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Sedimentation & Stratigraphy ( 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 Course objectives This course aims at: • Providing 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 and structures of sediment and sedimentary 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 and their importance) 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 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 Scientific Publications - Levis, D.W. and Mc Conchie, D. (1994), Practical Sedimentology. Chapman & Hall, N.Y. - Sam Boggs (2006): Principles of Sedimentology and Stratigraphy, 4th Ed. Prentice Hall, 662 pp. Introduction to Sediments & Sedimentary Rocks WHAT ? Definition of sediments & sedimentary rocks WHY ? Importance of sediments and sedimentary rocks HOW ? Sedimentary rocks are formed? WHAT ? Textures and Mineralogy of sediments & sedimentary rocks HOW ? Sedimentary rocks are classified? Introduction & Definitions Sedimentology is the branch of the geological science which deals with the study of sediments, sedimentary deposits and sedimentary rocks. All rocks at the Earth's surface (igneous, metamorphic and sedimentary) disintegrate slowly by chemical and physical weathering. The products of weathering are both solid particles, such as grains of sand, and ions dissolved in water. These weathering products erode and are carried away by running water, wind, glaciers, and gravity to lower elevations where they collect and deposit physically, chemically or biologically. These loose, unconsolidated particles are referred collectively by sediment. When the sediment is lithified and cemented together and form new consolidated materials, it is called sedimentary rock. Stratified Sedimentary rocks Sediment Introduction & Definitions So, Sediments are loose, unconsolidated particles formed at or near the earth's surface under low T & P, in response to processes of chemical and physical weathering, erosion, transportation and deposition. Sediments may be: • A fragment of rock, mineral or shell (clastic sediments) • Crystals which chemical precipitated directly from water (chemical sediments) • Shells of marine organisms (biochemical sediments) • Reef (Growth in position by organisms) Shell fragments Shells Rock fragments crystals Quartz grains reef The clasts sediments include: • Terrigenous clastic sediments: The solid grains derived from weathering of previous rocks (sedimentary, igneous, metamorphic). • volcaniclasts sediments: those generated by volcanic eruptions. • bioclastic sediments: those generated by the mechanical, chemical and biological breakdown of skeletal parts. - All these grains can be transported, redistributed and deposited by surface processes to form clastic sedimentary rocks Chemical sediments: derived from precipitation of authigenic minerals from solutions by biological and inorganic chemical processes to form rocks without the transportation of the particles and form chemical and biochemical sedimentary rocks. Growth in position by organic processes (e.g., carbonate reefs, coals) Definitions Sedimentology is the science of study of sedimentary deposits and rocks. Sedimentary Rocks are secondary rocks formed from preexisting rocks. They are layered or stratified rocks formed at or near the earth's surface under the low temperatures and pressures normally characteristic of this environment, in response to transportation, deposition and lithification of loose sediment. Sedimenatry deposit is a body of sediments accumulated at/or near the earth’surface within a specific sedimentary environment Sedimentary petrology is that branch of petrology, which deals with the genesis of the sedimentary deposits and rocks. Sedimentary petrography is the description of the sedimentary rocks. Sedimentation is the process of sediment accumulation in the earth surface under the influence of certain forces (e.g. gravity, winds, streams currents, glaciers, tubidity currents, ..) and is primarily applied to the settling of solid particles from a fluid. 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 Continental Sediment & Sedimentary Rocks Marine Sediment & Sedimentary Rocks Why ? Importance of sediments and sedimentary rocks: Academic importance: deduce the environments and processes of deposition, palaeogeography and palaeoclimatology understanding of the Earth's geological history. Stratigraphic correlation. Why ? Importance of sediments and sedimentary rocks: Economic importance: • Source rock for petroleum oil and gas. • Reservoirs for the petroleum oil, gas, freshwater • Cap rocks (Seal) for reservoirs such as evaporites, mudrocks. • Raw materials for the Mud Fluids (e.g. Bentonite (clay minerals). •Source of ores of many metals such as: iron, manganese, lead ores, and others.… • Raw materials for the ceramic and Portland cement 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 Cycle • 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 explains: 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, Uplift - It the process of uplifting of the rocks by the effect of tectonic movements. - It leads to the exposing of the rocks whose are subjected to the effect of weathering processes again. 2 3 1 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 processes into 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. They accumulate where they form and overlie unweathered bedrock A) Physical (Mechanical) Weathering rock disintegration (physical breakdown) It is the breakdown of rock materials into smaller and smaller pieces with no change in their chemical and mineralogical 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 rocks. Pressure release fracturing - As uplifting proceeds, the overlying rocks gradually erode, and thus the pressure on the underlying rocks decreases. This unloading of pressure causes the rocks to expand and fracture “exfoliation domes” Expansion/contraction of rock 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 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. Frost wedging -Repeated cycles of freezing & thawing in Mountainous regions can break rock into smaller fragments - It develops large accumulations of loose angular sediments beneath cliffs (talus slopes) Talus Slope Salt cracking • 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 (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. • Most important agents in chemical weathering: Water, Carbon dioxide and Oxygen Factors Controlling Chemical Weathering: (Rates of weathering) 1. Bedrock characteristics a. Rock Composition: 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. This will result in differential weathering. b. Particle size Smaller the particle size the greater the surface area and hence the more rapid the weathering. 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. 