandstone; most stable of all minerals. ⮚The average sandstone contains about 65% quartz, but some are practically 100% quartz derived from granitoid rocks, acid gneisses and schists. Source: https://vitex.us/ourservice/countertopinstallation/quartz/ Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Feldspar ⮚K feldspars (orthoclase and microcline) are much more common in sandst. than plagiocl. because Kfeldsp. are chemically more stable and are more common in continental basement rocks. Plagioclases is more common in sandstones derived from uplifted oceanic and island-arc terranes. ⮚Aside from suitable source rock, feldsp content is largely controlled by rate of erosion and climate. ❑Humid climate - promotes destruction; ❑Arid - fresh feldsp survive. ❑Rapid erosion (e.g. high relief) will produce some feldsp. grains in spite of a humid climate. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Feldspar ⮚Feldspars average 10-15%, but in arkoses, commonly reach 50%. ⮚Feldspar less stable than Quartz. ⮚Chemical stability is also low (hydrolysis leads to replacement by sericite, kaolinite and illite). Can be replaced also by calcite. Incipient alteration - dusty appearance. Complete replacement - clay-mineral pseudomorphs after feldspar. Alteration takes place at site of weathering and during diagenesis. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Mica and Clay Minerals ⮚Phyllosilicates are common in the matrix of sandstones and coarse clastics. ⮚Muscovite and biotite are derived from many igneous rocks but are especially from metamorphic rocks. Muscovite is more common. ⮚Clay - both detrital and authigenic. Kaolinite, illite, chlorite, smectites and mixed-layer clays. ⮚Detrital clays reflect source geology, climate and weathering processes. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Heavy Minerals ⮚ HV min are accessory (<1%). Sp.gr. > 2.9. They are chiefly silicates and oxides, many of which are very resistant to chemical weathering and abrasion. ⮚ The common non-opaque HV min: apatite, epidote, garnet, rutile, staurolite, tourmaline, and zircon. ⮚ Common opaque HV min: magnetite and ilmenite. Source: https://www.researchgate.net/figure/_fig2_27 4007613A-representative-image-of-theidentified-heavy-mineral-through-agemological-microscope ⮚ Study of HV min give indications of provenance and of events in the source area. ⮚ Metamorphic terranes epidote, and staurolite. - garnet, ⮚ Igneous rocks - rutile, apatite, and tourmaline Source: https://www.sciencedirect.com/science/article /pii/S0037073812000978 Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Heavy Minerals ⮚Major changes in the source-area geology, such as the uplift and unroofing of a granite, may be recorded in the HV min assemblage. HV min suites can be used to identify petrographic provinces within a formation, where, for e.g., sediment was supplied by two or more rivers draining areas of different geology. ⮚HV min assemblage in source rocks is affected by weathering in the same way as other minerals. By and large, most HV min are sufficiently strong mechanically to resist loss by abrasion during transport. Terrigenous clastic sediments and sedimentary rocks • Sand and Sandstone o Major Components: Detrital Minerals • Others ⮚Fossils ⮚Non-skeletal grains (ooids, peloids, intraclasts) ⮚Skeletal phosphate ⮚Glauconite - iron potassium phyllosilicate (mica group) ⮚Chamosite - the Fe2+ end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore ⮚Organic matter Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Dott’s Scheme • Point counting is a method whereby a thin section on a petrographic microscope is examined by stepping across the thin section at equal intervals and identifying the material (quartz, feldspars, rock fragments or matrix) that lies immediately beneath the cross hairs. Counting 250 to 300 grains will accurately yield the proportion of each component Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Quartz Arenite • Sandstones with at least 95% quartz grains are the most compositionally mature, usually consist of well-rounded and well-sorted grains so that textural maturity is also very high. • In many cases, quartz arenites are the products of extended periods of sediment reworking so that all grains other than quartz have been broken down by mechanical abrasion. Climate also a factor - warm, humid climate will lead to removal of many unstable grains. If this is couples with low relief and slow sedimentation, quartz will dominate. Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Quartz Arenite • Many quartz grains in these arenites are 2nd cycle (derived from older sediments). • Quartz arenites produced by persistent wave or current reworking were deposited on stable cratons and passive margins. • They can also be formed in situ by extreme chemical weathering; through leaching of sandy sediment by organic acids descending from peat. Source: https://www.pinterest.ph/pin/585186545301376783/ Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Arkose • Arkoses - >25% feldspar, much quartz and some rock fragments. • Detrital micas are also present and some fine-grained matrix. Feldspar is chiefly K-feldspar and much is this is microcline. Feldsp usually fresh although some may be altered to kaolinite and sericite. • Arkoses are typical red or pink, through the feldspar’s color, but also through the presence of finely disseminated hematite. • Arkoses are derived from granites and gneisses and vary from in situ weathering products to sandstones which have undergone long transport. • Texture is typically poorly- to well-sorted, with very angular to subrounded grains (depending on degree of transportation). Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Arkose • Arkoses are clearly derived from feldsp- rich rocks, particularly Kfeldsp bearing granites and gneisses. However, climate and relief are also important factors. • Humid conditions - feldspars weather to clay, so that semi-arid and glacial climates favor arkose formation. If erosion is very rapid particularly where source has high relief, then arkosic detritus can be produced in spite of intense chemical weathering. • Many arkoses were deposited in fluvial environments. Source: http://www.alexstrekeisen.it/english/sedi/arkosicsandstone s.php Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Litharenites • L > F. These rocks range widely in composition (both grain types and chemistry), depending largely on the rock fragments present. Lithics chiefly of mudrock and their low grade metamorphic equivalents and volcanic grains; other components are flakes of mica, some feldspar and mica quartz. • Immature composition implies high rates of sediment production followed by short transport distances. Many fluvial and deltaic sandstones are litharenites. • The nature of rx fragments in litharenites may change through time reflecting uplift in the source area and the availability of different rock types. Source: http://archives.datapages.com/data/specpubs/memoir109/data/39_aa pg-sp1980039.htm Terrigenous clastic sediments and sedimentary rocks • Sandstone Classification o Graywackes • fine-grained matrix, which consists of an intergrowth of chlorite, sericite and silt-sized grains of quartz and feldspar. • Feldspar grains are chiefly Na plag. • Greywackes are dark gray or black. • Origin of the matrix = “greywacke problem.” 2 possibilities: (a) fine-grained sediment deposited along with the sand fraction and (b) diagenetic alteration of unstable rock fragments. Terrigenous clastic sediments and sedimentary rocks MUDROCK Source: https://en.wikipedia.org/wiki/Mudrock Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Silt is defined as the grain size of material between 0.0625mm and 0.0039mm in diameter. o This size range is subdivided into coarse, medium, fine and very fine. o Clay is a textural term to define the finest grade of clastic sedimentary particles, those less than 0.0039mm in diameter. Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Mud – refer to when clay- and silt-sized particles are mixed in unknown proportions as the main constituents in unconsolidated sediment. o The general term mudrock can be applied to any indurated sediment made up of silt and/or clay. o If it can be determined that most of the particles (over twothirds) are clay-sized the rock may then be called a claystone and if silt is the dominant size a siltstone; mixtures of more than one-third of each component are referred to as mudstone. o Shale is sometimes applied to any mudrock (e.g. by drilling engineers) but it is best to use this term only for mudrocks that show a fissility, which is a strong tendency to break in one direction, parallel to the bedding. Terrigenous clastic sediments and sedimentary rocks • Silt and Clay Terrigenous clastic sediments