Download Ch6_Sedimentation

What is a sedimentary rock?
Sedimentary rocks result from
mechanical and chemical weathering
Comprise ~ 5% of Earth’s upper crust
Contain evidence of past environments
• Record how sediment is transported
• Often contain fossils
Fossil Fish - 50 Million Year Old Lakes in southern Wyoming
Sediment = fine-grained mudstone
Which of the following would retain the most detailed
impressions of fossilized organisms?
1. very fine-grained volcanic ash deposited in an ancient lake
2. an ancient, rainforest lateritic soil buried by basalt lava flows
3. quartz-rich sand deposited in shallow, beachfront
environment
4. gravel and sand stream deposits laid down during flash floods
Which of the following would retain the most detailed
impressions of fossilized organisms?
1. very fine-grained volcanic ash deposited in an ancient lake
2. an ancient, rainforest lateritic soil buried by basalt lava flows
3. quartz-rich sand deposited in shallow, beachfront
environment
4. gravel and sand stream deposits laid down during flash floods
Why do we care about sedimentary rocks?
They are important for economic reasons
because they contain
• Coal
• Petroleum and natural gas
• Iron, aluminum, uranium and manganese
• We use them to read Earth’s history
Changes occur in sediment after it is deposited
Diagenesis – chemical and physical changes that take place
after sediments are deposited
Diagenesis varies along Front Range
(feldspar cement makes Flatirons erode slowly and form
cliffs, same rocks north and south, but no cliffs)
Turning
sediment
into rock
Turning sediment into rock
Diagenesis
– Recrystallization – growth of stable minerals
from less stable ones
– Lithification – loose sediments is transformed
into solid rock by compaction and cementation
– Natural cements include calcite, silica, and iron
oxide
Quartz (new crystal)
Feldspar
(altered)
Quartz (sedimentary grain)
How do you know some quartz grains are new while others are
original sedimentary grains
A)
B)
C)
D)
Grain Shape
Grain Size
Grain Orientation
All of the above
Types of sedimentary rocks
• Detrital rocks – transported sediment as solid
particles
• Chemical rocks – sediment that was once in
solution
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
Common detrital sedimentary rocks
(in order of increasing particle size)
• Shale
– Mud-sized particles deposited in thin layers
called laminae
– Most common sedimentary rock
Shale containing plant fossils
Detrital sedimentary rocks
• Sandstone
– Made of sand-sized particles
– Forms in a variety of environments
– Sorting and composition of grains can be used
to interpret the rock’s history
– Quartz is the predominant mineral
(due to its durable nature)
Closeup view of sandstone (note large number of qtz grains)
Photomicrograph of quartz rich sandstone
Detrital sedimentary rocks
• Conglomerate and breccia
– Both composed of particles > 2mm in diameter
– Conglomerate consists largely of rounded clasts
– Breccia is composed of large angular particles
Conglomerate
Outcrop of conglomerate with boulder sized clasts
Outcrop of conglomerate
with cobble-sized clasts
interbedded with sandstone
Breccia (note this is not a particularly common sedimentary rock)
Note sharp angular clasts
Which of the following sedimentary rocks would you expect
to have originally been deposited by fast-moving streams?
1.
2.
3.
4.
mudstone
limestone
breccia
conglomerate
Which of the following sedimentary rocks would you expect
to have originally been deposited by fast-moving streams?
1.
2.
3.
4.
mudstone
limestone
breccia
conglomerate
Why are clasts in conglomerate rounded?
1. Chemical weathering rounds them before they are
transported
2. Chemical weathering rounds them after they are transported
and deposited
3. Mechanical abrasion caused while the clasts are being
transported rounds them
4. All of the above
Why are clasts in conglomerate rounded?
1. Chemical weathering rounds them before they are
transported
2. Chemical weathering rounds them after they are transported
and deposited
3. Mechanical abrasion caused while the clasts are being
transported rounds them
4. All of the above
Chemical sedimentary rocks
Precipitated material once in solution
Precipitation of material occurs two ways:
• Inorganic processes
• Organic processes (biochemical origin)
Chemical sedimentary rocks
Common chemical sedimentary rocks
• Limestone
– Most abundant chemical rock
– Made of the mineral calcite
– Marine biochemical limestones form as coral
reefs, coquina (broken shells), and chalk
(microscopic organisms)
– Inorganic limestones include travertine and
oolitic limestone
Coquina
Fossiliferous limestone
Note shells and lime matrix
Chalk Outcrops in SE USA
Chalk
Hand Specimen
Oolitic Limestone - Bahama Shoals
Oolitic Limestone - Hand Specimen
Ooids under microscope
07.03 Oolitic limestone is most likely to form in what type of
depositional environment?
1.
2.
3.
