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Lecture notes 2

  1. 1. Lecture 7: Sedimentary Rocks Processes related to the formation of sedimentary rocks: weathering, erosion and transportation, deposition and diagenesis Weathering: the physical breakdown (disintegration) and chemical alteration (decomposition) of rocks at or near the Earth’s surface Mechanical Weathering: The physical breaking up of rock into smaller pieces → leads to an increase in surface area; prevails in cold climates, high altitudes, dry regions • Frost wedging: repeated cycles of freezing and thawing; the expansion force of water as it freezes is sufficient to split any mineral or rock. • Heating and cooling: Differences in temperature in a rock give rise to differential expansion (heating) and contraction (cooling). • Wetting and drying: The disruption of soil results in the swelling and contracting of soil particles. • Organisms: Action of organisms, including animals and plants, reduces the size of rocks and minerals • Unloading: the removal of thick layers of sediments overlying deeply buried rocks by erosion or uplift. (a.k.a. exfoliation) Chemical Weathering: breakdown of minerals by chemical reactions with water, with chemicals dissolved in water or with gases in the air; progression from less stable minerals to more stable minerals • Hydration - combination of a solid mineral or element with water. • 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 • 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 • Dissolution - the dissolving of a solid in a liquid • Hydrolysis - process of minerals reacting with water to form hydroxides, which usually are more soluble than the original mineral. example - pyroxene to Fe oxide 4FeSiO3 + H2O + O2 → 4FeO(OH) + 4SiO2 • Acidification - Weathering is accelerated by the presence of hydrogen ion in water, such as that provided by carbonic and organic acids. Common Weathering Products Soluble Ions: Na, Ca, K, Mg Clay Minerals: kaolinite, montmorillonite Resistant Minerals: quartz, hematite, magnetite, garnet, diamond Alterations due to Chemical Weathering • Decomposition of unstable minerals • Generation or retention of those minerals which are in equilibrium with the Earth’s surface *account for predominance of certain minerals in soil. Weathering Factors Source Composition • Specifically involves the mineralogy, texture and rock structure
  2. 2. • Fine-grained rocks decompose chemically more readily than coarse-grained rocks Climate • Temperature fluctuations determine importance of ice-wedging and insolation • Precipitation governs the extent of hydrolysis, hydration and solution Topographic Relief • Influence the amount of rock exposed to the forces of weathering • Slope steepness controls the rate at which weathering products are eroded to be transported elsewhere • Angle of repose: the stable angle of the slope of unconsolidated material (i.e. sediments) Soil – combination of rock, air, water and organic material Soil type – varies in color, texture, mineral content • O Horizon - organic layer of soil, made up mostly of leaf litter and humus (decomposed organic matter) • A Horizon - made up of humus mixed with mineral particles • E Horizon - leaching layer; light in color; made up mostly of sand and silt, having lost most of its minerals and clay as water drips through the soil. • B Horizon – contains clay and mineral deposits (e.g. iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when water drips from the soil above. • C Horizon –it consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer because of very little organic material. • R Horizon - unweathered rock layer (regolith) Erosion – the removal of material by mobile agents such as water, wind, ice or man; involves movement of rock or soil Agents of Erosion and Transportation: gravity, ice, organism, water, wind Ways of physically transporting particles • Suspension: floating, especially, clay-sized particles • Saltation: “jumping” particles • Traction: most of the time in contact with the sediment floor; “rolling” or “dragging” Sorting - measure of variation of grain sizes Roundness: measures how rounded corners are Sphericity: measures sphere-like shape of clasts *well-sorted sediments, rounded and spherical grains indicate longer time of weathering and erosion and/or farther distance from source Deposition: transporting sediment requires energy; grain size has relationship with energy • Smaller grains take less energy; bigger grains take more • If river slows down, sediment will drop out; if river speeds up, water can pick up sediment Diagenesis: Physical, chemical, and biological processes which collectively result in transformation of sediments into sedimentary rock and modification of the texture and mineralogy of the rock Stages of diagenesis: Early diagenesis: takes place from sedimentation until shallow burial Late diagenesis: from deep burial to subsequent uplift The different types of diagenetic processes are:
