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Sedimentation & Stratigraphy g p y ( G 202 ) Course Coordinators: Dr. Ahmed Sadek Dr. Ahmed El-Sabbagh Geology department, Faculty of Science, Alexandria Univ. Part I ( Sedimentation ) Introduction to Sediments & Sedimentary Rocks Dr Ahmed Sadek Dr. Course objectives This course aims at: • Providing g an introduction to the basics of sedimentation processes and sedimentary deposits. • Generate in students an appreciation of the importance of the sediments and sedimentary rocks. • Assessing the textural and mineralogical properties of sediment and sedimentary y rocks. • giving a survey of the description and classification of sediment and sedimentary Rocks. • Providing a brief review of the full range of sedimentary environments. Course Contents Part I: Sedimentation 1. Introduction (definition of sediments and sedimentary rocks k and d th their i iimportance) t ) 2 Nature, 2. Nature sources and formation of sediments & sedimentary rocks, sedimentary rock cycle 3. Texture of sedimentary rocks (mechanical analysis grain fabric, roundness, sphericity and surface textures of grains, porosity & permeability) and its importance 4. Chemical and mineralogical composition and its significance 5. Simple genetic classification of sedimentary rocks Schedule of Course Assessments Methods of Assessment Weighting of Assessments Time Semester Work Mid-Term writing exam Final practical exam Final-Term Final Term writing exam Oral Exam 5% 10 % 15 % 60 % 10 % Weeks 4 & 11 Week 8 Week 16 Week 17 Week 17 Total 100 % List of References - Blatt, H. Middleton, G.V. and Murray, R.C. (1980), Origin of sedimentary Rocks, 634 p., Prentice-Hall, New Jersey. - Tucker, M.E. (1982), sedimentary Petrology An Introduction, 251 p., Blackwell S i ifi Publications Scientific P bli i - Levis, D.W. and Mc Conchie, D. (1994), Practical Sedimentology. Chapman & Hall, N.Y. - Sam S B Boggs (2006) (2006): Principles P i i l off Sedimentology S di t l andd Stratigraphy, St ti h 4th Ed. Ed Prentice P ti Hall, H ll 662 pp. Introduction to S di Sediments t & Sedimentary S di t Rocks R k WHAT ? Definition of sediments & sedimentary rocks WHY ? Importance of sediments and sedimentary rocks HOW ? Sedimentary rocks are formed? WHAT ? Textures and Mineralogy of sediments di & sedimentary di rocks k HOW ? Sedimentary rocks are classified? Introduction & Definitions What is Sedimentology ? • It is the branch of the geological science which deals with the study of: m , 1. sediments, 2. sedimentary deposits 3 sedimentation processes, 3. processes and 4. sedimentary rocks. Stratified Sedimentary rocks Sediment What are sediments? Sediments are: • loose, unconsolidated particles • formed at or near the earth's surface under low T & P, • formed in response to processes of chemical and physical weathering, erosion, transportation and deposition. What are sediments? Sediments may be: • A fragment of rock, mineral or shell (clastic sediments) Rock fragments Quartz grains Shell fragments • Crystals which chemical precipitated directly from water (chemical sediments) crystals • Shells and skeleton of marine organisms g ((biochemical sediments)) Shells The clasts sediments include: • Terrigenous clastic sediments: The solid grains derived from weathering of previous rocks (sedimentary, igneous, metamorphic) and transported, redistributed and deposited by surface processes to form clastic sedimentary rocks Rock fragments Q Quartz t grains i s mud d • bioclastic sediments: those generated by the mechanical, chemical and biological breakdown of skeletal parts. Shells Shell fragments • volcaniclasts sediments: those generated by volcanic eruptions. Chemical and biochemical sediments: Are derived from precipitation of authigenic minerals from solutions by biological and inorganic chemical processes to form rocks without the transportation p of the p particles and form chemical and biochemical sedimentary rocks. Shells Rock Salt Flint Growth in position by organic processes (e.g., carbonate reefs, coals) reef What is a sedimentary deposit? •It is a body of solid materials (sediments) accumulated