and sedimentary rocks • Silt and Clay Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Two groups for clay minerals • two layers, the kandite group o Kaolinite is the commonest member of the kandite group and is generally formed in soil profiles in warm, humid environments where acidic waters intensely leach bedrock lithologies such as granite. • three layers, the smectite group. o Clay minerals of the smectite group include the expandable or swelling clays such as montmorillonite, which can absorb water within their structure. Weathering product of volcanic rock and volcanic glass. o Chlorite, Illite Terrigenous clastic sediments and sedimentary rocks • Silt and Clay Terrigenous clastic sediments and sedimentary rocks • Silt and Clay Terrigenous clastic sediments and sedimentary rocks • Silt and Clay Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Clay Mineral Properties • Fine particle size (<2-4µm) o High surface area to volume • Electrostatic charge o Interlayer (between “T” and “O” layers) o Exterior surfaces due to broken bonds o Charge deficiency (ex: illite) • Result in: o Flocculation: attract other ions and polar molecules o Electrostatic forces may dominate (relative to gravity) the physical properties of clay mineral-rich systems o Cation exchange capacity (CEC) Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Clay Mineral Properties • The transport mechanisms of heavy metals through soil has long presented great interest to both environmental and soil scientists because of the possibility of groundwater contamination through metal leaching (Kanugo, 2000) • In general many soils contain a wide range of heavy metals with varying concentration ranges depending on the surrounding geological environment and anthropogenic and natural activities occurring or once occurred. Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Clay Mineral Properties • These metals can be Fe, Cr, Mn, Ni, Zn, Cu, Pb, Cd, Hg, etc. Metal transport is not only dependent on the physiochemical properties of the metals but mostly on the physical and chemical properties of the soil, like for example (Weber, 1991; Stevenson, 1992): a) soil organic matter (SOM) content, b) clay fraction content, c) mineralogical composition, d) pH, and e) more, all of which collectively determine the binding ability of soil. Terrigenous clastic sediments and sedimentary rocks • Silt and Clay o Clay Mineral Properties • Clay particles are usually negatively charged. This is a very important factor influencing sorption properties of the soil. • Soil organic matter (SOM) is the second main component of the soil solid fraction (Stevenson, 1992). Soil Organic Matter may range in soils from 0.1% in desert soils to 90% in organic soils. Humic substances make up approximately 85-90% of the total organic carbon in soils (Grim, 1968; Giesking, 1975). • The existence of humic material in soils strongly influences sorption of chemicals. Humic acids can exist in a dissociated form and thus are negatively charged (hydrogen can be replaced by metal ions). This source of negative charges in soil colloids is strongly pH-dependent so the sorption of heavy metals in organic soils or in soils with relatively high organic content is mostly pH dependent. The Cation Exchange Capacity (C.E.C.) is also very high for soil organic matter. Terrigenous clastic sediments and sedimentary rocks TEXTURAL PARAMETERS Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments o Textural Parameters 1. Grain size A. Individual B. Bulk (grain size distribution) 2. Grain shape 3. Grain Orientation 4. Porosity 5. Permeability Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain size individual A. By volume o Displacement method B. Settling velocity o Stoke’s law C. Direct measurement o Sieving D. By Grade Scale o Using chart Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Shape o Sphericity • is an inherited feature, that is, it depends on the shapes of the fragments which formed during weathering. A slab-shaped clast will become more rounded during transport and become disc-shaped, but will generally retain its form with one axis much shorter than the other two. o Roundness • During sediment transport the individual clasts will repeatedly come into contact with each other and stationary objects: sharp edges tend to be chipped off first, the abrasion smoothing the surface of the clast. A progressive