  3. 3. • Compaction: the process by which the volume of a sediment is reduced as the grains are squeezed together • Recrystallization: transformation of a mineral to another mineral of same chemical composition • Cementation: usual cements include quartz and calcite • Replacement: transformation of a mineral to another chemically distinct mineral • Bioturbation: disturbance of a organisms to the sediment Sedimentary Rock Classification • Siliciclastic/Clastic: based on dominant grain size on Udden-Wentworth scale • Chemical: Precipitated from solution; based on mineralogy o Limestone – calcite, dolomite, other carbonates (CaCO3) o Chert - quartz (SiO2) o Rock salt – halite (NaCl) o Gypsum – gypsum (CaSO4 . H2O) • Biochemical: chemically precipitated with the help of organisms; contains fossils o Peat and coal: plant remains o Limestone o Chert Sedimentary Structures: formed during sediment deposition (e.g., cross bedding, graded bedding and ripple marks) Lecture 8: Sedimentary Environments Capacity: refers to the maximum load of sediment that a stream can transport Competence: refers to the heaviest particles a stream can erode and therefore transport Sedimentary Environments: a part of the earth’s surface, physically, chemically, and biologically distinct from adjacent terrain (different sedimentary facies); defined by fauna and flora, geology, geomorphology, climate, weather, temperature, and if sub-aqueous, the depth, salinity, and current system of the water. Terrestrial Environments • Fluvial - rivers, streams; main transporters of sediment on land Meandering River System: consist of one single channel; low gradient and high sinuosity • sediments deposited at the inner sides of meander bends; deposition of sediments takes place in the channel, on the levees and in the basins. • lag deposits: gravel and coarse sand found on channel floor • point bars: finer sand settles along the inner bends of the river • avulsion: changing of meanders • oxbow lakes: truncated meanders
  4. 4. Braided River System: have one single channel of low sinuosity and high gradient, with multiple `thalwegs' (deepest part of the channel) and bars. • high sediment load • during times of maximum discharge, the channel is completely inundated; in times of low discharge, multiple thalwegs and bars reappear within the channel • Lacustrine – lakes; landlocked body of standing, non-marine water • Well-sorted sands - Silt/clay • Water is not disturbed often so fossils are easily preserved • Also, laminae (very thin layers of sediment) are preserved • Sometimes have fossils made from freshwater organisms • Paludal – swamp, marshes • Silt/clay • Shallow and frequent floods • Rocks have high organic content due to the number of organisms in swamp • Lots of plant fossils • Glacial –Glaciers; flowing ice erodes rocks and transports sediment • Ice-contact deposits characterized by extremely poor sorting and lack of stratification • Alpine/Mountain Glaciers - Relatively small glaciers at higher elevations in mountainous regions. • Ice Sheets (Continental glaciers): the largest types of glaciers on Earth. o cover large areas of the land including mountain areas. o modern ice sheets cover Greenland and Antarctica. • Ice Shelves: sheets of ice floating on water and attached to land. o usually occupy coastal embayments. • Can also be classified as: o Temperate Glaciers - ice near melting point. o Polar Glaciers - ice below melting point. Landforms produced by mountain glaciers o Cirques - bowl shaped depressions that occur at the heads of mountain glaciers o Glacial Valleys - Valleys that once contained glacial ice become eroded into a "U" shape in cross section. o Arêtes - If two adjacent valleys are filled with glacial ice, the ridges between the valleys can be carved into a sharp knife-edge ridge, called an arête. o Horns - Where three or more cirques are carved out of a mountain, they can produce a sharp peak called a horn. o Hanging Valleys - A valley that has greater elevation than the valley to which it is tributary. o Fjords - narrow inlets along seacoasts once occupied by a fjord glacier. Ice Laid Deposits o Till - nonsorted glacial drift deposited directly from ice. A till that has undergone diagenesis and has turned into a rock is called a tillite. o Erratics - a glacially deposited rock or fragment that now rests on a surface made of different rock.
