Sediments and Sedimentary Rocks Chapter 6

Home , Lithification, Sediment, Sedimentary rock, Sedimentation, Sediment transport

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?

This breakdown occurs Weathering is the mechanical and chemical because the parent alteration of Earth materials at or near the surface material reacts with its Erosion involves removing weathered materials new physical and from their place of origin-by running water or chemical environment wind, for example. 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, Weathering and erosion take place at different rates ions in solution, and solid particles

These products of weathering can be eroded and This can occur even on become sedimentary rock or modified in place to the same body of rock become soils. because rocks are not compositionally and structurally homogenous throughout, thereby producing uneven surfaces.

Fig. 6.1, p. 134 Geo-inSight 9., p. 137

How Are Earth Materials Altered? How Are Earth Materials Altered?

Mechanical Weathering Mechanical Weathering Frost Action

Frost action When water freezes in cracks in rocks it expands and then it Pressure release contracts when it thaws, thus Thermal expansion and exerting pressure and contraction opening the cracks wider. Crystal growth Repeated freezing and Activities of organisms. thawing disaggregates rocks into angular pieces that may tumble downslope and The products of mechanical weathering are accumulate as talus. chemically the same as their parent materials.

Fig. 6.9d, p. 142 Fig. 6.3a, p. 138

4. Physical weathering: frost wedging 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 Mechanical / Physical weathering: joints in rocks

Exfoliation occurs where large flat & curved sheets Breakage along natural bedding joints plus cracking of rock fracture and detach from outcrop from expansion due lowered pressure at surface

How Are Earth Materials Altered? 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 The force of the growing roots pry the cracks apart

How Are Earth Materials Altered? How Are Earth Materials Altered? Chemical Weathering Chemical weathering These processes cause a change in the chemical composition.

The parent material is transformed into products Solution including ions in solution, soluble salts and clay Oxidation minerals. 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: Chemical weathering: carbon dioxide 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 Chemical weathering: iron and oxygen Pyroxene dissolves, releasing silica and ferrous iron. Pyroxene (FeSiO ) ● Role of oxygen in weathering: ferrous iron. 3 from iron silicates to iron oxides

Silica Ferrous ● ferric and ferrous iron Ferrous iron is oxidized, iron forming ferric iron. ● hematite, a common mineral Ferric iron precipitates Ferric iron a solid, iron oxide. ● red and brown – the colors of

oxidized iron Iron oxide (hematite) Fe2O3

Chemical weathering: red means iron 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 Chemical weathering: the disintegration of granite 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 Chemical weathering: the disintegration of granite disintegration of granite The decay progresses, Granite is made up Granite is made up and the rock weakens of several minerals of several minerals and disintegrates. that decay at that decay at Cracks form along Cracks form along different rates. different rates. crystal boundaries. crystal boundaries.

Feldspar Feldspar Magnetite Magnetite Biotite Biotite Quartz Quartz

How Are Earth Materials Altered? Chemical weathering: the role of Chemical Weathering increasing surface area 24 sq cm 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. 2 cm Because chemical weathering is a surface process, the more surface exposed, the faster the weathering.

2 cm

Fig. 6.8 a-c, p. 141

Chemical weathering: the role of Chemical weathering: the role of increasing surface area 24 to 48 sq cm 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…

6 Chemical weathering Chemical stability: a speed control for 2 cm weathering 1 cm

2 cm 1 cm • Solubility (halite high, quartz low) • rate of dissolution (feldspar higher than quartz) • relative stability of common rock- forming minerals (halide to iron oxide) Large rocks have less …than small rocks do, surface area for chemical so smaller rocks weather weathering… 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

How Does Soil Form and Deteriorate? weathering 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 The Soil Profile chemical processes operate faster where it is warm and wet.

Soil formation produces horizons that are known in descending order as O, A, B, and C. These horizons differ from one another in texture, Soils known as pedalfers develop in humid climates such structure, composition and as that of the eastern United color. 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 Fig. 6.11, 6.12, p. 144-145

How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Factors that Control Soil Formation Laterite is a deep red soil typical of the tropics where Other Factors That Control Soil Formation chemical weathering is intense.

Laterites are made up of clays and the most Parent material insoluble compounds Organic activity that were present in the Relief and slope parent material. Time

Fig. 6.12, p. 145 Fig. 6.7, p. 141

How Does Soil Form and Deteriorate? How Does Soil Form and Deteriorate? Soil Degradation - Any soil losses, physical Soil Degradation changes, or chemical alteration is called soil Soil erosion is caused mostly by sheet and degradation, and all lead to reduced soil productivity. rill erosion.