rounding of the edges occurs with prolonged agitation of the sediment and hence the roundness is a function of the transport history of the material. Roundness is normally visually estimated Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Shape Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Shape o Form • Provides a consistent terminology for describing the overall form of particles Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Shape o Significance of form and roundness • Potential use as guides to provenance and transport histories of siliciclastic sediment. • Shape have influence on the settling velocity of particles in a fluid; departure of a grain from a spherical shape causes a decease in settling velocity. • Shape affects the transportability of particle moving by traction. Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Shape o Factors affecting shapes of sediments • Lithology • Hardness Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Orientation ▪ Sorting - a description of the distribution of clast sizes present: a well-sorted sediment is composed of clasts that mainly fall in one class on the Wentworth scale (e.g. medium sand); a poorly sorted deposit contains a wide range of clast sizes. Sorting is a function of the origin and transport history of the detritus. With increased transport distance or repeated agitation of a sediment, the different sizes tend to become separated. A visual estimate of the sorting may be made by comparison with a chart or calculated from grain-size distribution data. Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Grain Orientation ▪ Sorting Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Porosity ▪ the volume of void space (available to contain fluid or air) in a sediment or sedimentary rock. ▪ Factors affecting porosity 1. Packing density - the arrangement of the particles in the deposit. The more densely packed the particles the lower the porosity. Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Porosity ▪ Factors affecting porosity 2. Grain Size and shape When grains settle through a fluid the large grains will impact the substrate with larger momentum, possibly jostling the grains into tighter packing (therefore with lower porosity). Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Porosity ▪ Factors affecting porosity 3. Sorting Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Porosity ▪ Factors affecting porosity 3. Post Burial a. Compaction b. Cementation c. Clay formation d. Solution e. Pressure Solution Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Permeability ▪ related to how easily a fluid will pass through any granular material. ▪ Darcy’s law ▪ Tortuosity Terrigenous clastic sediments and sedimentary rocks • Texture of terrigenous clastic sediments ⮚Permeability ▪ Factors controlling permeability ▪ Packing density ▪ Porosity ▪ Grain Size ▪ Grain Shape ▪ Post – Burial process Terrigenous clastic sediments and sedimentary rocks • Maturity o This refers to the extent to which the material has changed when compared with the starting material of the bedrock it was derived from. o Maturity can be measured in terms of texture and composition. Terrigenous clastic sediments and sedimentary rocks • Maturity o Textural maturity • The texture of sediment or sedimentary rock can be used to indicate something about the erosion, transport and depositional history. The determination of the textural maturity of a sediment or sedimentary rock can best be represented by a flow diagram Terrigenous clastic sediments and sedimentary rocks • Maturity o Mineralogical maturity • Compositional maturity is a measure of the proportion of resistant or stable minerals present in the sediment. The proportion of highly resistant clasts such as quartz and siliceous lithic fragments in a sandstone, compared with the amount of less resistant, labile, clast types present, such as feldspars, most other mineral types and lithic clasts, is considered when assessing compositional maturity. A sandstone is compositionally mature if the proportion of quartz grains is very high and it is a quartz arenite according to the Pettijohn classification scheme: if the ratio of quartz, feldspar and lithic fragments meant that the composition falls in the lower part of the triangle it is a mineralogically immature sediment. Terrigenous clastic sediments and sedimentary rocks SANDSTONE DIAGENESIS Terrigenous