4.
quiet, muddy, lagoons and bays
shallow, clear, marine waters with vigorous current activity
deep, marine waters below most wave action
acidic, organic-rich waters in freshwater swamps and bogs
07.03 Oolitic limestone is most likely to form in what type of
depositional environment?
1.
2.
3.
4.
quiet, muddy, lagoons and bays
shallow, clear, marine waters with vigorous current activity
deep, marine waters below most wave action
acidic, organic-rich waters in freshwater swamps and bogs
Travertine forming in Hot Spring
Travertine
Hand Specimen
Chemical sedimentary rocks
Common chemical sedimentary rocks
• Dolostone
– Typically formed secondarily from limestone
– Common in ancient rocks, rare today
• Chert
– Made of microcrystalline quartz
– Usually deposited as siliceous ooze in deep
oceans (can be diatomaceous)
chert
Diatomaceous chert
Chemical sedimentary rocks
Common chemical sedimentary rocks
• Evaporites
– Evaporation triggers deposition of chemical
precipitates
– Examples include rock salt and rock gypsum
Chemical sedimentary rocks
Common chemical sedimentary rocks
• Coal
– Different from other rocks because it is
composed of organic material
– Stages in coal formation (in order)
– 1. Plant material
– 2. Peat
– 3. Lignite
– 4. Bituminous
Successive stages in coal formation
If sediment keeps
accumulating in a
place like this ----->
it will ultimately
turn into:
a. Sandstone
b. Conglomerate
c. Limestone
d. Coal and/or shale
If sediment keeps
accumulating in a
place like this ----->
it will ultimately
turn into:
a. Sandstone
b. Conglomerate
c. Limestone
d. Coal and/or shale
Sedimentary environments
Geographic setting where sediment
accumulates
Determines nature of sediment that
accumulates (grain size, shape, etc.)
Sedimentary environments
Types of sedimentary environments
• Continental
– Dominated by erosion and deposition
associated with streams
– Glacial
– Wind (eolian)
• Marine
– Shallow (to about 200 meters)
– Deep (seaward of continental shelves)
Sedimentary environments
Types of sedimentary environments
• Transitional (shoreline)
– Tidal flats
– Lagoons
– Deltas
Continental (left) and marine (right)
depositional environments
Sedimentary environments
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
Sedimentary facies
Limestone Reef
Muddy Lagoon
Sandy Beach
A rock outcrop reveals the following rock sequence:
pebble conglomerate, well-rounded
sandstone & shale containing root casts
well-sorted sandstone containing shallow-marine fossils
shale containing worm burrows
Up is Up…
Which of the following is the most plausible interpretation?
1. The shoreline receded seaward as sea levels fell
2. The shoreline advanced landward as sea levels rose
3. The environment changed from high energy to
low energy
4. An ice age occurred
A rock outcrop reveals the following rock sequence:
pebble conglomerate, well-rounded
sandstone & shale containing root casts
well-sorted sandstone containing shallow-marine fossils
shale containing worm burrows
Which of the following is the most plausible interpretation?
1. The shoreline receded seaward as sea levels fell
2. The shoreline advanced landward as sea levels rose
3. The environment changed from high energy to
low energy
4. An ice age occurred
Sedimentary structures
Tells us something about past environments
Types of sedimentary structures
• Strata, or beds (most characteristic of
sedimentary rocks)
• Bedding planes that separate strata
• Cross-bedding
Cross bedding in Sandstone in Zion National Park
Sandstone deposited
in ancient sand dunes
Sedimentary structures
Types of sedimentary structures
• Graded beds: waning flow
• Ripple marks: low energy environment
• Mud cracks: drying of mud-filled pools
Graded Beds: stream energy wanes during flood cycles
End of Chapter 6
Mud Cracks: clay layer shrinks during drying, cracks
fill with younger sediment in next flood
Mud cracks in a sedimentary rock
indicate that:
a. The environment was an ocean in
the past
b. Deposition of sand dunes by wind
occurred at the site
c. The site experienced wetting and
drying
d. Dinosaur tracks are likely to be
present
Slabs of eroding sandstone with ripple marks
Fossils: Evidence of past life
By definition, fossils are the traces or
remains of prehistoric life now preserved
in rock
Fossils are generally found in sediment or
sedimentary rock (rarely in metamorphic
and never in igneous rock)
Fossils: Evidence of past life
Geologically fossils are important for
several reasons
• Aid in interpretation of the geologic past
• Serve as important time indicators
• Allow for correlation of rocks from
different places
3 Billion Year Old Algal Mounds (Stomatolites) Australia
Natural casts of
shelled invertebrates
Extinct Organisms can get pretty weird! Anomalocarid from
China, an extinct Phyllum of early predatory swimmers