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Rocks: Earth’s Rocks 2:Sedimentary and Metamorphic Environments

Sedimentary Rocks

Sedimentary rocks are formed by the accumulation and hardening of sediment.

Three kinds of sediment:

Clastic sediment - consisting of particles (clasts) derived from pre-existing rocks (e.g. sand)

Chemical sediment - consisting of mineral matter precipitated from a solution (e.g. salt)

Biogenic sediment- consisting of materials produced by organisms (e.g. shells, bone, teeth, leaves, wood, etc.)

Clastic Sediment:Weathering

When exposed at Earth’s surface, rocks are broken down by processes of weathering (due to dissolution and physical breakdown)

Mechanical weathering: physical breakup or disintegration of rocks without changes in their composition. This is accomplished mainly by physical agents such as water, wind and ice but can be aided by biological factors (e.g. widening ofcracks in bedrock by tree roots).

Chemical weathering: breakdown or decomposition of minerals due to chemical reaction of minerals with water or gases in the air.

Clastic Sediment:Mechanical Weathering

Mechanical weathering basically serves to break rock into smaller particles.

In areas where the degree of chemical weathering is very low (e.g. in cold, dry, regions of the Canadian Arctic), clastic sediment can consist almost entirely of small fragments of rock with no change in mineral makeup.

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Clastic Sediment:Chemical Weathering

Some minerals are more susceptible to weathering than others.

For minerals in igneous rocks, resistance to weathering basically follows the reverse trend of same trend as the order of crystallization in Bowen’s Reaction Series.

This is because high-temperature minerals are less stable at Earth’s surface than low-temperature minerals.

Note that Earth’s surface is a lot cooler than the environments in which minerals form from magma; low-temperature minerals are most stable at Earth’s surface.

Bowen’s Reaction Series Revisited

Note that quartz, muscovite mica and potassium feldspar (orthoclase) are formed at a much lower temperature than the ferromagnesian minerals (olivine, pyroxene, amphibole, biotite)..

Thus the light-coloured minerals are more stable at Earth’s surface than the ferromagnesian minerals and are more resistant to weathering

This is why most beach sand is mostly quartz (it is most resistant to weathering).

Products of Chemical Weathering

Chemical weathering produces a number of minerals and free ions.

Solid components are left as a residue (becoming sediment)

The remaining material is dissolved in water, in the form of ions.

For example:

carbonic acid(in water) attacks feldspar

Residual product:Kaolinite (clay)

Produces… + potassium (K+)and

silica (SiO44- )

Ions in solution This block of dolostone on the top of the Niagara Escarpment is being split apart by roots of cedar trees.

Biologically-aided weathering

Living things can also influence weathering.

For example, plant roots can widen cracks in the rock.

Also, the decomposition of plant matter can produce organic acids that aid in chemical weathering.

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Clastic Sediment:Transportation

Once dislodged (eroded), sedimentary particles can be transported away from their source area by:

WaterRivers

WindSand/Dust Storms

GravityRockslidesMudflows

Etc.

IceGlaciers

Clastic Sediment: Sedimentary Structures

Some physical structures in clastic sedimentary rocks can also provide information about how sediment was deposited. For example:

Ripple marks(produced by wave ripples)

Hummocky crossbedding(produced by storm waves)

Crossbedding(produced by migrating dunes in current)

Mudcracks(indicating wetting and drying on a mudflat)

Clastic Sediment:Deposition

Sedimentary particles ultimately come to rest once the transporting medium can no longer carry them.

As a general rule, smaller/lighter particles are deposited in less-agitated conditions than larger/heavier particles when transported by wind or water. Sedimentary particles can therefore experience some degree of sorting. Beach sand tends to be well sorted, due to constant wave action.

Sedimentary particles transported and deposited rapidly by events such as mudflows tend be poorly sorted.

beach

Well-sortedsediment

mudflow

Poorly sortedsediment

Depositional Environments

Clastic sediments can be deposited in a wide variety of settings-characteristics of clasticsedimentary rocks can provide information on where their constituent sediments were originally deposited.

We will look at some of these environments in greater detail as the term progresses.

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Sediment to Sedimentary Rock: Lithification

Once buried, sediment undergoes changes that transform it into rock. This transformation is called lithification (lithos = stone)

Compaction: As sediment layers are buried to deeper and deeper levels under successive sediment layers, sedimentary particles are squeezed together as the spaces between them decrease in size. The material thus becomes more rock-like.

Cementation: Sediment grains can also become cemented together by minerals that precipitate from water remaining in the pore spaces between the grains. Grains therefore effectively become glued together.