at/or near surface of the earth within a specific sedimentary environment. Sand Dune deposits p Swamp deposits Lake deposits p Beach deposits What is Sedimentation? Sedimentation is the process of sediment accumulation accu u at o ((falling a g do down)) in tthe e ea earth t su surface ace u under de the influence of certain forces (e.g. gravity, winds, streams currents,, glaciers, g , tubidityy currents,, ..)) and is primarily applied to the settling of solid particles from a fluid. What are Sedimentary Rocks? • Sedimentary rocks are secondary rocks (i.e. they are formed from pre-existing rocks) • They are formed via the deposition and lithification (diagenesis) of loose sediments, which are the product of f chemical h i l and d physical h i l weathering th i of f All rocks k (igneous, metamorphic and sedimentary). • They form at or near the earth's earth s surface at relatively low temperatures and pressures primarily. Stratified Sedimentary rocks Sediment Reefal limestone Bedded conglomerate Beach rock Limestone & Sandstone Limestone & shale Limestone Abundance of Sedimentary Rocks - Cover 75 % of the Earth's surface - Make up about 5% of the total volume of the Earth'crust. - Have an average thickness of about 1800 m on the continents Marine Sediment & Sedimentary Rocks Continental Sediment y Rocks & Sedimentary Why ? Importance of sediments and sedimentary y rocks: Academic importance: deduce the environments and processes off deposition, palaeogeography and palaeoclimatology understanding d di off the h E Earth's h' geological l i l hi history. Stratigraphic correlation. Economic importance: • S Source rock k for f petroleum l oilil and d gas. • Reservoirs for the petroleum oil oil, gas, freshwater • Cap p rocks ((Seal)) for reservoirs such as evaporites, mudrocks. • Raw materials for the Mud Fluids ( (e.g. Bentonite B t it (clay ( l minerals). i l ) •Source of ores of many metals such as: iron, manganese, lead ores, and others.… • Raw materials for the ceramic and Portland cement industries industries. • Some of them are used as fertilizers such as phosphates, some nitrates. • Some of them used as raw materials for building as sand and gravel. How a sedimentary rock is formed? Sedimentary y Cycle y • It is a part of the major rock cycle which shows the manner to form new rocks (igneous, metamorphic and sedimentary rocks) from old ones through geologic time. • Sedimentary cycle takes place at or near the surface of the earth under normal conditions of temperature and pressures. Sedimentary Cycle y explains: p How a Sedimentary rock is formed? Weathering - it is the decomposition and breakdown of rocks and minerals at the earth's surface by mechanical and chemical processes. Erosion - it is the removal and movement of weathered material from the site of weathering by moving water, wind, glaciers, or gravity. Transportation - The weathering products may still located in situ or undergone variable long or short transportation. Transporting medium are water (including river, streams, waves, ..), wind, glaciers and gravity. Deposition – the accumulation of transported sediment in the basin of deposition (ocean, lakes, lagoons, inland valleys) as layers of loose sediment. Lithification - it is the process of transforming a loose sediment into a hard sedimentary rock. - It involves different processes: cementation, compaction, recrystallization, t lli ti Uplift - It the process of uplifting of the rocks by the effect of tectonic movements. movements - It leads to the exposing of the rocks whose are subjected to the effect of weathering processes again. Origin g of Sedimentary y Rocks Volcanic ejecta Ground Water Parent Rock Decomposition & Leaching Transportation Sediments Ocean Water Diagenesis Sedimentary Rocks Atmosphere h Cosmic matter 1 2 3 4 Sedimentary Cycle I. WEATHERING PROCESSES Weathering is the breakdown and decomposition of rocks and minerals (at or near the Earth's surface) by mechanical, chemical and biological g processes into p p products that are more in equilibrium with the conditions found in this environment. Weathering itself involves little or no movement