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Sediments and Sedimentary Rocks Chapter 6 Weathering, Soil and Sedimentary Rocks Sediments, Soils & Sedimentary Rocks Processes of the rock cycle • Weathering (Soils) • Erosion • Transportation • Deposition (sedimentation) • Burial • Diagenesis Introduction Introduction Rocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering. How does weathering differ from erosion? Weathering is the mechanical and chemical alteration of Earth materials at or near the surface Erosion involves removing weathered materials from their place of origin-by running water or wind, for example. This breakdown occurs because the parent material reacts with its new physical and chemical environment transforming it into a new equilibrium state. Geo-inSight 4., p. 136 Fig. 6.2, p. 135 1 How Are Earth Materials Altered? How Are Earth Materials Altered? The products of weathering include soluble salts, ions in solution, and solid particles Weathering and erosion take place at different rates These products of weathering can be eroded and become sedimentary rock or modified in place to become soils. This can occur even on the same body of rock because rocks are not compositionally and structurally homogenous throughout, thereby producing uneven surfaces. Fig. 6.1, p. 134 How Are Earth Materials Altered? Geo-inSight 9., p. 137 How Are Earth Materials Altered? Mechanical Weathering Mechanical Weathering Frost Action When water freezes in cracks in rocks it expands and then it contracts when it thaws, thus exerting pressure and opening the cracks wider. Repeated freezing and thawing disaggregates rocks into angular pieces that may tumble downslope and accumulate as talus. Frost action Pressure release Thermal expansion and contraction Crystal growth Activities of organisms. The products of mechanical weathering are chemically the same as their parent materials. Fig. 6.9d, p. 142 4. Physical weathering: frost wedging Fig. 6.3a, p. 138 How Are Earth Materials Altered? Mechanical Weathering Pressure Release and Sheet Joints Sheet joints are fractures that more or less parallel exposed rock surfaces, especially rocks now at the surface that formed under great pressure at depth. These joints form in response to pressure release; that is, when the rocks formed, they contained energy that is released by outward expansion. Frost wedging due the expansion of freezing water can turn small cracks into large ones Fig. 6.4 a-b, p. 138 2 Mechanical /Physical weathering: exfoliation Exfoliation occurs where large flat & curved sheets of rock fracture and detach from outcrop How Are Earth Materials Altered? Mechanical / Physical weathering: joints in rocks Breakage along natural bedding joints plus cracking from expansion due lowered pressure at surface Mechanical / Physical weathering: tree roots Mechanical Weathering How do organisms contribute to mechanical and chemical weathering? Any organic activity such as tree roots growing in cracks contributes to mechanical weathering Organic acids and the tendrils of mosses and lichens aid in the chemical alteration of parent material. Fig. 6.5b, p. 139 How Are Earth Materials Altered? The force of the growing roots pry the cracks apart How Are Earth Materials Altered? Chemical Weathering These processes cause a change in the chemical composition. Chemical weathering The parent material is transformed into products including ions in solution, soluble salts and clay minerals. Solution Oxidation Hydrolysis Hot and wet environments accelerate chemical weathering. Chemical weathering occurs in all environments, except, possibly, permanently frozen polar regions. Fig. 6.7, p. 141 Fig. 6.6, p. 140 3 How Are Earth Materials Altered? Chemical Weathering Solution – rocks dissolve Carbonate Rocks Rocks such as limestone (CaCO³) are nearly insoluble in neutral or alkaline solutions, but they rapidly dissolve in acidic solutions The atoms making up the minerals dissociate, that is, they separate and the rock dissolves. Chemical weathering: carbon dioxide Chemical weathering: carbon dioxide How Are Earth Materials Altered? Chemical Weathering Oxidation – rocks rust Rocks such as sandstone may contain iron minerals that will breakdown when exposed to the atmosphere The atoms making up the minerals dissociate, that is, they separate as the rock rusts away. Geo-inSight 4., p. 136 4 Chemical weathering ● Role of oxygen in weathering: from iron silicates to iron oxides ● ferric and ferrous