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, moisture) Processes of weathering are more rapid in humid and hot climate than cold or very dry one. Ex. limestones in a dry desert climate are very resistant to weathering, but limestones in tropical climate weather very rapidly. 4. Amount of vegetation and organic materials The more organic mass is in the zone of weathering the more intensively the chemical decomposition of rocks takes place. Main processes of chemical weathering: 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 CaSO4. 2H2O (gypsum) Fe2O3 (hematite) + H2O 2FeOOH (geothite) Dehydration involves removal of H2O or OH- ion from a mineral. 3. Hydrolysis (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. 2KAlSi3O8 + 2H+ + H2O → Al2Si2O5(OH)4 + 2K+ +4H4SiO4 (K-Feldspar) kaolinite (clay mineral) Kaolinite 4. Carbonation (Carbon dioxide Hydrolysis, type of dissolution) 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+ + (carbonic acid) Example: Calcite dissolution CaCO3 + CO2 + H2O HCO3(bicarbonate) Ca2+ + 2 HCO3- (bicarbonate) 5. Oxidation • 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 structure to be less rigid and increasingly unstable. • This is more common in Fe & Mn bearing minerals. Examples: 1. Rusting: Iron combines with oxygen (dissolved in water) to form iron oxide -4Fe° + 3O2 = 2Fe 2O3 (hematite) 2. Oxidation of organic 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 compound (reaction with organic carbon. Fe2O3 . H2O (limonite) + C → FeCO3 (siderite) C) Biological Weathering Weathering of rock from activities of: organisms plants roots 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. They include quartz and feldspar and all types of rock fragments. They form clastic rocks such as sandstones, conglomerates, mudrocks or form soils. Gravel Conglomerate Sand Sandstone Mud Shale Products of Weathering 2. Soluble materials Ions or molecules such as (Ca2+, Na+, CO32-, SO42-,..) 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. CO3-2 Coral L.S. Flint Rock Salt Products of Weathering 3. Secondary minerals Neoformed minerals, which are formed after chemical weathering and alteration of some minerals. Ex., chemical weathering of feldspars produces clay 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. Soil Profile O Horizon – decayed and loose organics (topsoil) A Horizon - inorganic mineral 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), - 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: olivine, pyroxene, amphibole H2CO3 alteration, oxidation Clay minerals, Mg, Ca, Si Fe-oxides Feldspars H2CO3 alteration Clay Minerals Quartz Little change, some none dissolution Si Calcite Dissolves easily Ca none K, Na, Ca Weathering of Granite Quartz Clay Minerals Clay Minerals + + K, Na, Ca Mg, Ca, Si 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. • The weathering products may still located in situ and not transported or undergone variable long or short transportation. • 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 sizes. The major natural agents of erosion and transportation are gravity, wind, running water, glaciers, waves, and rain. Wind Gravity Rockslide Glacial Rock fall from a steep slope or cliff. River Beach Transport Mechanisms: Suspended Load: Fine-grained sediment (clay and silt) transported in suspension due to turbulence Bed (or traction) Load: 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 in water).. Factors Affecting Transportation of Sediments: 1. Agent of transportation: - Glaciers transports all sizes together and deposit angular and poorly sorted sediment. - Wind transports clay-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. 3. Distance of transportation Long transportation of sediments by water and air causes: more abrasion of the grains (more rounded). well sorting (coarse particles 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 of loose sediment. This process is called a sedimentation Depositional environment : - A geographic setting where sediment is accumulating (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 easily interpret the rock record along the geologic time. How can determine the type of Depositional environment? It can be determined by looking at – 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, fossil 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 • salinity Chemical factors such as • Eh and pH of water • Shape and location of basin of deposition • Plate tectonic • topography • affect and control the living organisms or type of the sediment. • affect organisms and control mineral precipitation. • affect the composition, fossil 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 Continental Sedimentary Environments 1. Glacial -- deposits may have wide range of grain sizes (poorly sorted). made of glacial till. 2. Fluvial -- 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 Marine (Nearshore) Sedimentary Environments (Transitional) 1. Deltas -- where rivers meet the sea -- clastic sediments are deposited 2. Beaches -- deposits of sand (siliciclastic and carbonate ) at the coast 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 2. Slope and rise -- clastics and mixed carbonates/clastics 3. Deep marine -- finely layered mud Delta Desert Beach Ocean Swamp Lakes Deposition of sediment occurs in a wide variety of geological environments. Streams deposit clastic sediments in streambeds, in floodplain, and on deltas. Wind deposits sand and silt on land surfaces and form dunes. Glaciers deposit large volume of angular and unsorted sediment of gravel, sand and clay, where they melt. Dissolved ions (Ca, silica,..) in water may be carried to the ocean where they are absorbed by some organisms to form shells and skeleton of calcite or silica,... When the organisms die, the skeletal materials deposited and form bioclastic sediment. IV. Lithification Turning sediment into rock Diagenesis = chemical, physical, and biological changes that take place to sediment after it is deposited. It includes 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 convert loose sediment to hard sedimentary rocks. 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 • Lithification requires increased pressures and temperatures (with depth) Burial 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: - 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 clastic sediments, such as clays and silts. •Burial of a clay may result in a 40% reduction in volume Burial 2. Cementation • Cementation is the most important process for turning 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-grained 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 is a 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 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 to the effect of weathering processes again. Simple Model for the Evolution of Sedimentary Rocks 1. Weathering 2. Erosion 3. Transport 4. Deposition 5. Compaction / Cementation 6. Uplift