Causes include erosion, compaction, and any kind It is a problem in some areas, especially of chemical pollution that inhibits plant growth. where accelerated by human activities such as construction, agriculture, ranching, and deforestation.

Fig. 6.14, p. 147 Fig. 6.13, p. 146

8 How Does Soil Form and Deteriorate?

Soil Degradation The Dust Bowl – An Nutrient depletion American Tragedy Loss of nutrients is most prevalent in areas of land overuse. Improper disposal of chemicals and concentrations of insecticides can destroy soil.

Geo-Focus Fig. 1 a-c, p. 149 Fig. 6.14, p. 147

Weathering and Resources Sedimentary rocks are produced by surface processes in the rock cycle. Intense chemical weathering causes the concentration of valuable mineral resources • Weathering processes break up rock to create sediment. • Physical - Mechanical breakage and disintegration. Residual concentrations – bauxite and other valuable minerals are concentrated by selective • Chemical - Decomposition by reaction with water. removal of soluble substances during chemical • Weathering processes occur at Earth’s surface. weathering Bauxite, which forms in lateritic soils in the tropics, occurs - Rocks react with hydrosphere, atmosphere & biosphere. in areas where chemical weathering is so intense that only the most insoluble compounds accumulate in the soil. - Low temperature and pressure. Aluminum is just such an insoluble compound. Laterites Weathering are the primary source of aluminum oxide, called bauxite. to >>>>>> It is the main source of aluminum ore. 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 Physical Weathering The two primary types of sediment are detrital and chemical. Sedimentary rock is simply rock • Mechanical breakup; doesn’t change mineral makeup. made up of consolidated sediments. • Creates broken fragments or “detritus.” • Detrital fragments classified by size. Detrital sediment consists – Coarse grained – Boulders cobbles and pebbles. of solid particles, products – Medium grained – Sand-sized. of mechanical weathering. – 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. CHEMICAL MECHANICAL WEATHERING • Dissolution WEATHERING (clay minerals and ions, (gravel, sand, silt, TRANSPORT – halite, gypsum, & clay–sized particles) compounds in solution) Transport calcite dissolve. Transport • Hydrolysis Precipitation Used by Deposition from solution organisms (detrital sediments) – Water breaks apart cations that hold silicates together. Deposition – Dissolved cations - Clay minerals. Lithification (chemical sediment) Lithification – Alteration residues - Iron oxides (rust). Detrital sedimentary rocks (e.g.,sandstone) Chemical sedimentary rock Stepped Art (e.g., limestone) Fig. 6-15 (top), p. 150

Sediment and Sedimentary Rocks Sediment and Sedimentary Rocks

Sediment Transport and Deposition Sediment Transport and Deposition

Sedimentary material weathers, undergoes erosion Eventually the sediment comes to rest in a and transport to a new location. depositional environment. Transportation of sediment results in rounding and sorting. Depositional environments are areas of sediment deposition that can be defined by their physical Why are rounding and sorting important in characteristics (topography, climate, wave and sediments and sedimentary rocks? current strength, salinity, etc.). Both are important in determining how fluids move They provide geologist with clues as to how the rock through sediments and sedimentary rocks formed and what the geologic past was like. The amount of rounding and sorting depends on particle size, distance of transportation, and depositional processes.

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 Processes forming Weathering Erosion carries Processes - breaks down breaks down away particles. rocks. sedimentary rock rocks. Weathering Glacier Glacier then Delta Delta Desert Desert Erosion Playa Playa lake lake

Sedimentary Sedimentary rocks rocks Metamorphic Metamorphic rocks rocks Plutons Plutons

Weathering Erosion carries Weathering Erosion carries Process - breaks down away particles. Process - breaks down away particles. rocks. rocks. Transportation moves Transportation moves particles downhill. particles downhill. transport Deposition Glacier Glacier Delta Delta Desert Desert Playa Playa Deposition occurs lake lake when particles settle out or precipitate.

Sedimentary Sedimentary rocks rocks Metamorphic Metamorphic rocks rocks Plutons Plutons

Weathering Erosion carries Process – Weathering Erosion carries Process – breaks down away particles. breaks down away particles. rocks. Transportation moves rocks. Transportation moves particles downhill. Burial particles downhill. Diagensis

Glacier Glacier Delta Delta Desert Desert Playa Deposition occurs Playa Deposition occurs lake when particles lake when particles settle out or settle out or precipitate. precipitate.

