Chapter 7: Sediments and Sedimentary Rocks: Archives of Earth’s History.

Chapter 7: Sediments and Sedimentary Rocks:

Archives of Earth’s History

Introduction: 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.

Sedimentation, Stratification, and Bedding

Stratification, or bedding, is the arrangement of sedimentary particles in layers.

Each stratum (plural = strata) or bed is a distinct later of sediment.

The top or bottom surface of a bed is a bedding plane.

Figure 7.1

Sediments Types And Characteristics

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

produced by physical weathering. Sand and clay.

– Chemical sediment precipitates from solution in water.

Calcium carbonate and salt.

– Biogenic sediment is composed of the fossilized remains of plants or animals.

Coal, oil, and natural gas.

Figure B7.1

Characteristics of Clastic Sediment (1)

Clastic sediment is divided into four main size classes:– Gravel.– Sand.– Silt.– Clay.

Characteristics of Clastic Sediment (2)

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

Sorting (1)

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

Figure 7.3

Figure 7.4

Sorting (2)

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

Unusually dense minerals (e.g. gold, platinum,magnetite) are deposited first when stream velocity slows.

Lighter particles are carried onward.

Figure 7.5

Sorting (3)

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.

Figure 7.6

Varves

Rhythmic layering occurs in varves.– A varve is a pair of sedimentary layers deposited over

the cycle of a single year.– In spring, the inflow of sediment laden water

increases and coarse sediment is then deposited throughout the summer.

– During winter, very fine sediment slowly settles to form a thinner, darker layer.

Figure 7.7

Rhythmic Lamination and Cross Bedding

Rhythmic laminations may result from a succession of large storms.

Cross bedding creates beds that are inclined with respect to a thicker stratum within which they occur.– Cross bedding is the work of turbulent flow in the

transporting agency.

Graded Bedding

In graded bedding, the particles are sorted more or less according to size, grading upward from coarser to finer.

Most coarse clastic sediments consist of mineral grains and rock least susceptible to chemical and physical breakdown.– High content of quartz and potassium feldspar.

Figure 7.9

Figure 7.10

Characteristics of Chemical Sediments

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).

Salts (1)

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).

Salts (2)

Lake water precipitates are used in the production of:

Paper.Soap.Detergents. Antiseptics.Tanning and dyeing.

Salts (3)

The most important salts that precipitate from seawater are halite (rock salt: NaCI) and gypsum (CaSO4.2H2O).

Much of the common table salt we use daily and the gypsum used for plaster and construction materials are recovered from marine evaporate deposits.

Characteristics of Biogenic Sediments (1)

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

bioclastic sediment. When floating microscopic marine organisms die,

their remains settle and accumulate on the seafloor to form a muddy sediment called deep-sea ooze.

Characteristics of Biogenic Sediments (2)

Siliceous ooze resembles calcareous ooze, but is composed mainly of the siliceous remains of tiny floating protozoa (radiolarians) and algae (diatoms).– Calcareous ooze is a fine-grained, deep-sea deposit of

skeletal material containing more than 30% calcium carbonate.

– Siliceous ooze is made of skeletal material made mainly of silica.

Sedimentary Environments And Facies Changes

Vertical changes in strata reflect the passage of time.

Horizontal strata change over distance, but the changes occur within the same stratum.– Such a lateral change from one depositional environment

to another is called a change of facies.

Each environment has distinctive physical, chemical, and biological characteristics.

Figure 7.14

Figure 7.15

Nonmarine Depositional Environments: Streams

Stream sediments.– Widespread.– The type depends on:

The energy available for doing work.The nature of the sedimentary load.

Nonmarine Depositional Environments: Lakes

Lake sediments.– Sediments deposited in a lake accumulate both on the

lakeshore and on the lake floor.– Sand and gravel form beaches, bars, and spits across

the mouths of bays.– Finer sediment forms a delta.

Nonmarine Depositional Environments: Glaciers

Glacial sediments. – Sedimentary debris eroded and transported by a

glacier. – Random mixture of particles that range in size from

clay to boulders.– Stones in glacially deposited sediment often are

angular and some are striated;

Nonmarine Depositional Environments: Wind

Eolian (windblown) sediments.– Sediments carried by the wind tends to be finer than

that moved by other erosional agents.– Sand may pile up to form dunes. – When powdery dust picked up and moved by the

wind is deposited, it becomes progressively thinner and finer with increasing distance downwind.

Shoreline and Continental Shelf Environments (1)

Deltaic sediments. Lakes and marine deltas.

Estuarine sediments Sediments trapped in an estuary. When the stream water meets seawater, the clay particles tend to

aggregate into clumps.

Beach sediments. Quartz is typical of beach sediments.


Offshore sediments.– Fine grained sediment, carried in suspension, settle

slowly to the seafloor.– Only about 10 percent of the sediment reaching the

continental shelves remains in suspension long enough to arrive in the deep sea.

Carbonate shelves.– Carbonate sediments accumulate mainly on broad,

flat carbonate shelves that border a continent.

Figure 7.16


Marine evaporate basins.– Ocean water occupying a basin with restricted

circulation that lies in a region of very warm, dry climate will evaporate.

Deep-sea fans.– Some large submarine canyons on the continental

slopes are aligned with the mouths of major rivers, like the Amazon, Congo, Ganges, and Indus.

At the base of many such canyons is a huge deep-sea fan.

Figure 7.19

Figure 7.17

Figure 7.18

Figure 7.20


Sediments drifts;– Huge bodies of sediments up to hundreds of

kilometers long, tens of kilometers wide, and two kilometers high have been discovered along the continental margins bordering the North Atlantic ocean and Antarctica.

