Ch. 7 - Sedimentary Rocks

7.1 An Introduction to Sedimentary Rocks

Explain the importance of sedimentary rocks as indicators of past environments and sources of resources necessary for modern society. Summarize the part of the rock cycle that pertains to sediments and sedimentary rocks. List the three categories of sedimentary rocks.

Sedimentary rocks

Presentation content:

• Sedimentary rocks are products of mechanical and chemical weathering
  • Sediments and soluble constituents are typically transported downslope by gravity (mass wasting)
  • The sediments are then deposited and subsequently buried
  • As deposition continues, the sediments are lithified into sedimentary rocks
• There are three types of sedimentary rocks:
  • Detrital/Clastic
  • Chemical/Crystalline
  • Organic sedimentary rocks

Notes from the lecture:

• Sedimentary rocks reflect environmental processes and conditions as well as help decipher ancient depositional systems

  • You can actually piece together environmental conditions
  • Traverse different time frames of deposition of the environment
  • You look at ancient rocks and environments

• Sediments are transported downslope

  • For the most part it is:
    • Detrial
    • Chemical
      • Fluid chemistry -> great salt lake at Utah, crystallization, precipitation
    • Organic
      • Coals, dictate our energy economy, black shells

Sediment coverage

Presentation content:

• Sediments and sedimentary rocks cover approximately 75% of land and virtually ALL of the ocean basins
• However, those only comprise about 5% (by volume) of Earth’s outer 10 miles
• Those contain evidence of past environments
• Those contain important economic resources
  • Coal, oil, and other fossil fuels
  • Uranium, iron, aluminum, manganese, phosphate
  • Groundwater resources

Notes from the lecture:

• Chances are that you dig a little bit and you find sediments
• If you dig deeper you will find sedimentary rocks
• Most land is covered by sedimentary rocks except for igneous topography
• Dust settling down
• Organisms that live, die, they make silica and settle down
• Deposition of sediments is like an hour glass

7.2 Classical Sedimentary Rocks

Describe the primary basis for distinguishing among clastic rocks and describe how the origin and history of such rocks might be determined.

Detrital sedimentary rocks

Presentation content:

• Detrital sedimentary rocks form from sediments that have been weathered and transported
  • Detrital rocks include: clay minerals, quartz, feldspars, and micas
  • Particle size is used to distinguish among the various rock types
    • It also presents important information about the environment of deposition
  • Common detrital sedimentary rocks include
    • Shale, sandstone, conglomerate, and breccia

Notes from the lecture:

• Broken up pieces of pre-existing rocks
  • Gravel
  • Sand
  • Mud
• Based on size
  • Impacts how fluids flow
    • Clay (Mud): impedes liquid to flow (impermeability)
    • Sands and Gravels: high porosity and permeability

Shale

Presentation content:

• Silt- and clay-sized (fine-grained) particles
• Form from gradual settling of sediments in low hydrodynamic energy (non-turbulent) environments. Deep ocean basins
• Sediments form in thin layers that are called laminae
• Has fissility (meaning the rock can be split into thin layers)
• Crumbles easily and tends to form gentle slopes
• Most abundant sedimentary rock

Notes from the lecture:

• One of our more common sedimentary rock
• If you look at it you will not be able to see any grains unlike Gravels, sand and clay
• Typically organic rich
  • That provides a darker color
  • Fossil impressions possible
• Form in deep ocean basins
  • low hydrodynamic energy -> swamp, etc.
• Are broken into thin layers
  • Crumble very easily
  • Easy to identify the shale layers vs. the sandstone layers

Sandstone

Presentation content:

• Sand-sized particles

• Forms in a variety of environments

• Second most abundant sedimentary rock

• Quartz is the most abundant mineral

  • Quartz sandstone is predominately composed of quartz
  • Arkose sandstone contains appreciable quantities of feldspar
  • Graywacke contains rock fragments and matrix, in addition to quartz and sandstone