  5. 5. o Moraines - are deposits of till that have a form different from the underlying bedrock. • Desert Environments: vast areas where sand is deposited in dunes; carried by the wind; likely to happen in areas with low rainfall, high atmospheric pressure, and low humidity (subtropics and middle latitudes). • Sediment becomes finer the longer and further it is carried by the wind. • Wind: a turbulent stream of air • low density - limits competence • unrestricted flow - enables spread over wide areas and high atmosphere • Two types of wind erosion (dry, loose particles of dust and salt are lifted and blown away) : • abrasion • deflation • deflation lag deposits - Coarsest clasts (desert pavement) • loess - Unconsolidated, unstratified aggregation of small, angular mineral fragments, usually buff in color. Generally believed to be wind-deposited. • dunes - Sand dunes form when there is (1) a ready supply of sand, (2) a steady wind, and (3) some kind of obstacle such as vegetation, rocks, or fences, to trap some of the sand. Sand dunes form when moving air slows down on the downwind side of an obstacle. Types of Sand Dunes: • Barchan dunes - crescent-shaped dunes. They form in areas where there is a hard ground surface, a moderate supply of sand, and a constant wind direction. • Transverse dunes- large fields of dunes that resemble sand ripples on a large scale. Consist of ridges of sand with a steep face in the downwind side, form in areas where there is abundant supply of sand and a constant wind direction. • Linear dunes - long straight dunes that form in areas with a limited sand supply and converging wind directions. • Parabolic dunes - are "U" shaped dunes with an open end facing upwind. Form in areas with abundant vegetation and constant wind. Most common in coastal areas. • Star dunes - dunes with variable arms and slip face directions. Form in areas with abundant sand supply and variable wind direction. • Alluvial Fans: a river or a stream drops a large amount of sediment in cone or fan-shaped deposits at the base of mountains. • lots of coarse particles; usually forming poorly-sorted sandstone and conglomerate • deposited by high energy floods or mudflows • cone-shaped • typically found in tectonically active regions (rifting continental grabens and foreland basins) Transitional Environment Depositional sedimentary environments at or near the transition between the land and the sea Deltas: prograding depositional bodies that form at the point where a river meets a lake or the sea; formed through continuous progradation (where rate of alluvium deposit > rate of erosion of sediments) Parts of a Delta: • delta plain - composed of meandering flood plains, swamps, and beach complex • delta front - steeper part
  6. 6. • prodelta - broadly sloping that grades into the open shelf Types of Delta Wave-dominated delta: smoothly arcuate; tidal action reworks sediment. • much sandier than the other types of delta • fan-shaped (Arcuate delta), with the wide portion of the fan farthest from the mainland. • Nile Delta, Egypt River-dominated delta: large sediment volume from rivers • Lobate to elongate in shape • If channels encroach delta, the shape of a Bird’s foot delta is apparent • Mississippi Delta, USA Tide-dominated delta: linear features parallel to tidal flow and perpendicular to shore • The waves force the sediment to spread outwards in both directions from the river's mouth, making a pointed tooth shape with sides that curve inward (Cuspate delta) • Tiber Delta, Italy Factors Affecting Delta Formation and Facies: • water and sediment yield of the fluvial system feeding the delta (climate, tectonics), • seasonal changes in water level and sediment yield (climate), • river-mouth processes (differences in river/sea water densities, buoyancy), • coastal configuration, mainly shelf slope and topography (delta gradient), • wave and tidal energy acting on the coast (climate, gradient, tidal range), • along shore winds and currents, • geometry and tectonics (subsidence) of the receiving basin. Marine Environment an environment related to the sea and other major water bodies including their surface interface and interaction Reef: a rock, sandbar or other feature lying beneath the surface of the low water. Coral Reefs: composed of carbonate structures formed by carbonate secreting organisms; builds up on continental shelves. They are wave resistant, mound like structures Types: • Abiotic Reefs: Erosion of sand due to great waves • Biotic Reefs: Composed of carbonate structures formed by carbonate secreting organisms • Artificial Reefs: Produced from man – made materials. Reef kinds in relation to an adjacent landmass • Fringing Reefs: coral reef that is directly attached or borders the shore of an island or continent • Barrier Reefs: a long narrow coral reef roughly parallel to the shore; separated from it at same distance by a lagoon • Atoll: continuous or broken circle of coral reef and low coral islands surrounding a central lagoon. Continental Margin Continental Shelf: continuous with the coastal plain sequences of the continents; part of the continental margin that is between the shoreline and the continental slope (~200m). • quartz and clay minerals are dominant; fossils are mostly marine invertebrates Continental Slope: sloping region between a continental shelf and a continental rise. A continental slope is typically about 20 km wide, consists of mud and silts