Common Clastic Sedimentary RocksClastic sedimentary rocks are classified primarily by grain size.

Clay(particles too small to be seen under low magnification)

Silt(particles barely discernible under low magnification)

Sand(sand-sized particles that are obviously gritty)

Gravel (pebbles, cobbles, boulders)

Rock TypeSediment type

Shale

Siltstone

Conglomerate

Sandstone

Protoceratops and Velociraptor preserved “locked in battle” in sandstone (apparently buried by windblown sand during a sandstorm or buried by sand that avalanched down the side of a sand dune)

Fossil preservation related to sediment transport and deposition by wind (or perhaps mass wasting ?)

Fossil preservation associated with sediment transport and deposition by rivers.

Ancient river deposits (mainly sandstone) in Dinosaur Provincial Park, Alberta

Centrosaurusbonebed in the river-deposited sandstone

Centrosaurus

The huge number of Centrosaurus bones deposited here (which are oriented roughly parallel to one another, thus indicating current flow after the death of the animal) are interpreted to represent a herd of Centrosaurus that attempted to cross a wide river, but ultimately drowned.

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Chemical Sediment

Water on Earth’s surface contains dissolved ions (electrically charged atoms or groups of atoms of various elements) principally derived from weathered minerals and volcanic gases.

Dissolved Components in Seawater (Percent by Weight)

Positive IonsSodium (Na+): 30.61 %Magnesium (Mg 2+ ): 3.69 %Calcium (Ca2+): 1.16 %Potassium (K+): 1.10 %

Negative IonsChloride (Cl-): 55.04 %Sulphate (SO4

2-): 7.68 %

Others: 0.72 %Include trace amounts of metals(gold, silver, zinc, lead, copper, etc.)

Due to evaporation, dissolved ions can become too concentrated for the water to hold, so positive and negative ions join together and precipitate as minerals.

Water highly concentrated in salt (in isolated basin or on mud flat) cannot escape back to the ocean, and becomes further concentrated to the point that “evaporite minerals” are deposited.

Chemical Sedimentary Rocks: Evaporites

Salt flat in California

Two common minerals in evaporite deposits are:

Gypsum (calcium sulphate) and Halite (sodium chloride).

In some cases, rocks can be composed exclusively of one of theseminerals.

A rock formed exclusively of gypsum is called “rock gypsum”

A rock formed exclusively of halite is called rock salt

480 million years ago 430 million years ago 400 million years ago

Southern Ontario indicated by red dot !

Salt deposits in southern Ontario (e.g. Goderich, Windsor) were produced under arid, tropical, conditions when southern Ontario was south of the equator and covered by a shallow inland sea that was somewhat restricted from the open ocean

Salts deposited from concentratedSeawater in inland sea

Southern Ontario occupied dry land

Drying up of sea, again exposing southern Ontario on dry land

Ancient Example of Salt Deposition: Extensive Salt Deposits in Ontario

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Chemical Sedimentary Rocks: Chemical Limestones

Some kinds of limestone form as chemical sedimentary rocks

Travertine forms in caves and hot springs.

A travertine ample Travertine stalactites on the ceiling of a cave

Biogenic Sediment

Biochemical sediment consists of materials that are produced by chemical processes associated with biological activity.So one can think of biochemical sediment as being chemical sediment formed through biological activities.

Biogenic components include:

BonesShells Teeth Plant Remains

Biogenic Sedimentary Rocks

Two common rocks that are formed from biogenic sediment are fossiliferous limestone, chert, and coal.

Fossiliferous limestone is composed almost entirely of calcite shells (skeletons) of organisms. In many cases, the identity of the former owners of skeletal material can be identified.

This piece of limestone contains broken “stem” segments of animal called a crinoid.

Biogenic Sedimentary Rocks

A familiar rock that, in a loose sense, can also be considered a variety of fossiliferous limestone is chalk, which is made of microscopic skeletons of algae.

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Other Fossils in Chalk

Tylosaurus, a mosasaur (marine lizard), preserved in the Smoky Hill Chalk, Kansas

Reefs

Fossil coral reef(about 400 million years old)

Coral reef(present day)

Chert is another common biochemical sedimentary rock.

A few organisms, such as some microscopic planktonicorganisms (as well as some sponges), have a skeleton made of silica. The silica can dissolve and form a “gel” on the seafloor. When this gel solidifies, it forms a finely crystalline rock called chert. As it is basically made of very fine-grained quartz crystals, it is very hard.