Sedimentary Burial occurs Sedimentary Burial occurs rocks as layers of rocks as layers of Metamorphic sediment Metamorphic sediment rocks accumulate. rocks accumulate. Plutons Plutons

Diagenesis causes lithification of the sediment, making sedimentary rocks.

11 Sediment Classes Sedimentary rocks are produced by Sediments are diverse, as are the rocks made from them. surface processes in the rock cycle. Sedimentary rocks divide to groups based on sediments type. 1) Siliciclastics – Made from weathered rock fragments Transport agents - oceans, wind (minor/yr), (clasts primarily of silicates). rivers (25 billion ton/yr), etc 2) Biological & Chemical (Bio/Chemical) - subdivided as Current strength distance affect: particle size – Bioclastic seds.– Shells of organisms (reefs, clams, etc) • strong >50cm/s – gravel – Chemical seds.– Minerals crystallized directly from water • weak <20cm/s - muds – Organic seds.– Carbon-rich remains of plants (coal). Transport distance affect: ClasticClastic BiochemicalBiochemical OrganicOrganic ChemicalChemical •Sizeof clastic particles • Sorting of clastic particles • Rounding of clastic particles

Size & rounding versus transport distance Sorting examples : Well vs Poor

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) • Tularosa Basin, NM (~65-50 ma, white sands (CaSO4) precipitate) More rounding with longer transport, stronger current, low rock hardness, clay minerals

12 Sedimentary basins Sedimentary environments

• Sediments tend to accumulate in depressions in the Earth’s crust. Types of environments: 1. Continental • Depressions are formed by subsidence. 1. Continental Lake • Sedimentary basins are depressions filled River (alluvial) with thick accumulations of sediment. They Desert are sinks for sediment. are sinks for sediment. Glacier

3. Sedimentary environments 3. Sedimentary environments

Types of environments: Types of environments: 2. Shoreline 3. Marine Delta Continental shelf Tidal flat Organic reef Beach Continental margin Continental slope Deep sea

Sedimentary environments 3. Sedimentary environments

13 Sedimentary environments Sedimentary environments

Environments of siliciclastic Environments of chemical and sediments: biological sediments: 1. Continental (alluvial, desert, 1. Carbonate deposits (organic reefs, lake, and glacial) beaches, shelves, and tidal flats) 2. Shoreline (deltas, beaches, 2. Siliceous environments (deep and tidal flats) sea) 3. Marine (shelf, margin, slope, 3. Evaporite environments (lakes) and deep sea)

Sediment and Sedimentary Rock Sediment and Sedimentary Rock How Does Sediment Become Sedimentary Rock? Thru the process of lithification of sediment is How Does Sediment Become Sedimentary Rock? converted into sedimentary rock.

Lithification involves two Lithification involves two processes processes 2. Cementation is a process 1. Compaction -The volume that glues the sediments of a deposit of sediment together. decreases as the weight of The most common cements overlying sediment causes a are calcium carbonate and silica, but iron oxide and iron reduction in pore space (open hydroxide are important in space) as particles pack more some rocks. closely together. Compaction alone will not form Compaction alone is sufficient rocks from sand and gravel. Cementation is necessary to for lithification of mud into glue the particles together into shale. rocks. Fig. 6.19c, p. 153 Fig. 6.18, p. 152

Sediment Process Rock

Gravel > 2 mm Types of Sedimentary Rock Compaction/cementation Conglomerate

Rounded clasts Detrital Sedimentary Rocks are made of solid Sedimentary particles of pre-existing rocks. Angular clasts breccia

Sand 2 mm–1/16 mm Compaction/cementation Quartz sandstone (mostly quartz) Detrital sedimentary particles are classified according to Sandstone grain (particle) sizes, in decreasing diameter: Arkose Silt 1/16 mm–1/256 mm Compaction/cementation (> 25% feldspars) Gravel (including boulders, cobbles and pebbles) Siltstone Mostly silt Sand Silt Clay < 1/256 mm Compaction Mudstone Silt and clay Shale if Clay (or mud). Mudrocks fissile* Claystone Mostly clay

*Fissile refers to rocks capable of splitting along closely spaced planes. 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 Chemical and Biochemical particle size. Sedimentary Rocks

Examples include conglomerate, breccia, sandstone, siltstone, mudstone, and shale. Chemical and biochemical sedimentary rocks are substances derived from solution by How do conglomerate and sedimentary breccia differ? inorganic or biochemical processes. Both begin as detrital gravel. Conglomerate consists of rounded gravel, breccia consists of gravel with sharp edges. 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 Chemical Sedimentary Rocks on the basis of composition.