  7. 7. Continental Rise: between continental slope and abyssal plain; gentle incline and generally smooth topography; may bear submarine canyons Abyssal Plain: flat region of the ocean floor; covered with pelagic mud with fine sand layers from distal turbidites Longshore drift: the movement of sand along a beach. Waves approaching the beach create a zigzag current that transports the sediments parallel to the length of the beach. Longshore current is the zigzag movement of water particles parallel to the shore Spit: a long, narrow sandbar that is connected to the shore at one end. It is deposited and shaped by currents flowing along the shore, and often forms an arc around the mouth of water. Tombolos and Bars: A bar of sediments that connects an island to the mainland. Lagoons and Barrier Islands: Lagoons are bodies of water on the landward side of barrier islands. Tidal flats: periodically flooded and drained by tides (usually twice each day). Laminated or rippled clay, silt, and fine sand (either terrigenous or carbonate) may be deposited. Sabkhas: tidal flats or pools that are periodically inundated with water. The water evaporated leaving behind deposits of evaporate minerals like halite (salt) or gypsum. Resources Sediments are used as: • Sand and gravel: used as road base, components of concrete, fill, sand for sandboxes; if pure sand, for Glass making pure clays for ceramics (dinner ware/plate ware) • Fossil fuels (e.g. coal, petroleum, natural gas) • Food additives (e.g. sodium and potassium salts) • Placer deposits (grains of economic minerals) e.g. gold, diamond, garnet Sedimentary rocks can be used as • Building stone, filtering materials • Construction and manufacturing • Petroleum and natural gas • Uranium Lecture 9: Metamorphic Rocks Agents of Metamorphism • Heat • Geothermal gradient - temperature increases with depth (20o – 30o C per km in the crust) • Large bodies of molten rock or intrusive bodies • Pressure • When subjected to confining pressure, minerals may recrystallize into more compact forms. • Confining pressure – equal stress in all directions; from overlying rock • Differential stress – unequal pressure in different directions • Chemically active fluids • Water trapped in pore spaces of the original rock. • Water released during dehydration of minerals, such as amphibole or mica. • Water from magmatic bodies (hydrothermal fluids). Types • Contact Metamorphism: occurs when magma invades pre-existing rock. A zone of alteration called an aureole (or halo) forms around the emplaced magma . • Takes place at shallow depths (0-6km) and low pressure.
  8. 8. • Regional Metamorphism: Takes place at considerable depths over an extensive area (5-20 km, sometimes more than 30 km) under high pressure and is associated with the process of mountain buliding. • When continents collide or oceanic crust subducts • Hydrothermal Metamorphism or Metasomatism: Chemical alteration at high temperatures and moderate pressures by hot, ion-rich (hydrothermal) fluids that circulate through fissures and cracks. • Rich ore deposits are often formed as a result of hydrothermal metamorphism • Shock/Impact Metamorphism: When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock • forms SiO2 polymorphs coesite and stishovite. • Dynamic/Cataclastic Metamorphism: a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. • Burial Metamorphism: When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300°C may develop in the absence of differential stress. • New minerals grow like zeolite, but the rock does not appear to be metamorphosed. Classification is based on: • Texture – size, shape and relationships of constituent minerals (foliated and non-foliated) • Foliated – exhibits a pervasive planar structure known as foliation which is due to the nearly parallel alignment of minerals and/or compositional and mineralogical layering in the rock • Non-foliated – no preferred mineral orientations; metamorphism leads to transformation of one mineral to another • Protolith – original rock that was transformed by metamorphism (in low-grade metamorphic rocks, original textures may still be preserved and might allow determination of likely protolith) • Mineralogy – mineral assemblage present Characteristic Metamorphic Minerals: garnet, kyanite, andalusite, sillimanite, staurolite Metamorphic grade: indicates the pressure and temperature conditions based on the mineralogy Metamorphic facies: indicates which pressure and temperature conditions present during metamorphism Resources: 1. Marble is used for statues and ornamental building stone. 2. Slate is used for roofing, flooring, billiard/pool tables, and blackboards. 3. Talc is ground into powder.