or removal of the decomposed rocks and minerals. minerals They accumulate where they form and overlie unweathered bedrock A) Physical (Mechanical) Weathering Rock disintegration g (p (physical y breakdown)) It is the breakdown of rock materials into smaller and smaller pieces with no change in their chemical and mineralogical g composition. Processes of Mechanical Weathering serve to increase surface area, which enhances chemical weathering. A) Physical (Mechanical) Weathering Exfoliation and spheroidal weathering (“sheeting”) -Process in which Slab-like layers/sheets of rock are split from the main rock mass due to reduction in pressure as a result of removal/ erosion of overlying rocks -Exfoliation is largely restricted to granitic i i rocks. k “exfoliation domes” P Pressure release l ffracturing t i - As uplifting proceeds, the overlying rocks gradually erode, and thus the pressure on the underlying rocks decreases This unloading of decreases. pressure causes the rocks to expand and fracture A) Physical (Mechanical) Weathering Expansion/contraction pa s o /co t act o of o rock oc Alternate thermal heating/cooling as a result of daily and seasonally changes in temperatures for long time can cause fracturing and exfoliation of the rocks. The surface of the rock expands more than its i t i interior. Abrasion & Collision Abrasion is the mechanical grinding of rock surfaces by friction and impact of rock fragments which are carried by moving current and glaciers. Also the collision between different particles with each other cause their abrasion and wearing. A) Physical (Mechanical) Weathering Frost wedging g g -Repeated cycles of freezing & thawing in Mountainous regions can break rock into smaller fragments - It develops d l llarge accumulations l ti off lloose angular l sediments di t beneath cliffs (talus slopes) Talus Slope S lt cracking Salt ki • It is formed when salt water penetrates into cracks and evaporates. This leads to growing of salt crystals in the cracks which causing expansion of these cracks and pushes the rock apart. • It is common in desert environment. B) Chemical Weathering Rock decomposition p ((Chemical alteration)) • It is the decomposition of rocks by chemical agents that can alter and change both their chemical and mineralogical composition. • New minerals may be formed formed. • Most important agents in chemical weathering: Water, Carbon dioxide and Oxygen Factors Controlling Rates of weathering 1. Bedrock characteristics a Rock Composition: a. the rate of weathering of rocks is related to relative stability of their mineral composition to chemical weathering. Ex. Limestones (calcite) & Sandstones (quartz). The more susceptible parts of the rock will weather faster than the more resistant portions of the rock. rock This will result in differential weathering. b. Particle size Smaller the particle size the greater the surface area and dh hence th the more rapid id the th weathering. th i c. Structure: Bedding planes, joints, and fractures, all provide pathways for the entry of water. A rock with these features will weather more rapidly than a massive and structureless rock rock. 2. Topography e.g., gentle or steep slope On gentle slopes water may stay in contact with rock for longer periods of time, and thus result in higher weathering rates. 3 Climate: (Temperature 3. (Temperature, moisture) Processes of weathering are more rapid in humid and hot climate than cold or very dry one. E lilimestones iin a d Ex. dry d desert climate li are very resistant i to weathering, h i b but limestones in tropical climate weather very rapidly. 4 Amount of vegetation and organic materials 4. The more organic mass is in the zone of weathering the more intensively the chemical decomposition of rocks takes place. Major Factors Controlling Rates of weathering B) Chemical Weathering M i processes off chemical Main h i l weathering: th i 1. Dissolution Dissolving of minerals by a liquid agent (i.e. water) example: -Halite dissolution NaCl = Na+ + Cl-CaSO4. 2H2O (gypsum) = SO42- + 2H2O 2. Hydration Hydration involves attachment of H2O or OH- ion to a mineral, resulting in formation of a new mineral. CaSO4 (anhydrite) + 2H2O Fe2O3 (hematite) + H2O CaSO4. 