iron ● hematite, a common mineral ● red and brown – the colors of oxidized iron Chemical weathering: red means iron Chemical weathering: iron and oxygen Pyroxene dissolves, releasing silica and ferrous iron. Pyroxene (FeSiO3) Ferrous iron is oxidized, forming ferric iron. Silica Ferric iron precipitates a solid, iron oxide. Ferrous iron Ferric iron Iron oxide (hematite) Fe2O3 How Are Earth Materials Altered? Chemical Weathering Hydrolysis – breakdown to clays Potassium Feldspar During hydrolysis hydrogen ions react with and replace positive ions in potassium feldspar The result is clay minerals and substances in solution such as potassium and silica. Chemical weathering: the disintegration of granite Chemical weathering: the disintegration of granite Granite is made up of several minerals that decay at different rates. Feldspar Magnetite Biotite Quartz Mr. Granite 5 Chemical weathering: the disintegration of granite Chemical weathering: the disintegration of granite Granite is made up of several minerals that decay at different rates. The decay progresses, and the rock weakens and disintegrates. Granite is made up of several minerals that decay at different rates. Cracks form along crystal boundaries. Feldspar Feldspar Magnetite Magnetite Biotite Biotite Quartz Quartz Cracks form along crystal boundaries. Chemical weathering: the role of increasing surface area 24 sq cm How Are Earth Materials Altered? Chemical Weathering Factors That Control the Rate of Chemical Weathering Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby increasing the surface area for chemical reactions. Because chemical weathering is a surface process, the more surface exposed, the faster the weathering. 2 cm 2 cm Fig. 6.8 a-c, p. 141 Chemical weathering: the role of increasing surface area 24 to 48 sq cm Chemical weathering: the role of increasing surface area 24 to 48 sq cm 2 cm 2 cm 1 cm 1 cm 2 cm 1 cm 2 cm 1 cm Large rocks have less surface area for chemical weathering… weathering… 6 Chemical weathering Chemical stability: a speed control for weathering 2 cm 1 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering… weathering… • Solubility (halite high, quartz low) • rate of dissolution (feldspar higher than quartz) • relative stability of common rockrockforming minerals (halide to iron oxide) …than small rocks do, so smaller rocks weather more quickly. Weathering factors A. duration of weathering B. bedrock type - stability of minerals C. climate i. water & temperature >>> chemical weathering; ii. lower temperature >>> mechanical weathering; iii. more acidity >>> chemical weathering D. topography i. steep slopes >>> mechanical/physical weathering; ii. gentle slopes >>>chemical weathering weathering How Does Soil Form and Deteriorate? The Soil Profile Soils consist of weathered materials, air, water, humus and also the plants which they support. Fig. 6.10a, p. 143 7 How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Factors That Control Soil Formation Climate - Certainly climate is the most important factor because chemical processes operate faster where it is warm and wet. The Soil Profile Soil formation produces horizons that are known in descending order as O, A, B, and C. These horizons differ from one another in texture, structure, composition and color. Soils known as pedalfers develop in humid climates such as that of the eastern United States and much of Canada. Soils of arid and semiarid regions are known as pedocals, and may contain hard, irregular masses of caliche (calcium carbonate) in horizon B. Fig. 6.10b, p. 143 How Does Soil Form and Deteriorate? Fig. 6.11, 6.12, p. 144-145 How Does Soil Form and Deteriorate? Factors that Control Soil Formation Other Factors That Control Soil Formation Laterite is a deep red soil typical of the tropics where chemical weathering is intense. Laterites are made up of clays and the most insoluble compounds that were present in the parent material. Parent material Organic activity Relief and slope Time Fig. 6.12, p. 145 How Does Soil Form and Deteriorate? Fig. 6.7, p. 141 How Does Soil Form and Deteriorate? Soil Degradation - Any soil losses, physical changes, or chemical alteration is called soil degradation, and all lead to reduced soil productivity. Causes include erosion, compaction, and any kind of chemical pollution that inhibits plant growth. Fig. 6.14, p. 147 Soil Degradation Soil erosion is caused mostly by sheet and rill erosion. It is a problem in some areas, especially where