• Particles are classified by sorting and shape

  • Sorting is the degree of similarity in particle size
    • If all the grains in a rock are of similar size, the rock is well sorted
    • If the grains in a rock are different sizes (both large and small grains), the rock is poorly sorted
    • Sorting can help decipher the depositional environment of the rock
  • Particle shape varies from rounded to angular
    • The degree of rounding is indicative of how far the sediments have been transported
    • Rounded sediments are typically transported to great distances
    • Angular sediments are only transported a short distance

Notes from the lecture:

• Our second most abundant sedimentary rock
• Dessert, beach, etc.
• Sediments predominantly deposited and transported by the winds (Aeolian processes)
  • Aeolian environment = wind blowing environment
• Wind washes away fine particles and things like sand and gravel is what is left behind, it is sorted.
• We can determine what the current was with sediments
• Sphericity:
  • How circular or flat are the sediments

Conglomerate and Breccia

Presentation content:

• Conglomerate consists of rounded, gravel-sized (or larger) sediments
• Breccia consists of angular, gravel-sized (or larger) sediments
• Both types of rocks are usually poorly sorted

Notes from the lecture:

• Conglomerate are more rounded class
• Breccia are more angular class
• These are close to the source train
  • Experiencing a lot pf physical and mechanical weather

7.3 Chemical Sedimentary Rocks

Explain the processes involved in the formation of chemical sedimentary rocks and describe several examples.

Chemical sedimentary rocks and precipitation

Presentation content:

• Chemical sedimentary rocks form from precipitation
• Precipitation of material occurs by:
  • Inorganic processes: evaporation or chemical activity
  • Organic processes from water-dwelling organisms form biochemical sedimentary rocks
• Chemical sedimentary rocks include:
  • Limestone, chert, rock salt

Notes from the lecture:

• Forming through a process of precipitation
• As result of a high saturation state of the fluid, it precipitates as salt

Limestone

Presentation content:

• Most abundant chemical sedimentary rock

• Mainly composed of the mineral calcite

• Can form from inorganic and biochemical origins

• Has economic value

• Biochemical limestone originates from the shells of marine organisms

  • Large quantities of marine limestone are formed from corals
    • Corals secrete a calcium carbonate skeleton and create reefs
      • Australia’s Great Barrier Reef is the largest coral reef on Earth
  • Coquina is composed of cemented fragments of shell material
  • Chalk is composed of the hard parts of microscopic marine organisms

Notes from the lecture:

• Most abundant chemical sedimentary rock
• Over time chemical sedimentary rocks may turn into calcite
• They form by biomineralization
• Sea shells made of calcium carbonate
  • They lithify very early on
  • Form as rocks at the surface
    • Sea water interacts and sements up the rocks
• Example: Great Barrier Reef
  • Australia very diverse system largely from different corals that as they die those skeletons are deposited and turned into limestone
  • 200 M years ago outside sediments may have been a reef
• Example: The massive White Chalk Cliffs
  • Chalk is a biochemical limestone made up almost entirely of the tiny hard parts of microscopic marine organisms, mainly plankton.
  • Plankton called coccolithonphores made out of calcite (calcium carbonate)
  • Natural system facilitated a lot of carbon stored in the rock record to actually change the climatic condition during that time
    • Climate self regulating naturally

Inorganic Limestone

Presentation content:

• Inorganic limestone forms when chemical changes increase the calcium carbonate content of the water until it precipitates
  • Travertine is a type of limestone found in caves
    • Is precipitated when the water in the cave loses carbon dioxide
  • Oolitic limestone is composed of small spherical grains called ooids
    • Ooids form as tiny “seeds” roll in shallow marine water supersaturated with calcium carbonate

Notes from the lecture:

• Water is saturated with respect to carbonate

Dolostone and Chert

Presentation content:

• Dolostone
  • Similar to limestone but contains magnesium
  • Origin of dolostone is unclear
    • Significant quantities of dolostone are created when magnesium-rich waters circulate through limestone
• Chert
  • Composed of microcrystalline quartz
  • Forms when dissolved silica precipitates
  • Flint, jasper, and agate are varieties of chert

Notes from the lecture:

• Dolostone
  • Take calcite, dissolve it and replace it with magnesium, making a more stable structure, carbonate is stored in the rock record
• Chert
  • Water saturated with respect to silica
  • What you find in petrified wood (this is how it is formed)
    • Water going into the wood and precipitating
  • Latinoamerican civilizations used it for arrowheads

Evaporites

Presentation content:

• Form when restricted seaways become over-saturated and salt deposition starts
• Rock salt and rock gypsum are two common evaporites
• Occasionally, evaporites form on salt flats when dissolved materials are precipitated as a white crust on the ground

7.4 Coal: An Organic Sedimentary Rock

Coal

Presentation content:

• Coal is different from other sedimentary rocks

• Organic sedimentary rocks form from the carbon-rich remains of organisms

• Occasionally, plant structures (leaves, bark, and wood) are identifiable in coal

• Four stages of Coal Formation:

  1. Accumulation of plant remains
  2. Formation of peat and lignite
  3. Formation of bituminous coal
  4. Formation of anthracite coal

Notes from the lecture:

• Because of organic rich matter that has been deposed
• Swamp environment -> Burial -> Compaction -> Greater burial -> Compaction -> Metamorphism
• Anthracite
  • It is the cleanest form -> generates the most amount of energy
• As you go up you generate less amount of energy, because it is not as clean and was not more exposed to heat

7.5 Turning Sediment into Sedimentary Rock: Diagenesis and Lithification

Describe the processes that convert sediment into sedimentary rock and other changes associated with burial.

Sediment after deposition

Presentation content:

• Many changes occur to sediment after it is deposited
  • Diagenesis — chemical, physical, and biological changes that take place after sediments are deposited
    • Occurs within the upper few kilometers of Earth’s crust
    • Examples:
      • Recrystallization of more stable minerals from lass stable ones (e.g., aragonite to calcite)
    • Formation of coal!
  • Lithification — unconsolidated sediments are transformed into solid sedimentary rocks
    • Compaction — as sediments are buried, the weight of the overlying material compresses the deeper sediments
    • Cementation — involves the crystallization of minerals among the individual sediment grains

7.6 Classification of Sedimentary Rocks

Summarize the criteria used to classify sedimentary rocks.

Classification based on type and texture

Presentation content:

• Sedimentary rocks are classified according to the type and texture of material
• Two major groups:
  • Detrital
    • Has clastic texture
    • Composed of discrete fragments cemented together
  • Chemical/organic
    • Has nonclastic or crystalline texture
    • The minerals form patterns of interlocked crystals

7.7 Sedimentary Rocks Represent Past Environments

Distinguish among three broad categories of sedimentary environments and provide an example of each. List several sedimentary structures and explain why these features are useful to geologists.

Sediments and the Environment

Presentation content:

• An environment of deposition or a sedimentary environment is a geographic setting where sediment is accumulating
  • Characterized by combinations of geologic processes and environmental conditions
• Environment determines the types sediments that accumulate (e.g., size and shape)
  • Three broad categories of sedimentary environments:
    • Continental
    • Marine
    • Transitional

Continental Environments

Presentation content:

• Dominated by stream erosion and deposition
  • Streams are the dominant agent of landscape alteration
• Glacial
  • Deposits are typically unsorted mixtures of sediments that range from clay to boulder-sized
• Wind (eolian)
  • Well-sorted, fine sediments

Marine Environments

Presentation content:

• Shallow marine (to about 200 m)
  • Borders the world’s continents
  • Receives huge quantities of terrestrial sediments
  • Warm seas with minimal terrestrial sediments have carbonate-rich muds and debris from coral reefs
• Deep marine (seaward of continental shelves)
  • Primarily fine sediments that accumulate on the ocean floor
  • Turbidity currents — submarine landslides — are the exception

Transitional Environments

Presentation content:

• The shoreline is the transition zone between marine and continental environments
  • Examples include:
    • Beaches
    • Tidal flats
    • Spits, bars, and barrier islands
    • Lagoons
    • Deltas

Sedimentary Facies

Presentation content:

• Different sediments often accumulate in adjacent environments
  • For example, when sand is depositing on a beach, mud is being deposited offshore
• Changes in past environments can be seen when a single layer of sedimentary rock is traced laterally
  • Each unit (facies) possesses a distinctive set of characteristics reflecting the conditions of a particular environment
  • Transitions between different facies are gradual

Sedimentary Structures

Presentation content:

• When present, they provide additional information for interpreting Earth’s history
• Types of sedimentary structures
  • The layers of the sedimentary rocks are called strata or beds
    • Single most common and characteristic feature of sedimentary rocks
  • Bedding planes separate strata
  • Cross-bedding occurs when the layers in the sedimentary rocks are inclined
    • Characteristic of sand dunes, deltas, and some stream deposits

Graded beds

Presentation content:

• Graded beds are a unique situation where the sediments in a strata gradually change from coarse at the bottom to fine at the top
• Often associated with turbidity currents

Ripple marks, mud cracks and fossils

Presentation content:

• Ripple marks are small waves that are lithified in the sedimentary rocks
• Mud cracks indicate sediments form in an alternatively wet and dry environment
• Fossils are the remains of prehistoric life

7.8 The Carbon Cycle and Sedimentary Rocks

Relate weathering processes and sedimentary rocks to the carbon cycle.

Movement of CO2

Presentation content:

• CO2 moving from the atmosphere to the biosphere and back again is one of the most active parts of the carbon cycle
• Plants absorb CO2 through photosynthesis
• When plants die, some CO2 is deposited in sediments
  • Over geologic time, a small amount of CO2 is deposited as sediment—but considerable amounts of plant biomass is converted into fossil fuels
  • When fossil fuels are burned, the CO2 is released back into the atmosphere
  • Volcanoes also release CO2
• Limestone is Earth’s largest repository of carbon

End of Chapter 7 - Concept Checks

7.1 An Introduction to Sedimentary Rocks

1. How does the volume of sedimentary rocks in Earth’s crust compare to the volume of igneous and metamorphic rocks?
2. List two ways in which sedimentary rocks are important.
3. Outline the steps that would transform an exposure of granite in the mountains into various sedimentary rocks.
4. List and briefly describe the differences among the three basic sedimentary rock categories.

7.2 Clastic Sedimentary Rocks

1. What minerals are most abundant in clastic sedimentary rocks? In which rocks do these minerals predominate?
2. What is the primary basis for distinguishing among clastic rocks?
3. Describe how sediments become sorted. What would cause sediments to be poorly sorted?
4. Distinguish between breccia, conglomerate, sandstone, and shale.

7.3 Chemical Sedimentary Rocks

1. Explain how the formation of biochemical sediments differs from the formation of sediments by inorganic processes. Use examples as part of your explanation.
2. Distinguish among limestone, dolostone, and chert. Describe several varieties of each.
3. How do evaporites form? What are some examples?

7.4 Coal: An Organic Sedimentary Rock

1. What is the “raw material” for coal? Under what circumstances does it accumulate?
2. Outline the successive stages in the formation of coal.

7.5 Turning Sediment into Sedimentary Rock: Diagenesis and Lithification

1. If lithos means “stone,” then what is lithification?
2. Compaction is most important as a lithification process with which clastic sediment size?
3. List three common cements of sedimentary clasts. How might each be identified?

7.6 Classification of Sedimentary Rocks

1. What is the primary basis for distinguishing (naming) different chemical sedimentary rocks? How is the naming of clastic rocks different?
2. Distinguish between the texture of clastic and nonclastic sedimentary rock.

7.7 Sedimentary Rocks Represent Past Environments

1. What are the three broad categories of sedimentary environments? List a specific example associated with each category (see Figures 7.25 and 7.26).
2. Why might a single sedimentary layer be made up of different types of sedimentary rock? What term applies to these different parts of such a layer?
3. What is the single most characteristic feature of sedimentary rocks?
4. How might mud cracks and ripple marks be useful clues about the geologic past?

7.8 The Carbon Cycle and Sedimentary Rocks

1. Describe how chemical weathering and the formation of biochemical sediment remove carbon from the atmosphere and store it in the geosphere.
2. Provide an example by which carbon moves from the geosphere to the atmosphere.