2H2O (gypsum) 2FeOOH (geothite) Dehydration involves removal of H2O or OH- ion from a mineral. B) Chemical Weathering 3. Hydrolysis 3 d l i (water) ( ) • Hydrolysis represents a substitution of H+ or OH- of water for an ion in the mineral. • It is especially effective in the weathering of common silicate minerals. 2KAlSi S 3O8 + 2H+ + H2O → Al2S Si2O5(O O5(OH))4 + 2K+ +4H4S SiO O4 (K-Feldspar) kaolinite (clay mineral) Kaolinite B) Chemical Weathering 4 Carbonation (Carbon dioxide 4. dio ide Hydrolysis, H drol sis type t pe of dissolution) dissol tion) It is a dissolution due to the reaction of the minerals with carbonate and bicarbonate ions formed as a result of dissolving of carbon dioxide in water: CO2 + H2O ↔ H2CO3 H+ + ( b i acid) (carbonic id) Example: Calcite dissolution CaCO3 + CO2 + H2O HCO3(bi b (bicarbonate) t ) Ca2+ + 2 HCO3- (bicarbonate) B) Chemical Weathering 5 Oxidation 5. • • • It is the reaction that occurs between compound and oxygen. It may react with minerals to change the oxidation state of an ion, which causes the th structure t t tto b be lless rigid i id and d iincreasingly i l unstable. t bl This is more common in Fe & Mn bearing minerals. Examples: E l 1. Rusting: Iron combines with oxygen (dissolved in water) to form iron oxide -4Fe° + 3O2 = 2Fe 2O3 (hematite) 2. Oxidation of organic g matter: CH2O + O2 → CO2 + H2O 6. Reduction It is simply the reverse of oxidation, and is thus caused by the addition of one or more electrons producing a more stable p ((reaction with organic g carbon. compound Fe2O3 . H2O (limonite) + C → FeCO3 (siderite) C) Biological Weathering Weathering of rock from activities of: organisms plants roots p burrowing animals humans Products of Weathering 1. Solid particles Formed from mechanical weathering of parent rocks differ in grain size (gravel, sand, silt, clay). Accumulations of these products called clastic or detrital sediments. Th include They i l d quartz t and d feldspar f ld and d allll ttypes off rock k ffragments. t They form clastic rocks such as sandstones, conglomerates, mudrocks or form soils. Gravel Conglomerate Sand Sandstone Mud Shale Products of Weathering 2. Soluble materials 2 , Na+, CO 22 2 2 Ions or molecules such as (Ca (C 2+ 3 , SO4 ,..)) dissolved in water. They removed through chemical weathering They produce some chemical and biochemical rocks such as: evaporites, chert, limestones. Ca+2 Na+ Mg+2 Fossiliferous L.S. CO3-22 Coral L.S. Flint Rock Salt Products of Weathering 3. Secondary y minerals Neoformed minerals, which are formed after chemical weathering and alteration of some minerals. E chemical Ex., h i l weathering th i off ffeldspars ld produces d clay l minerals, aluminum hydroxides, ferric oxides. 2KAlSi3O8 + 2H+ + H2O → Al2Si2O5(OH)4 + 2K+ +4H4SiO4 (K-Feldspar) (kaolinite) Oxidation of Iron to hematite, limonite,.. 4Fe° (Iron) + 3O2 = 2Fe 2O3 (hematite) Products of Weathering 4. Soil loose, uneroded sediment material (residual materials). It is a product of mechanical and chemical weathering of rock plus addition of organic material material. Soil Profile O Horizon – decayed and loose organics (topsoil) A Horizon - inorganic g mineral p particles mixed with some organics B Horizon – clays with little organics C Horizon – transition between bedrock and soil The most common residual deposits are - Bauxite (aluminium ore) ore), - Laterite (iron-rich soil). -Calcrete, Ferricrete Decomposition of Main Minerals in Igneous Rocks Original Minerals Weathering Process New Mineral Material Washed Away In Solution Fe, Mg minerals: H2CO3 alteration, olivine, oxidation pyroxene pyroxene, amphibole Clay minerals, Fe oxides Fe-oxides Mg, Ca, Si Feldspars H2CO3 alteration Clay Minerals K Na, K, Na Ca Quartz Little change change, some dissolution none Si Calcite Dissolves easily none Ca Weathering of Granite Quartz Clay Minerals + Mg, Ca, Si Clay Minerals + K, Na, Ca II. Erosion and Transportation * Erosion is the removal of weathered material from the site of weathering. * Transportation is the movement of weathered