clastic sediments and sedimentary rocks Terrigenous clastic sediments and sedimentary rocks Quartz Overgrowth –is the most common type of silica cement. Silica cement is precipitated around the quartz grain and in optical continuity,. The syntaxial overgrowth commonly gives the grain euhedral crystal faces. In many cases the shape of the original grain is delineated by a thin iron-clay coating between the overgrowth and the grain. Terrigenous clastic sediments and sedimentary rocks Terrigenous clastic sediments and sedimentary rocks Terrigenous clastic sediments and sedimentary rocks Table 5.7. Common diagenetic processes which affect detrital grain types, after McBride (1985). For each mineral, processes are list with the most common first. In: Tucker (1991) Terrigenous clastic sediments and sedimentary rocks Hematite Cementation & Pigmentation • Hematite typically occurs as a very thin coating around grains, but also impregnates authigenic minerals (clays, quartz, feldspars) • Absence of hematite at grain contacts indicate diagenetic origin • Origins of Hematite Pigment: o Detrital – lateritic weathering o Diagenetic – Fe from intrastratal dissolution of detrital silicates (hbl, aug, oliv, chl, biot, magnet) Terrigenous clastic sediments and sedimentary rocks Telegenetic (Uplift) Environment Semi-Arid o Oxidation of sulfides and Fe carbonates to Fe oxides (goethitelimonites) which may age to hematite Humid - Leaching (due to low Eh and acid pH) of feldspars, carbonates and heavy minerals can raise porosity significantly - Extent of leaching depends largely on porosity and permeability, which may well have been largely occluded during burial diagenesis Sedimentary Petrology and Sedimentology Sediment origin and classification VOLCANICLASTIC SEDIMENTARY ROCK • Volcanic eruptions are the most obvious and spectacular examples of the formation of both igneous and sedimentary rocks on the Earth’s surface. During eruption volcanoes produce a range of materials that include molten lava flowing from fissures in the volcano and particulate material that is ejected from the vent to form volcaniclastic deposits Biogenic, Chemical Sediment • Calcium carbonate (CaCO3) is the principal compound in limestones, which are, by definition, rocks composed mainly of calcium carbonate. • Limestones, and sediments that eventually solidify to form them, are referred to as calcareous Sedimentary rocks may also be made of carbonates of elements such as magnesium or iron, and there are also carbonates of dozens of elements occurring in nature. • This group of sediments and rocks are collectively known as carbonates to sedimentary geologists, and most carbonate rocks are sedimentary in origin. • Exceptions to this are marble, which is a carbonate rock recrystallised under metamorphic conditions, and carbonatite, an uncommon carbonate-rich lava. Biogenic, Chemical Sediment • Calcite (CaCO3) o The most familiar and commonest carbonate mineral o Reacts with dilute (10%) hydrochloric acid (HCl) o Biogenic in origin, it has formed as a part of a plant or animal. • Aragonite o No chemical difference between calcite o Calcite has a trigonal crystal form, aragonite has an orthorhombic crystal form. o Slightly denser than calcite (a specific gravity of 2.95, as opposed to a range of 2.72–2.94 for calcite), and is slightly harder (3.5–4 on Mohs’ scale) Biogenic, Chemical Sediment Biogenic, Chemical Sediment • Dolomite o Calcium magnesium carbonate (CaMg(CO3)2) o Dolostone is used for the lithology to distinguish it from dolomite. o There is usually little or no reaction between cold HCl and dolomite. • Siderite o iron carbonate (FeCO3) o the same structure as calcite o very difficult to distinguish between iron and calcium carbonates on mineralogical grounds. It is rarely pure, often containing some magnesium or manganese substituted for iron in the lattice. Biogenic, Chemical Sediment • Carbonate-forming animals o Molluscs - large group of organisms that have a fossil record back to the Cambrian and commonly have calcareous hard parts. o Bivalve molluscs - have a distinctive layered shell structure consisting of two or three layers of calcite, or aragonite, or both. o Gastropods - have a calcite or aragonite layered structure, and are distinctive for their coiled form. o Brachiopods - shelly organisms with two shells and are hence superficially similar to bivalves. o Echinoids – Sea urchins o Crinoids – Sea lilies Biogenic, Chemical Sediment • Carbonate-forming animals Biogenic, Chemical Sediment • Carbonate-forming animals o Foraminifera are small, single-celled marine organisms that range from a few tens of microns in diameter to tens of millimetres across. They are either floating in life (planktonic) or live on the sea floor (benthic) and most modern and ancient forms have hard outer parts (tests) made up of high- or low magnesium calcite. o Corals (Cnidaria) - largest calcium carbonate biogenic structures Biogenic, Chemical Sediment • Carbonate-forming plants o Algae and microbial organisms - are an important source of biogenic carbonate and are important contributors of fine-grained sediment in carbonate environments through much of the geological record o Red algae(rhodophyta) - are otherwise known as the coralline algae: some forms are found encrusting surfaces such as shell fragments and pebbles. They have a layered structure and are effective at binding soft substrate. o Green algae (chlorophyta) - have calcified stems and branches, often segmented, that contribute fine rods and grains of calcium carbonate to the sediment when the organism dies. Biogenic, Chemical Sediment • Carbonate-forming plants o Nanoplankton - are planktonic yellow green algae that are extremely important contributors to marine sediments in parts of the stratigraphic record. o Cyanobacteria Biogenic, Chemical Sediment • Non-biogenic constituents of limestone o Ooids - are spherical bodies of calcium carbonate less than 2 mm in diameter. • A rock made up of carbonate ooids is commonly referred to as an oolitic limestone. o Pisoids - Concentrically layered carbonate particles over 2 mm across. o Peloids - are commonly the faecal pellets of marine organisms such as gastropods and may be very abundant in some carbonate deposits, mostly as particles less than a millimetre across. o Intraclasts - are fragments of calcium carbonate material that has been partly lithified and then broken up and reworked to form a clast which is incorporated into the sediment. Biogenic, Chemical Sediment • Non-biogenic constituents of limestone Biogenic, Chemical Sediment • Dunham Classification John O'Brien and Kevin Hefferan (2010) Biogenic, Chemical Sediment • Folk Classification Evaporites • These are minerals formed by precipitation out of solution as ions become more concentrated when water evaporates. On average, seawater contains 35 g/L (35 parts per thousand) of dissolved ions, mainly chloride, sodium, sulphate, magnesium, calcium and potassium • The least soluble compounds are precipitated first, so calcium carbonate is first precipitated out of seawater, followed by calcium sulphate and sodium chloride as the waters become more concentrated. Potassium and magnesium chlorides will only precipitate once seawater has become very concentrated. Evaporites • Gypsum and anhydrite o The most commonly encountered evaporite minerals in sedimentary rocks are forms of calcium sulphate. o Calcium sulphate is precipitated from seawater once evaporation has concentrated the water to 19% of its original volume. o Gypsum (CaSO4-2H2O) o Anhydrite (CaSO4) o Selenite and Alabaster • Halite (NaCl) o Precipitates out of seawater once it has been concentrated to 9.5% of its original volume. o It may occur as thick crystalline beds or as individual crystals that have a distinctive cubic symmetry, sometimes with a stepped crystal face Evaporites • Other evaporite minerals o Potassium chloride, sylvite (KCl), is an important source of industrial potash that occurs associated with halite and is interpreted as the product of extreme evaporation of marine waters. o trona (Na2CO3.NaHCO3.2H2O) o mirabilite (Na2SO4.10H2O) o Epsomite (MgSO4.7H2O). Facies Other Non-clastics • Chert o are fine-grained siliceous sedimentary rocks made up of silt-sized interlocking quartz crystals (microquartz) and chalcedony, a form of silica which is made up of radiating fibers a few tens to hundreds of microns long. o On the floors of seas and lakes the siliceous skeletons of microscopic organisms may accumulate to form a siliceous ooze. o These organisms are diatoms in lakes and these may also accumulate in marine conditions, although Radiolaria are more commonly the main components of marine siliceous oozes. Radiolarians are zooplankton (microscopic animals with a planktonic lifestyle) and