Chapter 7: Sediments and Chapter 7: Sediments and Sedimentary Rocks Sedimentary Rocks

Sedimentary RocksSedimentary RocksTwo Categories: Two Categories:

Clastic(Detrital) and ChemicalClastic(Detrital) and Chemical

Sedimentary RocksSedimentary Rocks• Clastic ( or detrital) sedimentary rocks reflect

several processes. – Weathering – Generation of detritus via rock

disintegration.– Erosion – Removal of sediment grains from rock.– Transportation – Dispersal by wind, water and ice.– Deposition – Accumulation after cessation of

transport. – Lithification – Transformation into solid rock.

• Burial – Continued deposition buries sediments. • Compaction – Squeezing by the weight overlying material.• Cementation – Grains “glued” by pore-filling minerals.

Sediments In The Rock Cycle Sediments In The Rock Cycle

• Sediments are transported by:– Water.– Ice.– Wind.– Gravity.

• The transport and deposition of sediments are initial phases of the rock cycle.

• The ultimate fate of most sediment is burial and conversion to sedimentary rock.

Sediments Types And Sediments Types And Characteristics Characteristics

• Three broad classes:– Clastic sediment is loose fragments of rock

debris produced by physical weathering. – Chemical sediment precipitates from solution in

water.– Biochemical/Biogenic/Bioclastic sediment is

composed of the fossilized remains of plants or animals.

Characteristics of Clastic Characteristics of Clastic Sediment (1)Sediment (1)

• Clastic sediment is divided into four main size classes:– Gravel and larger ( 2mm and larger)– Sand (0.0625 to 2 mm)– Silt (0.0625 to 0.0039 mm)– Clay (less than 0.0039 mm)

Characteristics of Clastic Characteristics of Clastic Sediment (2)Sediment (2)

• Gravel is subdivided into:– Boulder gravel.– Cobble gravel.– Pebble gravel.

SortingSorting

• Sorting identifies sediment in terms of the variability in the size of its particles.– Poorly sorted (wide range of particle size),– Well sorted (range is small).

• Changes of grain size typically result from fluctuations in the velocity of the transporting agent, such as water or wind.

Figure 7.2 Sorting in Sediments

Figure 7.4 Sorting, rounding and sphericity

SortingSorting

• The greater the speed and energy involved, the larger or heavier are the particles that can be transported.

Sorting Sorting • Nonsorted sediment is a mixture of

different sizes arranged chaotically.– Till is a nonsorted sediment of glacial origin.

• Smooth and rounded particles are transported by water or air.

TransportTransport• Sediments move by air, wind, rain, water,

organisms - With increasing transport, average grain size

decreases.

Sedimentary RocksSedimentary RocksChemical/BiochemicalChemical/Biochemical

Characteristics of Chemical Characteristics of Chemical SedimentsSediments

• Chemical sediments are formed by precipitation of minerals from solution in water.

• They form in two principal ways:– Through biochemical reactions resulting from the

activity of plants and animals in the water.– Inorganic reactions in the water.

• When water from a hot spring cools, it may precipitate opal (a hydrated silicate) or calcite (calcium carbonate).

Some Chemical Sedimentary Some Chemical Sedimentary RocksRocks

SaltsSalts• Evaporation of sea water or lake water

forms salts.• Lake waters precipitate sodium

carbonate (Na2CO3), sodium sulfate Na2SO4), borax (Na2B4O7.10H2O),and trona (Na2CO3.NaHCO3.2H2O).

Biochemical/Biogenic Biochemical/Biogenic SedimentsSediments

• Biogenic sediments contain fossils.• If the remains are broken and scattered it

is bioclastic sediment.

Depositional EnvironmentsDepositional Environments

Sedimentary or Depostional Sedimentary or Depostional EnvironmentsEnvironments

• Each environment will have distinctive physical, chemical, and biological characteristics.

Figure 7.14

Sediments to RocksSediments to Rocks

How do we do this?How do we do this?

How Sediment Becomes Rock How Sediment Becomes Rock • Lithification is the overall process of

creating sedimentary rock.

How Sediment Becomes RockHow Sediment Becomes Rock

• Processes involved include:– Compaction.

• As the weight of an accumulating sediment forces the grains together. The pore space is reduced.

– Cementation.• Substances dissolved in water precipitate to form a

cement that binds the sediment grains together.

What do we call these What do we call these rocks?rocks?

Clastic Sedimentary RocksClastic Sedimentary Rocks

• The four basic classes of clastic sedimentary rocks are:– Conglomerate/Breccia, a lithified gravel.– Sandstone, consisting mainly of sand grains.– Siltstone, composed mainly of silt-size mineral

fragments, commonly quartz and feldspar.– Mudstone, still-finer grain size.

Figure 7.24A

Figure 7.24B

Figure 7.24C

Chemical Sedimentary RocksChemical Sedimentary Rocks

• Chemical sedimentary rocks result from Lithification of organic or inorganic chemical precipitates.

Figure 7.27B

Figure 7.27C

Rock Salt

Rock Gypsum

Figure 7.27D

Chert

Sedimentary StructuresSedimentary Structures

Clues to the EarthClues to the Earth

Environmental clues in Environmental clues in sedimentary rocks sedimentary rocks

• Sedimentary rocks reflect the environment in which they were formed.

• Clues from bedding planes.– Ripples marks preserved in sandstones and

siltstones.– Footprints, trails, and raindrop impressions.

Clues in the RocksClues in the Rocks

• Clues from fossils.– Some animals and plants are restricted to

warm, moist climates, whereas others are associated only with cold, dry climates.

Figure 7.30 Ripples

Figure 7.31 Mud cracks

Crossbedding

Layering and fining upward

Tilted Bedding or strata

Trace Fossil

Trace Fossil from Mt Cristo Rey

Where do we go next?Where do we go next?

• Metamorphic Rocks