Evaporites Carbonate rocks consist primarily of minerals containing the carbonate ion, such as limestone Bedded rock salt (halite) and and dolostone. rock gypsum are chemical Dolostone forms when magnesium replaces evaporite sediments formed by calcium in limestone. precipitation of minerals during the evaporation of water.

Fig. 6.20b-d, p. 154 Fig. 6.21a-b, p. 155

Types of Sedimentary Rocks Types of Sedimentary Rocks

Chemical Sedimentary Rocks Biochemical Sedimentary Rocks

Coal is a biochemical sedimentary rock composed Bedded Chert largely of altered land plant remains 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 Marine Transgression and Regression composition and/or texture.

Bodies of sediment or sedimentary rocks which are A marine transgression recognizably different from adjacent sediment or occurs when sea level rises sedimentary rocks and are deposited in a different with respect to the land, depositional (sub) environment are known as resulting in offshore facies sedimentary facies. overlying nearshore facies. Today we recognize modern facies changes when A marine regression, we go from an inland area with rivers to the beach. 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. Fig. 6.22, p. 156

Three Stages of Marine Transgression Three Stages of Marine Regression Offshore Near shore Low-energy High-energy Land Limestone Shale Sandstone surface facies facies facies Time line

Time lines

Time lines Cross-bedded Sandstone

Old land surface Old land Stepped Art surface Peter Kresan Fig. 7.6 Fig. 6-22, p. 156

Sedimentary structures Reading the Story in Sedimentary Rocks Sedimentary Structures Sedimentary structures – all kinds Some sedimentary structures, such as ripple marks, bedding, cross-bedding, and mud cracks form shortly after deposition. of features in sediments formed at Sedimentary structures the time of deposition. are useful in determining the types of environments in which Bedding (stratification) the sediments were deposited. Cross-bedding Sediments are most commonly deposited flat Graded bedding in water. One of the most Ripples common is strata or Ripples bedding. Bioturbation structures

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. 6.23b-c, p. 158 Fig. 7.7

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 Graded Beds Fossils-Remains and Traces of Ancient Life

Fossils are the remains of past life and are usually found Depositional environment: Submarine fans – tell us only in sediments and sedimentary rocks. the location of the ancient shelf margin They provide the only record of prehistoric life, and are used by geologists to correlate strata, and to interpret depositional environments.

Fig. 6.24a-b, p. 158 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 Organics Chert Phosphorite Evaporite

21 Organic reef development

Organic reef development Organic reef rock

Foraminifer in the Eye of a Fossiliferous Limestone Needle

Chevron Corporation Fig. 7.17 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 Determining the Environment of Deposition How do we know that the Navajo Sandstone formed as a desert dune deposit? Sedimentary Rocks in the Grand Canyon

Fig. 6.28 a, p. 161 Fig. 6.28 b, p. 161

Important Resources in Important Resources Sedimentary Rocks in Sedimentary Rocks Many important natural resources are Petroleum and Natural Gas sedimentary rock deposits. These include: Most oil and gas reserves are found within sedimentary rocks.

Sand and gravel What are stratigraphic and structural traps? Both are areas where petroleum, natural gas, or both accumulate in economic quantities. Coal Stratigraphic traps form because of facies changes in the rock layers Clay (strata). Evaporites (like salt) Banded-iron formations. Oil and gas

Fig. 6.29a p. 162

23 Important Resources in Important Resources in Sedimentary Rocks Sedimentary Rocks

Petroleum and Natural Gas Petroleum and Natural Gas

Structural traps form as the result of folding or fracturing Oil shale is a fine-grained sedimentary rock (faulting) of rocks. 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 Fig. 6.29c p. 162

Important Resources in Important Resources Sedimentary Rocks in Sedimentary Rocks

Uranium Banded Iron Formation

Most uranium is used in nuclear reactors. The uranium comes from the minerals carnotite and uraninite. Why is banded iron The richest ores are found in Wyoming, Utah, Arizona and New formation such an Mexico in ancient stream deposits. important sedimentary Large reserves of low grade ore is found in the Chattanooga rock? Shale, which covers portions of several states.

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