material from the site of weathering to the site of deposition deposition. • The weathering products may still located in situ and not transported or undergone variable long or short transportation. • Mostly, Mostly no chemical changes take place for the sediments during transportation processes but many physical changes may occur as in rounding of particles, sorting of sediments and particle ti l sizes. i The major natural agents of erosion and transportation are gravity, wind, running water, glaciers, waves, and rain. Wind Gravityy Rockslide Glacial Rock fall from a steep slope or cliff cliff. River Beach Transport Mechanisms: Suspended p Load: Fine-grained g sediment ((clay y and silt)) transported in suspension due to turbulence Bed (or traction) Load: Coarse-grained Coarse grained sediment (sand and gravel) transported on the bottom of the stream bed by rolling and sliding Saltation: Sediment (typically sand) transported by intermittent jumps Solution: as dissolved particles (soluble ions and molecules i water).. t ) in Factors Affecting Transportation of Sediments: 1. Agent of transportation: - Glaciers transports all sizes together and deposit angular and poorly sorted sediment. - Wind transports clay clay-sand sand size sediments and deposits rounded and well sorted sediments. 2. The average velocity (speed) of the flow. As the velocity of the flow increases, the size of the particles carried in the flow also increases increases. 3. Distance of transportation Long transportation of sediments by water and air causes: more abrasion of the grains (more rounded). wellll sorting i (coarse ( particles i l are transported for short distance and deposited near the source, but the fine particles are transported for long distance and deposited far from the source). III. Deposition Deposition is the accumulation and settling of sediment in the basin of deposition (ocean, lakes, lagoons, inland valleys) as layers y of loose sediment. This p process is called a sedimentation Depositional D iti l environment i t - A geographic setting where sediment is accumulating (deposited). (deposited) - Each setting is characterized by a particular combination of geologic processes - The geologic setting may change with time (e.g. marine to continental) - By studying present day environments, geologist can more easilyy interpret eas te p et the t e rock oc record eco d a along o g tthe e geo geologic og c ttime. e How can determine the type of Depositional Environment? It can be b determined d t i db by llooking ki att – Texture (grain size, shape and composition, etc..) – sedimentary structures – fossils content, – bed shape and vertical sequences within the sedimentary layers • Sedimentary Facies It is a body of sedimentary rock accumulated and modified in a particular environment. Each facies is characterized by features (sediment composition ( (lithology), ) grain size, texture, sedimentary structures, ffossil content and colour) that distinguish it from other facies How can Depositional environment affect the rock Facies? physical attributes such as: • Water type and depth • degree of agitation • affect and control the living organisms or type of the sediment. Chemical factors such as • Eh and pH of water • salinity • affect organisms and control mineral precipitation. • Shape and location of basin of deposition • Plate tectonic • topography • affect the composition fossil composition, contents, and textures Depositional Settings Continents: Desert, glacial, fluvial (rivers), lake, swamp, cave Mixed (Transition zone): Lagoon, river delta, beach, tidal flat Marine : Reefs, continental shelf, continental slope, deep water Delta Desert Beach Ocean Swamp Lakes Continental Sedimentary Environments 1. Glacial ‐‐ deposits may have wide range of grain sizes (poorly sorted). made of glacial till. (poorly sorted). made of glacial till. 2. Fluvial 2 Fluvial ‐‐ migration of ripples/dunes on riverbed produces X migration of ripples/dunes on riverbed produces X‐ stratification in deposits (mud and sand). 