Download Midterm Review 2

Sedimentary Rocks
•
•
•
•
•
•
•
•
•
•
The study of sedimentary rocks is divided into sedimentology, sedimentary
petrology, and stratigraphy
Types of sedimentary rocks are divided into clastic, biologic and chemical
Sedimentary rocks commonly form distinctive layers at the surface of the Earth
Sedimentary rocks are classified by composition and texture
The most common sedimentary rocks are the siliciclastics, which include
conglomerates, sandstones, mudstones and shales
Size, mineralogy and sorting indicate the ‘maturity’ of a sedimentary rock.
Limestones, composed of calcite, are the most common biologic sedimentary
rock
Coals are a common biologic sedimentary rock and are composed of ‘baked’
plant material
The type of rock and the sedimentary structures are characteristic of the
depositional environment
According to Walther’s Law, sedimentary rocks that are formed next to one
another will appear on top, or below, one as a result of transgression and
regression of the sea
Sedimentology
The study of the processes that erode,
transport and deposit sediments
Sedimentary Petrology
The study of the characteristics and origin of
sedimentary rocks.
Stratigraphy
The study of the origin, relationship, and
extent of rock layers (strata).
Types of Sedimentary Rocks
Clastic or Detrital
Chemical
Biologic
formed by and
animal or plant
Clastic
Texture
Crystalline
Texture
The Nature of Sedimentary Rocks
• Sedimentary rocks are common at the
Earth’s surface
– Cover ~75% of the continents
– Cover nearly all of the ocean floor
– Easily eroded
– Occur in distinct layers (strata)
The Nature of Sedimentary Rocks
• Layers are easily identified
– Majors layers (formations) easily recognized
over large distances
– Smaller layers within a formation are
separated by bedding planes
– Gradation in grain size, composition or
physical features may vary
Rock Identification is based on:
• Composition
What minerals make up
the rock?
• Silica? Calcite?
• Texture
What is the shape, size and
orientation of the mineral
grains that make up the
rock?
• Gravel, sand, clay etc.
• Rounded, sorted etc
SiliciClastic Sedimentary Rocks
• Classified by the size of the fragment
– Conglomerate : ‘gravel sized or bigger
– Sandstone: ‘just visible to the naked eye
– Shale if it is layered; Mudrock if not
• Grains are too small to see with the naked eye, but you
can taste them
• Also called Siltstone, Claystone, Siltshale etc.
Conglomerate
Sandstone
Shales
Fig. 7-7a, p. 207
Chemical/Biochemical Sedimentary Rocks
• Subdivided by composition and mode of
formation
• e.g., Limestone
– Biochemical formation by algae, coral, mollusks,
brachiopods, etc.
– Direct chemical precipitate from warm sea water –
oölites (German for egg)
– Chemical precipitate from springs and in caves,
e.g., stalactites and stalagmites, etc.
Limestones
Chalk
(Coccolithophores)
Fig. 7-11c, p. 210
Chert (Flint, Jasper, Agate…)
Evaporites:
Bonneville Salt Flats, Utah
Rock
Gypsum
Rock Salt
Sedimentary Rocks on Earth
Shale
Sandstone Siltstone Conglom. Limestone
Sedimentology
How do sediments get to the
place where they stop and
eventually become rocks?
From Sediment to Sedimentary
Rock
• Deposition
– Settling and coming to rest; all journeys stop
somewhere
– Accumulation of sediments, usually in water
– Environment of deposition is the location in which
deposition occurs
•
•
•
•
•
River Delta
Beach
Desert dunes
Deep Sea
Lake bottom
From Sediment to Sedimentary
Rock
• Preservation
– Sediment must be preserved or buried
• Lithification
– Processes of converting loose sediment into sedimentary rock
– Combination of compaction and cementation
Fig. 7-3, p. 202
A braided stream in its floodplain: Laramie River
Desert Dunes
Rocks from Desert Dunes
Moraines - Till
Tillite
River Deltas
Lagoons
Rocks and Coal from Lagoons
Rocks from Shallow Marine Environments
Mostly shales and mudstones
Continental Margins
p. 86
Turbidites
Fig. 4-15, p. 88
Turbidites
graded bedding
Turbidites
graded bedding
Turbidites
Sedimentary
Structures
Surface impressions
Preserve features indicating
past environment
•Ripple Marks
•Mudcracks
•Raindrop Impressions
•Flute Marks
•Salt Casts
•Worm trails and burrows
Fig. 7-18a, p. 215
Ripple
Marks
Raindrop
Impressions
Stratigraphy
Identifying and correlating the layers is
Stratigraphy. More on that later.