accelerated by human activities such as construction, agriculture, ranching, and deforestation. Fig. 6.13, p. 146 8 How Does Soil Form and Deteriorate? Soil Degradation The Dust Bowl – An American Tragedy Nutrient depletion Loss of nutrients is most prevalent in areas of land overuse. Improper disposal of chemicals and concentrations of insecticides can destroy soil. Fig. 6.14, p. 147 Geo-Focus Fig. 1 a-c, p. 149 Sedimentary rocks are produced by surface processes in the rock cycle. Weathering and Resources Intense chemical weathering causes the concentration of valuable mineral resources Residual concentrations – bauxite and other valuable minerals are concentrated by selective removal of soluble substances during chemical weathering Bauxite, which forms in lateritic soils in the tropics, occurs in areas where chemical weathering is so intense that only the most insoluble compounds accumulate in the soil. Aluminum is just such an insoluble compound. Laterites are the primary source of aluminum oxide, called bauxite. It is the main source of aluminum ore. • • • • Weathering processes break up rock to create sediment. Physical - Mechanical breakage and disintegration. Chemical - Decomposition by reaction with water. Weathering processes occur at Earth’s surface. - Rocks react with hydrosphere, atmosphere & biosphere. - Low temperature and pressure. Weathering to >>>>>> sediment Gossans - hydrated iron oxides formed on the earth’s surface by oxidation of iron. Sulfide minerals leach out and concentrate as deposits of iron ore, copper ore, lead and zinc ore beneath the gossan. Sediment and Sedimentary Rock The two primary types of sediment are detrital and chemical. Sedimentary rock is simply rock made up of consolidated sediments. Detrital sediment consists of solid particles, products of mechanical weathering. Physical Weathering • Mechanical breakup; doesn’t change mineral makeup. • Creates broken fragments or “detritus.” • Detrital fragments classified by size. – Coarse grained – Boulders cobbles and pebbles. – Medium grained – Sand-sized. – Fine grained – Silt and clay (mud). Chemical sediments consist of minerals precipitated from solution by inorganic processes and by the activities of organisms thru chemical weathering. Fig. 6.15, p. 150 9 SOURCE OF SEDIMENT Chemical Weathering • Weathering often forms stable from less stable minerals. – Dissolution. – Hydrolysis. – Oxidation. – Hydration. • Dissolution – halite, gypsum, & calcite dissolve. • Hydrolysis – Water breaks apart cations that hold silicates together. – Dissolved cations - Clay minerals. – Alteration residues - Iron oxides (rust). MECHANICAL WEATHERING (gravel, sand, silt, clay–sized particles) TRANSPORT CHEMICAL WEATHERING (clay minerals and ions, compounds in solution) Transport Transport Precipitation from solution Deposition (detrital sediments) Used by organisms Deposition (chemical sediment) Lithification Lithification Detrital sedimentary rocks (e.g.,sandstone) Chemical sedimentary rock (e.g., limestone) Sediment and Sedimentary Rocks Sediment and Sedimentary Rocks Sediment Transport and Deposition Sediment Transport and Deposition Sedimentary material weathers, undergoes erosion and transport to a new location. Transportation of sediment results in rounding and sorting. Why are rounding and sorting important in sediments and sedimentary rocks? Both are important in determining how fluids move through sediments and sedimentary rocks The amount of rounding and sorting depends on particle size, distance of transportation, and depositional processes. Stepped Art Fig. 6-15 (top), p. 150 Eventually the sediment comes to rest in a depositional environment. Depositional environments are areas of sediment deposition that can be defined by their physical characteristics (topography, climate, wave and current strength, salinity, etc.). They provide geologist with clues as to how the rock formed and what the geologic past was like. Sediment and Sedimentary Rocks Sedimentary environments Sediment Transport and Deposition Major depositional settings are continental, transitional, and marine. Glacier Delta Desert Playa lake Sedimentary rocks Metamorphic rocks Plutons Each of these depositional settings includes several specific subenvironments. Fig. 6.17, p. 151 10 Weathering breaks down rocks. Processes forming sedimentary rock Weathering breaks down rocks. Erosion carries away particles. Glacier Glacier Delta Delta Desert Playa lake Desert Playa lake Sedimentary