Sedimentary Rocks

Report
Lecture 6-7
Sedimentary and Metamorphic Rocks
Part 1
Sedimentary Rocks
Kyanite, Sillimanite, and Andalucite
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
• Record how sediment is transported
• Often contain fossils
What is the economic importance of
sedimentary rocks?
• They are important for economic reasons
because they contain
• Coal
• Petroleum and natural gas
• Iron, aluminum, uranium and manganese
• Geologists use them to read Earth’s history
Cementation
• 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
• Chemical rocks – sediment from
ions that were once in solution
• Detrital rocks –sediment
transported as solid particles
Detrital sedimentary rocks
• Constituents of detrital rocks can
include
• Clay minerals
• Quartz
• Feldspars
• Micas
• 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
– Includes Shale: mud-sized particles <.004
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
• Sandstone
–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
Energy
• 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
• Precipitated material once in solution
• Precipitation of material occurs two
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
• Limestone
–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)
http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg
• Common chemical sedimentary
rocks
•Evaporites
–Evaporation triggers
deposition of chemical
precipitates
–Examples include rock salt
and rock gypsum
Chemical Sediments: Coal
Sedimentary environment determines roundness sorting, mineral diversity
Character of detrital sediments depends on time,
6_5
distance, and
energy. For example, in streams:
Particles are large and
irregular, and consist of
a variety of lithologies,
including the least
resistant.
Particles are mid-sized
and of intermediate
sphericity, and include
resistant and nonresistant
lithologies.
HIGHLANDS
LOWLANDS
Particles are small and
nearly spherical, and
consist mainly of the
most resistant lithologies,
such as quartz.
NEAR-COASTAL
Graded bedding
Floods change the local
conditions
6_6
Fine-grained sediment
On floodplain
Older sediment
1
Pre-flood
Bounders on
bottom, sands and
muds suspended
Flood water
Erosion of uppermost
fine-grained sediment
2
Flood stage
Waning flow
Fine-grained above
Coarse-grained
below
Bedding plane
3
Post-flood
• Sedimentary Facies
• Different sediments accumulate
next to each other at same time
• Each unit (called a facies) possesses
a distinctive characteristics
reflecting the conditions in a
particular environment
• The merging of adjacent facies
tends to be a gradual transition
Some Facies in an oversimplified drawing
Abyssal Ooze
Stillwater muds
Nearshore sands
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
Navaho Sandstone
Sandstone deposited
in ancient sand dunes
Frosted Grains, well sorted
Mud Cracks: clay layer shrinks during drying, curls
upward; cracks fill next flood. Useful for right-side up
Terms for Marine (i.e. Ocean) Environments
6_27
and some characteristic
sediment facies
Continental
shelf
Continental
slope
Abyssal
Plain
Define Graded Beds
Submarine
volcanoes
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
• … is the transformation of rock by high
temperatures (heat) and pressure
• Metamorphic rocks are produced by
transformation of:
• Sedimentary and Igneous rocks, and by the
further alteration of other metamorphic rocks
• These are the source of many important
minerals
– Talc (lubricant, insulators, refractories),
Corundum, Garnet (abrasives), Kyanite
(ceramics), Micas (insulators), Chrysotile
(“asbestos” for fireproofing), etc., etc.
Minerals do not melt during metamorphism
Sedimentary
rock
0 km
Metamorphic
rock
Igneous
rock
Sediment
Increasing depth
and temperature
10 km
~200ºC
50 km
~800ºC
Sedimentary
rock
Metamorphism
Melting
Metamorphism
occurs
between about
10 and 50 km
of depth
What causes metamorphism?
1. Heat
• Most important agent
• Heat drives recrystallization - creates new,
stable minerals
• Increasing Heat with Depth
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
• Three types of 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
Shallow Lithosphere
Near trench
Deep Lithosphere
Metamorphic Grade and
Index Minerals
• Certain minerals, called index
minerals, are good indicators of
the metamorphic conditions in which
they form
Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
Notice Quartz and Feldspars are useless
Note Quartz and Feldspar are not index minerals: Why?
Some Useful as Thermometers and
Pressure Gauges
Sillimanite
Kyanite
Polymorphs of
Al2SiO5
Andalusite
CANADA
7_21
New England
MAINE
Dynamothermal
Augusta
Metamorphism
Montpelier
Mapped by index mineralsVERMONT NEW
HAMPSHIRE
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Concord
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Boston
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Albany
NEW YORK
ATLANTIC
OCEAN
MASSACHUSETTS
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Binghamton
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R.I.
Hartford
Providence
CONNECTICUT
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PENNSYLVANIA
Scranton
r
NEW
JERSEY
i
ft
va l
l
ey
Low
grade
Long Medium
Island grade
Newark
High grade
Unmetamorphosed
Chlorite/muscovite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Increasing pressure and temperature
DIAGENESIS
LOW GRADE
HIGH GRADE
INTERMEDIATE GRADE
Chlorite and muscovite
Biotite
Garnet
Staurolite
MELTING
Sillimanite
Common metamorphic rocks
• 1. Nonfoliated rocks
• Quartzite
– Formed from a parent rock of quartz-rich
sandstone
– Quartz grains are fused together
– Forms in intermediate T, P conditions
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
Metamorphism of a mudstone
Increasing Directed Pressure and increasing Temps
A mica garnet schist
Definition: Schist
Garnets are abrasives, long lasting bearings, and jewels
Gneiss displays bands of light
and dark minerals
Development of foliation due to
directed pressure
Granodiorite
Gneiss
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

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