Lecture 06 7 sed n met rx k

Mud: small particles easily kept in suspension – Settles in quiet water mm deposited in thin bedding layers called laminae Most common sedimentary rock 2.. Chemical sedimentary rockswa

Part 1 Sedimentary Rocks

Kyanite, Sillimanite, and Andalucite

Lecture 6-7 Sedimentary and Metamorphic Rocks

What is a sedimentary rock?

• Sedimentary rocks result from mechanical

and chemical weathering

• Comprise ~ 5% of Earth’s upper crust

• About 75% of rocks at surface

• Contain evidence of past environments

What is the economic importance of

sedimentary rocks?

• They are important for economic reasons

because they contain

• Precipitation of chemicals dissolved in water

binds grains of a sediment together.

• After the cements solidify, compaction drives

out the excess water.

• Important part of Lithification

• Remember where cements come from?

Types of sedimentary rocks

ions that were once in solution

transported as solid particles

Detrital sedimentary rocks

• Constituents of detrital rocks can

• Particle size is used to distinguish

among the various types of detrital rocks

Detrital sedimentary rocks

• Mudrocks: less than 063 mm

–1 Mud: small particles easily kept in suspension

– Settles in quiet water

mm deposited in thin bedding layers called laminae

Most common sedimentary rock

2 Larger mudrock grains called silts

silt-sized particles 004-.063 mm Gritty grains can be felt

Detrital sedimentary rocks

– Made of sand-sized particles larger

than 063 mm and less than 2mm

– Forms in a variety of environments – Sorting, angularity and composition

of grains can be used to interpret the rock’s history

– Quartz is the predominant mineral

(due to its durable nature)

Detrital sedimentary

rocks

• Conglomerate and breccia

–Both composed of particles > 2mm in

diameter

–Conglomerate consists largely of rounded clasts Rounded pebbles in high velocity areas

–Breccia is composed of large angular

particles Breccia is made of shattered rock that accumulates at the base of a cliff

• Coarse sediments are deposited in high

energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams.

• Fine sediments are deposited in low

energy environments, e.g the slow water

of deep lagoons, the abyssal plain, etc.

Chemical sedimentary rocks

ways:

• Inorganic processes: the minerals

precipitate out of water

• Organic processes: animals and plants

precipitate the minerals to use as shells

or skeletons

http://www.ucmp.berkeley.edu/collections/micro.html

• Common chemical sedimentary rocks

–Most abundant chemical rock –Made of the mineral calcite CaCO3–Marine biochemical limestones form

as coral reefs, coquina (broken shells), and chalk (microscopic organisms)

–Inorganic limestones include

travertine (caves) and oolitic limestone ( Bahamas )

• Common chemical sedimentary rocks

–Evaporation triggers

deposition of chemical precipitates

–Examples include rock salt

and rock gypsum

http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg

Chemical Sediments: Coal

Particles are large and

irregular, and consist of

Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz.

Character of detrital sediments depends on time, distance, and energy For example, in streams:

Sedimentary environment determines roundness sorting, mineral diversity

Floods change the local

conditions

Waning flow

Bounders on bottom, sands and muds suspended

Graded bedding

Fine-grained above

• Sedimentary Facies

next to each other at same time

a distinctive characteristics reflecting the conditions in a particular environment

tends to be a gradual transition

Nearshore sands Stillwater muds

Abyssal Ooze

Some Facies in an oversimplified drawing

Strata- Bedding Planes

Slabs of eroding sandstone with ripple marks

Cross Beds are ripples in cross section

• Irregularities lead to ripples, dunes,

sand bars.