rocks Sedimentary rocks Metamorphic rocks Plutons Weathering breaks down rocks. Metamorphic rocks Plutons Erosion carries away particles. Transportation moves particles downhill. Process transport Weathering breaks down rocks. Erosion carries away particles. Glacier Process Deposition Transportation moves particles downhill. Glacier Delta Delta Desert Playa lake Desert Playa lake Sedimentary rocks Deposition occurs when particles settle out or precipitate. Sedimentary rocks Metamorphic rocks Plutons Weathering breaks down rocks. Processes Weathering then Erosion Metamorphic rocks Plutons Erosion carries away particles. Process – Burial Transportation moves particles downhill. Weathering breaks down rocks. Erosion carries away particles. Glacier Glacier Delta Delta Desert Playa lake Sedimentary rocks Metamorphic rocks Plutons Process – Diagensis Transportation moves particles downhill. Desert Playa lake Deposition occurs when particles settle out or precipitate. Burial occurs as layers of sediment accumulate. Sedimentary rocks Metamorphic rocks Plutons Deposition occurs when particles settle out or precipitate. Burial occurs as layers of sediment accumulate. Diagenesis causes lithification of the sediment, making sedimentary rocks. 11 Sediment Classes Sediments are diverse, as are the rocks made from them. Sedimentary rocks divide to groups based on sediments type. 1) Siliciclastics – Made from weathered rock fragments (clasts primarily of silicates). 2) Biological & Chemical (Bio/Chemical) - subdivided as – Bioclastic seds.– Shells of organisms (reefs, clams, etc) – Chemical seds.– Minerals crystallized directly from water – Organic seds.– Carbon-rich remains of plants (coal). Clastic Clastic Biochemical Biochemical Organic Organic Chemical Chemical Sorting examples : Well vs Poor Sedimentary rocks are produced by surface processes in the rock cycle. Transport agents - oceans, wind (minor/yr), rivers (25 billion ton/yr), etc Current strength distance affect: particle size • strong >50cm/s – gravel • weak <20cm/s - muds Transport distance affect: • Size of clastic particles • Sorting of clastic particles • Rounding of clastic particles Size & rounding versus transport distance Sorting affected by strength, distance, time, agent More rounding with longer transport, stronger current, low rock hardness, clay minerals Size & rounding versus transport distance Sedimentary rocks are produced by surface processes in the rock cycle. Chemical mixing vats: • Oceans • Lakes Salinity varies with water input & evaporation. e.g. • Great Salt Lake, Ut (NaCl) NaCl) • Tularosa Basin, NM (~65(~65-50 ma, white sands (CaSO4) precipitate) More rounding with longer transport, stronger current, low rock hardness, clay minerals 12 Sedimentary basins • Sediments tend to accumulate in depressions in the Earth’ Earth’s crust. • Depressions are formed by subsidence. • Sedimentary basins are depressions filled with thick accumulations of sediment. They are sinks for sediment. Sedimentary environments Types of environments: 1. Continental Lake River (alluvial) Desert Glacier 3. Sedimentary environments 3. Sedimentary environments Types of environments: Types of environments: 2. Shoreline Delta Tidal flat Beach Sedimentary environments 3. Marine Continental shelf Organic reef Continental margin Continental slope Deep sea 3. Sedimentary environments 13 Sedimentary edimentary environments Sedimentary environments Environments of chemical and biological sediments: 1. Carbonate deposits (organic reefs, beaches, shelves, and tidal flats) 2. Siliceous environments (deep sea) 3. Evaporite environments (lakes) Environments of siliciclastic sediments: 1. Continental (alluvial, desert, lake, and glacial) 2. Shoreline (deltas, beaches, and tidal flats) 3. Marine (shelf, margin, slope, and deep sea) Sediment and Sedimentary Rock Sediment and Sedimentary Rock How Does Sediment Become Sedimentary Rock? How Does Sediment Become Sedimentary Rock? Thru the process of lithification of sediment is converted into sedimentary rock. Lithification involves two processes Lithification involves two processes 1. Compaction - The volume of a deposit of sediment decreases as the weight of overlying sediment causes a reduction in pore space (open space) as particles pack more closely together. Compaction alone is sufficient for lithification of mud into shale. 