GEY 101 - Introductory Geology: Exploring Planet Earth
Sedimentary Rocks

What is a sedimentary rock?

• Sedimentary rocks are products of mechanical and chemical weathering
  • Rocks are eroded and weathering at the surface of the Earth
  • Producing sand (quartz) and clay along with ions such as sodium, calcium, chloride, and carbonate.
  • These sediments are then transported by wind and water and are deposited
  • Lithification then compacts and cements the sediments into Sedimentary Rocks
• They account for about 5 percent (by volume) of Earth’s outer 10 miles
• Contain evidence of past environments
• Provide information about sediment transport
• Often contain fossils

Sedimentary rocks are important for economic considerations because they may contain

• Coal
• Petroleum and natural gas
• Sources of iron, aluminum, and manganese

Turning sediment into rock

Many changes occur to sediment after it is deposited

• Diagenesis – all of the chemical, physical, and biological changes that take place after sediments are deposited
• Occurs within the upper few kilometers of Earth’s crust

Diagenesis

Includes

• Recrystallization – development of more stable minerals from less stable ones
• Lithification – unconsolidated sediments are transformed into solid sedimentary rock by compaction and cementation
  Natural cements include calcite, silica, and iron oxide

Types of sedimentary rocks

• Sediment originates from mechanical and/or chemical weathering
• Rock types are based on the source of the material
  • Detrital rocks – transported sediment as solid particles
  • Chemical rocks – sediment that was once in solution

Detrital sedimentary rocks

The chief constituents of detrital rocks include

• Clay minerals
• Quartz
• Feldspars
• Micas

Common detrital sedimentary rocks (in order of increasing particle size)

• Shale
  • Mud-sized particles in thin layers that are commonly referred to as laminea
  • Most common sedimentary rock
  • Shale containing plant remains
• Sandstone
  • Composed of sand-sized particles
  • Forms in a variety of environments
  • Sorting, shape, and composition of the grains can be used to interpret the rock’s history
  • Quartz is the predominant mineral
• Conglomerate and breccia
  • Both are composed of particles greater than 2mm in diameter
  • Conglomerate consists largely of rounded gravels
  • Breccia is composed mainly of large angular particles

Chemical sedimentary rocks

Consist of precipitated material that was once in solution

Precipitation of material occurs in two ways

• Inorganic processes
• Organic processes (biochemical origin)

Common chemical sedimentary rocks

• Limestone
  • Most abundant chemical rock
  • Composed chiefly of the mineral calcite
  • Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)
  • Inorganic limestones include travertine and oolitic limestone
• Dolostone - Typically formed secondarily from limestone
• Chert
  • Made of microcrystalline quartz
  • Varieties include flint and jasper (banded form is called agate) Chemical sedimentary rocks
• Evaporites
  • Evaporation triggers deposition of chemical precipitates
  • Examples include rock salt and rock gypsum
• Coal - Different from other rocks because it is composed of organic material
  Stages in coal formation (in order)
  1. Plant material
  2. Peat
  3. Lignite
  4. Bituminous

Classification of sedimentary rocks

Sedimentary rocks are classified according to the type of material

Two major groups

Detrital
Chemical

Two major textures are used in the classification of sedimentary rocks

Clastic
• Discrete fragments and particles
• All detrital rocks have a clastic texture
Nonclastic
• Pattern of interlocking crystals
• May resemble an igneous rock

Sedimentary environments

A geographic setting where sediment is accumulating.
Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)

Types of sedimentary environments

• Continental
  • Dominated by erosion and deposition associated with streams
  • Glacial
  • Wind (eolian)
• Marine
  • Shallow (to about 200 meters)
  • Deep (seaward of continental shelves)
• Transitional (shoreline)
  • Tidal flats
  • Lagoons
  • Deltas

Sedimentary facies Different sediments often accumulate adjacent to one another at the same time.
Each unit (called a facies) possesses a distinctive set of characteristics reflecting the conditions in a particular environment.
The merging of adjacent facies tends to be a gradual transition.

Sedimentary structures

Provide information useful in the interpretation of Earth history.

Types of sedimentary structures

• Strata, or beds (most characteristic of sedimentary rocks)
• Bedding planes that separate strata, flat surfaces
• Cross-bedding – sediments deposited at inclined to the horizon
• Graded beds - Within the sedimentary layer the grain sizes change from coarse (on the bottom) to fine (at the top)
• Ripple marks - Small waves of sand that form at the surface from either wind or water
• Mud cracks - Formed in a wet and dry environment

Fossils: Evidence of past life

By definition, fossils are the traces or remains of prehistoric life now preserved in rock.

Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)

Geologically fossils are important for several reasons

• Aid in interpretation of the geologic past
• Serve as important time indicators
• Allow for correlation of rocks from different places

Additional Study Guide

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