• In cross section these look like lines

at an angle to the horizontal – “cross beds”

• Ripples can indicate direction of air or

water flow if asymmetrical, a tidal

environment if symmetrical Size and shape indicate fluid velocity

Cross bedding in Sand Dune deposits

Sandstone deposited

in ancient sand dunes

Frosted Grains, well sorted

Navaho Sandstone

Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood Useful for right-side up

Continental shelf

Continental slope

Shallow

marine

Deep marine

Submarine volcanoes

Terms for Marine (i.e Ocean) Environments

and some characteristic sediment facies

Abyssal Plain

Define Graded Beds

Fossils are traces of prehistoric life generally

preserved in sedimentary rock

Dinosaur footprint in mudstone

End of Sedimentary

Rocks

Part 2 Metamorphism and Metamorphic Rocks

Metamorphism

temperatures (heat) and pressure

transformation of:

• Sedimentary and Igneous rocks, and by the

further alteration of other metamorphic rocks

minerals

– Talc (lubricant, insulators, refractories), Corundum, Garnet (abrasives), Kyanite

(ceramics), Micas (insulators), Chrysotile

(“asbestos” for fireproofing), etc., etc.

0 km

Sedimentary rock

Metamorphic rock

Igneous rock

Sediment

Metamorphism occurs

What causes metamorphism?

1 Heat

stable minerals

What causes metamorphism?

2 Pressure (stress)

Increases with depth Pressure can be applied equally in all directions or differentially

All Directions = “Confining Pressure”

Differential = “Directed Pressure”

Origin of pressure in

metamorphism

(Burial)

(Convergent Margin)

Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation”

Source: Kenneth Murray/Photo Researchers Inc.

Main factors affecting

metamorphism

3 Parent rock

• Metamorphic rocks usually have the same

chemical composition as the rock they were formed from.

• Different minerals, but made of the same atoms.

• Exception: water carries in new atoms and

removes others Important at MOR and in subduction zones

Metamorphic Settings

• Contact metamorphism – due heat from adjacent rocks

• Hydrothermal metamorphism – chemical alterations from

hot, ion-rich water

• Regional metamorphism Occurs in the cores of

mountain belts and subduction zones (Converging Margins) Makes great volumes of metamorphic rock

Includes:

– Burial Metamorphism – e.g Burial of sediments

deeper than 10 km – non-foliated

– Dynamothermal Metamorphism – Directed pressure

in Plate Tectonic Processes - foliated

Contact metamorphism

Produced mostly by local heat source

2 Hydrothermal Metamorphism

• Due circulation of water near Magma

• Important at mid-ocean ridge

Hydrothermal Metamorphism

Metamorphism in a Subduction Zone

Near trench

Deep Lithosphere

Shallow Lithosphere

Metamorphic Grade and

Index Minerals

• Certain minerals, called index

the metamorphic conditions in which they form

Note Quartz and Feldspar are not index minerals: Why?

Some index minerals give us temperature info Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

Notice Quartz and Feldspars are useless

Some Useful as Thermometers and

7_21 CANADA

ATLANTIC OCEAN

Unmetamorphosed Chlorite/muscovite zone Biotite zone

Garnet zone Staurolite zone Sillimanite zone High grade

Medium grade

Low grade

Augusta

PENNSYLVANIA

NEW HAMPSHIRE

MAINE

MASSACHUSETTS

Concord

Montpelier VERMONT

CONNECTICUT

NEW JERSEY

R.I.

Providence Hartford

Sillimanite INTERMEDIATE GRADE HIGH GRADE MELTING

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Common metamorphic rocks

Common metamorphic rocks

• Nonfoliated rocks (cont.)

• Marble

– Coarse, crystalline – Parent rock usually limestone – Composed of calcite crystals – Fabric can be random or oriented

Change in metamorphic grade with depth

Increasing Directed Pressure and increasing Temps

Metamorphism of a mudstone

A mica garnet schist

Garnets are abrasives, long lasting bearings, and jewels

Definition: Schist

Gneiss displays bands of light

and dark minerals

Development of foliation due to

directed pressure

Migmatites- When Partial Melting Starts

• Heat the rock, when the minerals with the lowest

melting points (Quartz, Feldspar) at that pressure melt, then recrystallize We get separate regions

of Metamorphic (dark, mafic) and Igneous (light, felsic) rock

Part igneous, part metamorphic