diatoms are phytoplankton (free-floating algae). o Flint – black o Jasper – red (caused by hematite) Other Non-clastics • SEDIMENTARY PHOSPHATES o Most occurrences occur where there is high organic productivity and low oxygen, but not fully anoxic conditions. o Rocks with concentrations of phosphate (5% to 35% P2O5) are called phosphorites . o Mineralogically, phosphorites are composed of francolite, which is a calcium phosphate (carbonate hydroxyl fluorapatite). o In some cases the phosphate is in the form of coprolites, which are the fossilised faeces of fish or animals. Other Non-clastics • SEDIMENTARY IRONSTONE o Sedimentary rocks that contain at least 15% iron are referred to as ironstones or iron formations in which the iron is in the form of oxides, hydroxides, carbonate, sulphides or silicates o most of the iron ore mined today is from Precambrian rocks. Other Non-clastics • SEDIMENTARY IRONSTONE o Iron minerals in sediments o Magnetite(Fe3O4) is a black mineral which occurs as an accessory mineral in igneous rocks and as detrital grains in sediments. o Haematite(Fe2O3) is the most common oxide, bright red to black in colour, occurring as a weathering or alteration product in a wide variety of sediments and sedimentary rocks. o Goethiteis an iron hydroxide (FeO.OH) that is widespread in sediments as yellow-brown mineral, which may be a primary deposit in sediments, or is a weathering product of other iron-rich minerals, representing less oxidising conditions than haematite. Other Non-clastics • SEDIMENTARY IRONSTONE o Iron minerals in sediments o Limonite(FeO.OH.nH2O) is a hydrated iron oxide that is amorphous. o Pyrite(FeS2) is a common iron sulphide mineral that is found in igneous and metamorphic rocks as brassy cubic crystals o There are several silicate minerals that are iron-rich: • greenalite and chamosite are phyllosilicate minerals • Glauconite (glaucony) is also a phyllosilicate formed authigenically in shallow marine environments Other Non-clastics • SEDIMENTARY IRONSTONE o Banded Iron Formations o Banded Iron Formations (BIFs)are an example of a type of sedimentary rock for which there is no equivalent forming today. All examples are from the Precambrian, and most are from the period 2.5 to 1.9 Ga, although there are some older examples as well Other Non-clastics • Ferromanganese deposits o they are black to dark brown in colour and range from a few millimetres to many centimetres across as nodules or as extensive laminated crusts on hard substrates. Although these manganese nodules form at any depth, they form very slowly and are only found concentrated in deep oceans where the rate of deposition of any other sediment is even slower Other Non-clastics o A deposit is considered to be carbonaceous if it contains a proportion of organic material that is significantly higher than average (>2% for mudrock, >0.2% for limestone,>0.05% for sandstone). o Peats are forming at the present day in a wide range of climatic zones from subarctic boggy regions to mangrove swamps in the tropics and contain a range of plant types, from mosses in cool upland areas to trees in lowland fens and swamps. o Sapropel is the remains of planktonic algae, spores and very fine detritus from larger plants that accumulates underwater in anaerobic conditions: these deposits may form a sapropelic coal o Humic coals, formed from the in situ accumulation of woody plant material; Other Non-clastics o Coal • If over two-thirds of a rock is solid organic matter it may be called a coal. • Most economic coals have less than 10% non-organic, non-combustible material that is often referred to as ash o A nomenclature for the description of different lithotypes of coal has therefore been developed as follows: • Vitrain: bright, shiny black coal that usually breaks cubically and mostly consists of woody tissue. • Durain: black or grey in colour, dull and rough coal that usually contains a lot of spore and detrital plant material. • Fusain: black, fibrous with a silky lustre, friable and soft coal that represents fossil charcoal. • Clarain: banded, layered coal that consists of alternations of the other three types. Other Non-clastics o Origin of Coal All coal had their original in a WETLAND – an area partially or fully submerged most or all of the year. Types: Marsh (grass), Swamp (trees), Bog (mosses). The water in the best coal swamps was stagnant and acidic. As the