Chert

Silica skeletons of planktonic organisms

Coal is a special type of biogenic sedimentary rock that is largely composed of organic matter from plants. Coal seams represent large accumulations of organic matter that were deposited in swamps and were subsequently buried.

swamp coal seam

coal sample

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Metamorphic Rocks

Metamorphic rocks are formed by the transformation of pre-existing rocks under the influence of high temperatures and pressures and chemically active fluids

Three basic types of metamorphism:

Contact metamorphism

Regional metamorphism

Metasomatism

Contact metamorphism: occurs when a pre-existing rock is baked under relatively low pressures

Commonly occurs when rock is heated by igneous intrusion, forming a metamorphic halo or “aureole” in the adjacent rock – generally local in extent

Mineral grains recrystallize in random orientations (because pressure is uniform). Overall composition of the rock basically remains the same.

Regional metamorphism: occurs when pre-existing rock is subjected to heat and pressure on a regional scale. Commonly associated with mountain building events in which rocksare lowered to great depths and squeezed by compressive forces.

Regional Metamorphism: Foliation

Regional metamorphism generally occurs in areas where two lithospheric plates are pressing against one another, rocks are subjected to differential stress.

In response to this stress, platy/elongate minerals line up to produce a foliated texture (folium = leaf)

Platy mineral orientation in contact metamorphism(random)

Platy mineral orientation in regional metamorphism(aligned)

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Common Metamorphic Rocks

Non-foliated rocks

Some metamorphic rocks, whether formed by contact or regional metamorphism always have a non-foliated (also called granoblastic) texture. This is because they lack platy/elongate minerals required to define foliation.

Such rocks include quartzite (formed via metamorphism of quartz sandstone), and marble (formed via metamorphism of limestone).

In both cases, mineral form an interlocking texture. Fossils once present in the original sedimentary rock are obliterated due to this recrystallization.

Common non-foliated (granoblastic) metamorphic rocks

Heat (+ pressure)

Heat (+ pressure)

Limestone Marble

Quartz sandstone Quartzite

Common Foliated Metamorphic Rocks

Foliated rocks

Rocks containing platy/elongate minerals that subjected to regional metamorphism (and therefore) affected by differential pressure are typically foliated.

Increasing intensity of metamorphism (called “grade”) results in increased size of mineral grains, and the development of distinct types of foliation.

With increasing metamorphism, shale changes into the following rock types:

Shale Slate Phyllite Schist Gneiss

Example: Parent Rock: Shale (Sedimentary Rock)

Shale is “earthy-looking” and commonly contains fossilsPrimary components are grains of clay minerals and quartz

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Very low metamorphic grade (little increase in temperature and pressure)

Slate

Rock becomes harder and more compactDull to very slight sheenCleaves into plates (foliation called slaty cleavage)

Low to Medium metamorphic grade(moderately high temperature and pressure)

Phyllite

Rock attains silky sheen (due to growth of mica from clay)May have slightly wrinkled surfacesHas phyllitic foliation

Medium to high metamorphic gradehigh temperature and pressure)

Schist

Rock has sparkles due to large, aligned mica grains Mica grains may wrap around more spherical crystals of minerals (porphyroblasts) such as garnetHas schistose foliation

Very high metamorphic gradeVery high temperature and pressure

Gneiss

Rocks has distinct dark and light bandsPlaty minerals in dark bands are alignedHas gneissic foliation

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Common foliated rocks (showing relationship to depth)

Increasing temperature and pressure)

Slate: very slight sheen due to growth of platy mica grains

Phyllite: distinct sheen due to continued growth of platy mica grains

Schist: very sparkly appearance due to large mica grains

Gneiss: banded appearance due to separation of dark-and light-colouredminerals

Metasomatism: occurs when fluids (generally water or carbon dioxide) react with a pre-existing rock and alter the chemical composition of minerals within the rock. In some cases, the fluid itself is involved. In others, substances dissolved in the fluid are involved.

For example, the mineral olivine (which occurs in blocky crystals) reacts with water to form the mineral serpentine (with platy to fibrous crystals).

2Mg2SiO4 + 2H2O → Mg3Si2O5(OH)4 + MgOolivine water serpentine (in solution)

Fossils in Metamorphic Rock ?

As stated earlier, the vast majority of fossils are found in sedimentary rocks – if you find a rock with a fossil in it, you can be nearly certain that it is sedimentary.

Fossils are very rarely found in metamorphic rocks due to mineral recrystallization and intense deformation, which obscure details. Metamorphic rocks that do contain fossils have experienced very low grades of metamorphism, and are commonly deformed.

Imprint of soft-bodied animal preserved in quartzite

Clam (with splayed valves) preserved in slate

END OF LECTURE