2. Cementation is a process that glues the sediments together. The most common cements are calcium carbonate and silica, but iron oxide and iron hydroxide are important in some rocks. Compaction alone will not form rocks from sand and gravel. Cementation is necessary to glue the particles together into rocks. Fig. 6.19c, p. 153 Sediment Gravel > 2 mm Process Rock Compaction/cementation Types of Sedimentary Rock Conglomerate Detrital Sedimentary Rocks are made of solid Rounded clasts particles of pre-existing rocks. Sedimentary breccia Angular clasts Sand 2 mm–1/16 mm Compaction/cementation Quartz sandstone (mostly quartz) Sandstone Mudrocks Compaction Siltstone Mostly silt Mudstone Silt and clay Claystone Mostly clay Shale if fissile* *Fissile refers to rocks capable of splitting along closely spaced planes. Detrital sedimentary particles are classified according to grain (particle) sizes, in decreasing diameter: Arkose (> 25% feldspars) Silt 1/16 mm–1/256 mm Compaction/cementation Clay < 1/256 mm Fig. 6.18, p. 152 Gravel (including boulders, cobbles and pebbles) Sand Silt Clay (or mud). Stepped Art Fig. 6-18, p. 152 14 Types of Sedimentary Rocks Types of Sedimentary Rocks Detrital sedimentary rocks are classified on the basis of particle size. Examples include conglomerate, breccia, sandstone, siltstone, mudstone, and shale. How do conglomerate and sedimentary breccia differ? Both begin as detrital gravel. Conglomerate consists of rounded gravel, breccia consists of gravel with sharp edges. Chemical and Biochemical Sedimentary Rocks Chemical and biochemical sedimentary rocks are substances derived from solution by inorganic or biochemical processes. Some have a crystalline texture, meaning they are composed of a mosaic of interlocking crystals Others have a clastic texture, meaning that they are made of fragments, like shells that are glued together. Fig. 6.19 a and b , p. 153 Types of Sedimentary Rocks Types of Sedimentary Rocks Chemical Sedimentary Rocks Chemical sedimentary rocks are classified on the basis of composition. Chemical Sedimentary Rocks Evaporites Bedded rock salt (halite) and rock gypsum are chemical evaporite sediments formed by precipitation of minerals during the evaporation of water. Carbonate rocks consist primarily of minerals containing the carbonate ion, such as limestone and dolostone. Dolostone forms when magnesium replaces calcium in limestone. Fig. 6.20b-d, p. 154 Types of Sedimentary Rocks Fig. 6.21a-b, p. 155 Types of Sedimentary Rocks Chemical Sedimentary Rocks Biochemical Sedimentary Rocks Coal is a biochemical sedimentary rock composed largely of altered land plant remains Bedded Chert Marin County, California The origin of chert is highly debated. Fig. 6.21c, p. 155 Fig. 6.21d, p.155 15 Sedimentary Facies Sedimentary Facies Geologists realize that if they trace a sedimentary layer far enough, it will undergo changes in composition and/or texture. Marine Transgression and Regression Bodies of sediment or sedimentary rocks which are recognizably different from adjacent sediment or sedimentary rocks and are deposited in a different depositional (sub) environment are known as sedimentary facies. Today we recognize modern facies changes when we go from an inland area with rivers to the beach. A marine transgression occurs when sea level rises with respect to the land, resulting in offshore facies overlying nearshore facies. A marine regression, caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies Note the difference in the vertical rock sequence that occurs in a transgression versus a regression. Three Stages of Marine Transgression Offshore Near shore Low-energy High-energy Land Limestone Shale Sandstone surface facies Time line facies Fig. 6.22, p. 156 Three Stages of Marine Regression facies Time lines Cross-bedded Sandstone Time lines Old land surface Old land surface Stepped Art Peter Kresan Fig. 6-22, p. 156 Sedimentary structures Sedimentary structures – all kinds of features in sediments formed at the time of deposition. Bedding (stratification) CrossCross-bedding Graded bedding Ripples Bioturbation structures Fig. 7.6 Reading the Story in Sedimentary Rocks Sedimentary Structures Some sedimentary structures, such as ripple marks, bedding, cross-bedding, and mud cracks form shortly after deposition. Sedimentary structures are useful in determining the types of environments in which the sediments were deposited. Sediments are most commonly deposited flat in water. One of the most common is strata or bedding. Fig. 6.23 a, p. 158 16 Reading the Story in Sedimentary Rocks Formation of Cross-beds Sedimentary