3. Lacustrine ‐‐ deposition of mud, thin layers on lakebed; in arid regions forms evaporites 4. Aeolian ‐‐ large wind‐blown sand dunes produces thick cross‐stratification in deposits 5. Streams: deposit clastic sediments in streambeds, in floodplain, and on deltas. Marine (Nearshore) & Transtional Environments 1 D lt 1. Deltas ‐‐ where rivers meet the sea ‐‐ h i t th clastic l ti sediments are di t deposited 2 Beaches deposits of sand (siliciclastic 2. Beaches ‐‐ deposits of sand (siliciclastic &carbonate) at the &carbonate) at the coast 3 Shelf ‐‐ may be sand and mud or carbonate sediments 3. Shelf may be sand and mud or carbonate sediments 4. Reefs ‐‐ build‐up of limestone from coral skeletons Marine (Offshore) Sedimentary Environments (Deep Sea) 1 Shelf ‐‐ carbonates 1. Shelf ‐‐ carbonates 2. Slope and rise ‐‐ mixed carbonates/clastics 3 Deep marine ‐‐ finely layered mud 3. Deep marine ‐‐ finely layered mud IV. Lithification Turning sediment into rock Diagenesis = chemical, physical, and biological changes that take place to sediment di t after ft it is i deposited. d it d It includes i l d low temperature near-surface processes to higher temperature subsurface processes (<300C and 1-2 kb) Lithification is a diagenetic process that refers collectively to all the processes that h convert loose l sediment di to hard h d sedimentary di rocks. k loose sediment hard sedimentary rock IV. Lithification Turning sediment into rock Main Lithification Processes: 1. Compaction for sediments with clastic texture 2. Cementation 3. Recrystallization for chemical crystalline sediment • Burial Lithification requires increased pressures and temperatures p (with depth) 1. Compaction • It is a physical process of lithification by which buried sediment is reduced in volume by pressure from the weight of overlying sediments. •With compaction: p - the air and water between the grains are expulsed - the pore space is reduced -the loose sediment is converted into more cohesive rock. •Compaction affects all sediments, but changes are most pronounced in finegrained i d clastic l ti sediments, di t such h as clays l and silts. •Burial of a clay may result in a 40% reduction in volume Burial 2. Cementation • Cementation is the most important p p process for turning g sediments to rock. • It is the process by which sediment grains are bound together by minerals chemically precipitated from water circulating through sediment. • These cementing materials are precipitated in the pore spaces and cement the grains together. • Coarse-grained sediments, such as gravels and sands, are more likely to be cemented than fine fine-grained grained sediments sediments. • The most cementing materials are: Silica, Calcium carbonate, Iron oxides. 3. Crystallization & Recrystallization The process of formation of the chemical rocks with interlocked crystalline texture is called crystallization process. Recrystallization y is a p process in which the texture within the less stable minerals reorganizes and develop into new, more stable and more interlocked crystals with more size in solid state by the effect of heat and pressure with depth and the movement of fluids within pore spaces . Examples: • Amorphous silica to coarse crystalline quartz • fine lime mud into coarse sparry p y calcite • recrystallization of aragonite (unstable form of CaCO3) to calcite (more stable form of CaCO3). Aragonite Calcite V. Uplift It the process of uplifting of the rocks by the effect of tectonic movements. It leads to the exposing of the rocks t the to th effect ff t off weathering th i processes again. i Simple Model for the Evolution of Sedimentary Rocks 1 Weathering 1. 2. Erosion 3 Transport 3. 4. Deposition 5. Compaction / Cementation 6. Uplift Source area (any preexisting rock) Ch mi l weathering Chemical th in Weathering products Clay minerals Ions and d compound in solution Transportation Precipitation from solution Used by organisms Physical weathering Weathering products Gravel, sand, silt and clayclaysized particles Transportation Deposition (non(non-detrital sediments) Deposition (detrital sediments) Lithification Lithification Chemical & Biochemical sedimentary rocks (e.g. limestone) Detrital sedimentary rocks (e.g. sandstone)