Walther’s Law
Johannes
Walther
(1860-1937)
Marine Transgression = Sea Level Rise
Marine Regression = Sea Level Fall
Blue = No deposition
Cretaceous
Seaway
Walther’s Law
Sedimentary environments that started out side-by-side will end up overlapping
one another over time due to transgressions and regressions.
Facies
Limestone
Reef
Shale
Siltstone
Sandstone
Lagoon
Near Shore
Beach
Environment
Marine Trangression
Walther’s Law
Marine Regression
Weathering and Erosion
•
•
•
•
•
•
•
Weathering breaks down the rocks
Erosion and transport take them away
Weathering results in breakdown of rocks, dissolution of ions and formation of
new minerals such as clays and iron oxides
Types of weathering include physical and chemical
Physical weathering includes: frost wedging, pressure release, thermal cycling,
and the actions of plants and animals
Chemical weathering includes acid dissolution, hydrolysis, and oxidation.
Stability of many siliciclastic minerals is the reverse of Bowen’s reaction series,
i.e., the first minerals to form at high T are the first to weather at the lower
temperatures of the Earth’s surface
Weathering, Erosion and Transport
The Nature of Weathering
• Weathering is the physical and/or
chemical alteration of rocks and minerals
where the lithosphere, hydrosphere,
atmosphere, and biosphere meet
– In other words, its not just something that happens to
rocks, it also changes the atmosphere and the water.
– How do you think the sea got salty??
Products of Weathering
• Lithic (Rock) Fragments
(granite, basalt, schist, etc.)
• Dissolved Ions
(Calcium, Potassium, Sodium, etc.)
• Rust Minerals (Hematite, Goethite, etc.)
• Clay Minerals
(Bentonite, Montmorillonite, etc.)
• Residual Minerals
(Quartz, Orthoclase, Muscovite, etc.)
Physical Weathering
• Frost action
– Mechanic effect of freezing (and
expanding) water on rocks
• Pressure release
– Removal of overlying rock allows
expansion and fracturing
• Plant growth
– Growing roots widen fractures
• Burrowing animals
• Thermal cycling
– Large temperature changes fracture rocks
by repeated expansion and contraction
But mostly physical weathering is a matter of things just falling down.
So in a sense, gravity, is the primary cause of physical weathering.
Chemical Weathering
• Minerals are destroyed or altered by
chemical reactions
–Dissolution
–Hydrolysis
–Oxidation
Chemical Weathering
•
Oxidation
– Chemically active oxygen from atmosphere
– Iron oxides are common result
• Soil and sedimentary rocks often stained with
iron oxides
•
Acid dissolution
– Hydrogen cations replace others in minerals
– Carbonic acid from atmospheric CO2 dissolved
in water
– Sulfuric, hydrofluoric acids emitted by volcanic
eruptions
– Some minerals, such as calcite, may be totally
dissolved
– Human activity, such as mining and burning of
fossil fuels, produces acids
Relative stability of minerals
Stability of minerals at the Earth’s surface is
predicted by Bowen’s reaction series in Reverse, i.e.,
Quartz is most stable and Olivine is least.
Plate Tectonics and
Continental Drift
Plate Tectonics
1. Large crustal plates at the Earth’s surface move
about, colliding with one another.