– An exceptional rate of deposition, 200 cm/1000 years, has been measured for a sediment drift near Bermuda.

– Cores obtained from sediment drifts can provide a record of environmental change measurable at a decadal, rather than millennial, scale.

Deep-Sea Depositional Environments (1)

Deep-sea oozes.– Calcareous ooze occurs over wide areas of ocean floor

at low to middle latitudes where warm sea surface temperatures favor the growth of carbonate-secreting organisms in the surface water.

Calcareous ooze is rare where the water is deeper than 3-4 km.

Figure 7.21


Cold deep-ocean waters are under high pressure and contain more dissolved carbon dioxide than shallower waters.– At a level called the carbonate compensation depth, the

shells are dissolved, before they can reach the seafloor.


Land-derived sediment.– Deep-sea strata include sediments carried to the ocean

by:Rivers.Wave action.Wind (fine desert dust and volcanic ash), Floating ice.

Diagenesis: How Sediment Becomes Rock (1)

Lithification is the overall process of creating sedimentary rock.

Diagenesis is the collective term for all the chemical, physical, and biological changes that affect sediment as it goes from deposition through lithification.


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.

Figure 7.22


– Recrystalization. As sediment accumulates, less stable minerals recrystallize into

more stable forms. – The mineral aragonite in the skeleton of living corals recrystalizes

to its polymorph calcite.

– Chemical alteration. In the presence of oxygen, organic remains are quickly converted

to carbon dioxide and water. If oxygen is lacking (reducing or anaerobic environment), the

organic matter does not completely decay but instead may be slowly transformed to solid carbon.

Formation of Peat: Diagenesis Under Anaerobic Conditions

In stagnant swamp water, oxygen is used up and not replenished, creating an anaerobic environment (lacking air or free oxygen).

The oxygen and hydrogen (combined in gases, such as methane) escape, and the carbon gradually becomes concentrated in the residue.

In this manner, plant matter is gradually converted to peat.

Clastic Sedimentary Rocks

The four basic classes of clastic sedimentary rocks are:– Conglomerate, 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

Figure 7.24D

Figure 7.24E

Figure 7.24F

Chemical Sedimentary Rocks (1)

Chemical sedimentary rocks result from lithification of organic or inorganic chemical precipitates.– Evaporate deposits: salt domes.– Banded iron deposits.

Some of the Earth’s most important iron concentrations are in sedimentary rocks that formed billions of years ago.

Every aspect of banded iron deposits indicates chemical precipitation.

Figure 7.25

Chemical Sedimentary Rocks (2)

– Phosphorus deposits. A primary source of fertilizer. Form through the precipitation of apatite from seawater.

– Phosphorus rich sediments are forming today off the western coast of Africa and South Americ.,

– Extensive marine phosphates are found in Florida (world’s highest annual production).

– Chert is a hard, very compact sedimentary rock composed almost entirely of very fine-grained, interlocking quartz crystal (useful to early cultures for tool production).

Figure 7.26

Biogenic Sedimentary Rocks (1)

Limestone.The most important biogenic rock.One coarse-grained type, composed entirely of shelly

debris is called coquina.Reef limestone consists of cemented reef organisms.Chalk is composed of the compacted carbonate shells

of minute floating organisms. Limestone accounts for a major proportion of the

carbon dioxide stored in the Earth’ crust.

Figure 7.27A

Figure 7.27B

Figure 7.27C

Figure 7.27D


Coal.– Plant remains accumulate to form peat (carbon

content of about 60 percent).– Peat is compressed (causing water loss and generation

of organic gases such as methane CH4).– The peat is thereby converted into lignite.

The luxuriant plant growth needed to form thick, extensive coal seams required tropical or subtropical climate.

Coal deposits that we find today in the frigid lands of northern Alaska and Antarctica must have formed in warm, relatively low-latitude environments.


Oil shale.– An organic-rich clastic rock.– Dead organisms buried in marine or lake muds contain

organic oils and fats.– An alteration process occurs in which wax-like

substances are formed (oil shale).– Extensive oil shales occur in Alberta, Colorado, Utah,

and Wyoming.

Environmental clues in sedimentary rocks (1)

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 from fossils.– Some animals and plants are restricted to warm, moist

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

Figure 7.30

Figure 7.31


Clues from rock color.– Most dark colors are the result of iron sulfides and

organic detritus buried with sediment.– Reddish and brownish colors result mainly from the

presence of iron oxides.


Clues from isotopes and magnetic properties.– The ratio of two isotopes of oxygen, oxygen-18 and

oxygen-16, can provide important information about changing climate.

– Magnetic measurements in sediments and sedimentary rocks permit us to reconstruct changes in the Earth’s magnetic field through time.

Limestone, CO2, And Global Climate Change (1)

Carbon dioxide is one of the most important greenhouse gases.

Periods of high CO2, well above today’s value, occurred in intervals from about 550-390 and 180-90 million years ago.

These intervals, corresponding to much of the early Paleozoic and late Mesozoic eras, are well known for their extensive limestone formation.

Limestone, CO2, And Global Climate Change (2)

The earlier period followed an important interval of cold climate about 600 million years ago that coincided with low atmospheric CO2 values and important glaciation.

Figure 7.34

Chapter 7: Sediments and Sedimentary Rocks: Archives of Earth’s History.

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Transcript of Chapter 7: Sediments and Sedimentary Rocks: Archives of Earth’s History.