Structures Depositional environments are also inferred by comparison of these structures with present-day depositional environments. Cross-bedding preserves layers deposited at an angle. They are common in depositional environments like sand dunes, shallow marine deposits and stream-channel deposits How is cross-bedding used to determine ancient current directions? Understanding how physical features like cross-beds form today can reveal important ancient climate information such as current directions. Fig. 7.7 Fig. 6.23b-c, p. 158 Ripples Reading the Story in Sedimentary Rocks Sedimentary Structures Cross-bedding Depositional environment: streams or shallow marine? Streams have a current and leave behind asymmetric dunes. Shallow marine crossbeds exhibit a symmetrical shape from the rocking motion of the waves. Fig. 6.25 a-d, p. 159 17 Fig. 7.9 Reading the Story in Sedimentary Rocks Bioturbation structures Sedimentary Structures Mud cracks Depositional environment: Lagoons and mudflats Fig. 6.26 a-b, p. 159 Reading the Story in Sedimentary Rocks Reading the Story in Sedimentary Rocks Sedimentary Structures Fossils-Remains and Traces of Ancient Life Graded Beds Depositional environment: Submarine fans – tell us the location of the ancient shelf margin Fig. 6.24a-b, p. 158 Fossils are the remains of past life and are usually found only in sediments and sedimentary rocks. They provide the only record of prehistoric life, and are used by geologists to correlate strata, and to interpret depositional environments. Fig. 6.27 a-b, p. 160 18 Burial and diagenesis Burial is the preservation of sediments within a sedimentary basin. Diagenesis is the physical and chemical change that converts sediments to sedimentary rocks. 19 Burial and diagenesis Lithification includes: Compaction Cementation Classification of siliciclastic sediments and sedimentary rocks Classification of sediments by particle size Classification of sedimentary rocks by texture and composition 20 7. Classification of chemical and biological sedimentary rocks Limestone Chert Evaporite Organics Phosphorite 21 Organic reef development Organic reef rock Organic reef development Foraminifer in the Eye of a Needle Chevron Corporation Fig. 7.17 Fossiliferous Limestone Peter Kresan 22 One Model for the Formation of Evaporites Fig. 6-17, p. 109 Reading the Story in Sedimentary Rocks Reading the Story in Sedimentary Rocks Determining the Environment of Deposition How do we know that the Navajo Sandstone formed as a desert dune deposit? Determining the Environment of Deposition Sedimentary Rocks in the Grand Canyon Fig. 6.28 a, p. 161 Important Resources in Sedimentary Rocks Many important natural resources are sedimentary rock deposits. These include: Sand and gravel Coal Clay Evaporites (like salt) Banded-iron formations. Oil and gas Fig. 6.28 b, p. 161 Important Resources in Sedimentary Rocks Petroleum and Natural Gas Most oil and gas reserves are found within sedimentary rocks. What are stratigraphic and structural traps? Both are areas where petroleum, natural gas, or both accumulate in economic quantities. Stratigraphic traps form because of facies changes in the rock layers (strata). Fig. 6.29a p. 162 23 Important Resources in Sedimentary Rocks Important Resources in Sedimentary Rocks Petroleum and Natural Gas Petroleum and Natural Gas Structural traps form as the result of folding or fracturing (faulting) of rocks. Oil shale is a fine-grained sedimentary rock that contains kerogen from which liquid oil and combustible gases can be derived. None is mined at present in the United States because oil and gas from conventional sources are cheaper. Oil shale and tar sands are increasingly important petroleum reserves. Fig. 6.29b, p. 162 Important Resources in Sedimentary Rocks Fig. 6.29c p. 162 Important Resources in Sedimentary Rocks Uranium Banded Iron Formation Most uranium is used in nuclear reactors. The uranium comes from the minerals carnotite and uraninite. The richest ores are found in Wyoming, Utah, Arizona and New Mexico in ancient stream deposits. Large reserves of low grade ore is found in the Chattanooga Shale, which covers portions of several states. Why is banded iron formation such an important sedimentary rock? Banded iron formation consists of alternating thin layers of chert and iron minerals, mostly iron oxides. Nearly all of Earth’s iron ore is mined from ancient banded iron formations. Fig. 6.30 a-b, p. 163 Fig. 6.30b, p. 163 End of Chapter 6 24