2. There is geographic, geomagnetic, paleontologic and
other evidence that this occurs
3. Convection in the mantle is the main driver of plate
movement
4. Neighboring plates move relative to one another,
causing earthquakes and volcanic eruptions
5. Active plate boundaries produce mountains and
trenches
6. Continents have changed position
Continental Drift
• Wegner mechanism for drift was not
credible
– Less dense silicic rocks (the continents)
plowed through more dense ocean floor
– Earth’s rotation was driving force
• Other scientists didn’t buy it
Paleontological evidence
Evidence for Continental Drift
• Rock type & structures
– Distinct and similar rock types and geologic
structures on both sides of the Atlantic
Ocean
• Cape fold belt and equivalent – S.Africa &
Argentina
• Appalachian Mtns and equivalent – U.S.,
Canada, Scotland & Norway
• Only occur in rocks > 145 mya!!!!!!!!!
Rock type & structure evidence
Evidence for Continental Drift
• Paleoclimate
– Evidence of extreme changes in climate as
compared to the present
• Coal deposits in Antarctica
• Evidence from evaporite deposits, eolian
deposits & coral reefs
• Paleoclimate reconstruction shows strange
patterns unless continents are moved
Fig. 17.6. Paleoclimate evidence
Paleomagnetism
• Magnetization of ancient
rocks at the time of their
formation
• Declination
– Angle that a compass
needle makes with the line
running to the geographic
north pole
• Rocks lock in this orientation
at formation
70
Reconstruction from paleomagnetic data
Fig. 22.21. Cenozoic features of NW U.S.
Age of the sea floor
Fig. 17.15. Divergent plate margins
The Mid Atlantic
Ridge
Rates of Seafloor Spreading
FAST
SLOW
(East Pacific Rise)
(Mid Atlantic Ridge)
~10-20 cm/year
~1-2 cm/year
Life of a person
100 years
10 meters
1-2 meters
Civilization
10,000 years
1 km
100-200 m
Modern Humans
100,000 years
10 km
1-2 km
Stone tools
1,000,000 years
100 km
10-20 km
Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide.
How long would it take to create this much ocean crust.
Ocean-Ocean convergence
Ocean-Continent convergence
Continent-Continent Collision
Juan de Fuca
plate
Tectonic setting
Lavas and pyroclastics
Rock/sediment type
Felsic
Granites, Rhyolite and
pyroclastics
Turbidites, clays,
silts, sands
Marine sediments
(cherts, limestones,
red clays)
Basalts (Ophiolites)
Mafic
Fig. 18.1 Origin of Earthquakes by elastic rebound
Seismic Waves
• P-waves
– Primary waves, arrive first
• Alternating pulses of compression and dilation
(expansion) parallel to wave path
• P waves may pass through solids, liquids, and
gases
• Compression produces temporary changes in
volume & density of material
• These are Pressure Waves
Seismic Waves
• S-waves
– Secondary waves, arrive second
• S waves cause a shearing effect
• Waves are perpendicular to the direction of
travel
• Elastically change the shape of materials
• Liquids and gases do not behave elastically
so S-waves do not pass through them
Seismic Waves
• Surface waves
– Restricted to traveling along the Earth’s
surface
• Travel more slowly than P or S waves
• Similar to ocean waves but travelling through
rock
• Orbital or transverse motion
Fig. 18.3. Motion of seismic waves
Earthquake epicenter and focus
Earthquake Magnitude
• The Richter scale measures the
amplitude of seismic waves
– The Richter scale is logarithmic - Each unit
on the scale relates a 10 fold increase in the
amplitude of the seismic wave but about a
30-fold increase in energy released!
Fig. 10-10b, p. 309
Fig. 10-10a, p. 309
Seismic activity in the Aleutian Islands
The Benioff Zone
The dip angle at which the
earthquakes
Lake Hegben
Fig. 10-21b, p. 322
Fig. 10-21a, p. 322
Fig. 10-18b, p. 320
Fig. 10-18c, p. 320