  9. 9. 4. Graphite is used in pencils and lubricants. 5. Garnet and corundum used as gemstones and abrasives. 6. Kyanite, andalusite, sillimanite used as raw materials in the ceramics industry. 7. Sulfide deposits (bornite, chalcopyrite, galena, pyrite, and sphalerite) 8. Iron and tin oxide deposits (hematite, magnetite and cassiterite) 9. Tungsten deposits (wolframite and scheelite) 10. Precious metal deposits (gold) Lecture 10. Rock Deformation Stress: a measure of internal force applied to a deformable body Strain: material response (deformation) due to the stress applied; change in size and shape Types of stress: Confining stress/pressure: equal stress in all directions (in geology, lithostatic stress; in oceanography, hydrostatic stress) Differential stress: unequal stress Tensional stress: “stretching”, component perpendicular to given surface (e.g., Divergent Plate Boundaries) Compressional stress: “squeezing”, component perpendicular to given surface (e.g., Convergent Plate Boundaries) Shear stress: sidewards movement in opposite direction (e.g., Transform Plate Boundaries) Types of deformation (strains) Elastic: under stress, solid material deforms; however after stress is removed, solid material will return to its original shape (reversible) Ductile: under stress, solid material deforms and when stress is removed, solid material does not return to its original shape (irreversible); forms folds Brittle: under much stress, material breaks past its elastic and plastic (ductile) deformation ranges (irreversible); forms joints, fractures and faults Factors that affect deformation Type of force applied: the stronger the force, the higher the tendency to fail Pressure: higher confining pressure, less likely to fracture Temperature: higher temperature, less likely to fracture; material tends to behave like fluid Rock (mineral) composition: quartz, olivine, and feldspars are very brittle, while clay minerals, micas, and calcite are more ductile; water in chemical composition of minerals make it more ductile Measuring deformation in rocks Strike is the azimuth (degrees east of north) of the line formed by the intersection of a layer interface or bedding plane with the horizontal Dip is the angle between the layer interface or bedding plane and the horizontal measured perpendicular to the strike direction
  10. 10. Notation: strike = N x E/W, dip = x N/S E/W; where x is a number (however, the dip direction should always be perpendicular [90 degrees] from the strike direction, i.e. if the strike direction is NW, the dip direction should be either NE or SW only) Parts of a fold Axial plane: imaginary surface that divides a fold as symmetrically as possible, one limb on each side. Fold axis: the line made by the length-wise intersection of the axial plane with beds in the fold Limbs: the two sides of an anticline or syncline Anticline (“A”): oldest rocks at the core Syncline (“sink”): youngest rocks at the core Dome: Anticlines from all directions Basin: Synclines from all directions Joint: regular (same strikes and dips, regular spacing) breaks in rock with no movement Fracture: irregular breaks in rock with no movement Fault: breaks with movement Classification of faults: Strike-slip fault: strike-wise movement of blocks Right-lateral (Dextral) Fault: opposite block moves to the left Left-lateral (Sinistral) Fault: opposite block moves to the right Dip-slip fault: dip-wise movement of blocks Normal Fault: Hanging wall goes down, footwall goes up; tensional Reverse Fault: Hanging wall goes up; footwall goes down; compressional Thrust Fault: A reverse fault with an angle of depression less than 15 degrees Oblique-slip fault: faults with strike-slip and dip components