plant material dies and fell to the bottom of the swamp, it was partially decayed by bacteria forming thick peat beds. The peat was then buried by thousands of feet of sediments and the resulting heat converted it to coal. Other Non-clastics o Origin of Coal Coalification starts at 100ºC; water, oxygen, hydrogen are driven off. Organic matter becomes relatively enriched in fixed carbon. Other Non-clastics • CARBONACEOUS (ORGANIC) DEPOSITS o Oil shales and tar sands • Oil shales - Mudrocks that contain a high proportion of organic material that can be driven off as a liquid or gas by heating. • The organic material is usually the remains of algae that have broken down during diagenesis to form kerogen, long-chain hydrocarbons that form petroleum • Tar sands or oil sands are clastic sediments that are saturated with hydrocarbons and they are the exposed equivalents of subsurface oil reservoirs DIAGENESIS DIAGENESIS • Diagenesis - The physical and chemical changes that alter the characteristics of sediment after deposition o These processes occur at relatively low temperatures, typically below 250° and at depths of about 5000m DIAGENESIS o Lithification is the process of transforming sediment into sedimentary rock, and involves both chemical and physical changes that take place at any time after initial deposition. o Compaction happens when sediments are deeply buried, placing them under pressure because of the weight of overlying layers. This squashes the grains together more tightly. o Differential compaction occurs as one part of a sediment pile compacts more than the part adjacent to it. DIAGENESIS ❑ COMPACTION EFFECTS o Point contacts - subjected to very little overburden pressure the clasts will be in contact mainly at the point where they touch o Long contacts - grains are rotated and pushed closer together, and pore space is reduced o Concavo-convex contacts softer grains are compacted around harder ones, and grains start to dissolve in the pore waters at their contacts o Sutured contacts - irregular grain boundaries o Pressure solution -is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high pressure DIAGENESIS ❑ CEMENTATION - The nucleation and growth of crystals within pore spaces in sediments. o TYPES OF CEMENTS • Clay • Carbonates • Silicates • Iron Oxides ✔Eogenetic cements – forms very soon after deposition ✔Mesogenetic cements – chemical changes occur in sediment that is buried and saturated with pore waters ✔Telogenetic cementation - cement formation occurs during uplift, DIAGENESIS ❑ CEMENTATION - The nucleation and growth of crystals within pore spaces in sediments. o Factors affecting cement formation • the availability of different minerals in pore waters • the temperature • The acidity of the pore waters DIAGENESIS ❑ CEMENTATION DIAGENESIS ❑ DOLOMITIZATION - The process by which limestone is altered into dolomite (CaMg(CO3)2) ⮚Models have certain things in common: ✔the original rock must be limestone ✔the water that reacts with it must be marine, or pore water derived from seawater ✔elevated temperatures ✔Either enhanced or reduced salinities DIAGENESIS ❑ DOLOMITIZATION ⮚the reflux model - triggered by an evaporation phase in lagoonal and/or shallow marine settings DIAGENESIS ❑ DOLOMITIZATION ⮚The mixing-zone model – proposes that where fresh water mixes with marine waters then dolomitisation would occur DIAGENESIS ❑ DOLOMITIZATION ⮚burial models –involves prime mechanism which is the dewatering of basinal mud rocks due to compaction and removal of Mg-rich fluids into neighboring shelf edge. DIAGENESIS ❑ DOLOMITIZATION ⮚Seawater models – Thermally driven circulation, either by a geothermal heat source or by temperature differences between the interior of a platform and seawater References • Selected References o o o o o o o o o o Folk (1976) - Petrology of Sedimentary Rocks Pettijohn (1975) - Sedimentary Rocks Friedman and Sanders (1978) - Principles of Sedimentology Blatt et al. (1980) - Origin of Sedimentary Rocks Blatt (1982) - Sedimentary Petrology Leeder (1982) - Sedimentology, Process and Product Selley (1985) - Ancient Sedimentary Environments Selley (1988) - Applied Sedimentology Greensmith (1988) -Petrology of the Sedimentary Rocks Foronda, 2018 [Lecture: Power Point], Geology 253 Sedimentary Petrology and Sedimentology. University of the Philippines – National Institute of Geological Sciences, Diliman, Quezon City