Introduction
weathering
and erosion
Sediment Production and
Classification
Sediment Transport and
Refinement
Sediment Depositional
Environments
Classic Depositional Sites
of North America
The Lithification Process
Types of Sedimentary Rocks
Sedimentary Structures
Sedimentary Facies
Marine
Transgressions and Regressions
Reading the Sedimentary
Rock Record
Sedimentary-related Mineral
Resources
Important Vocabulary Words
II.
Introduction:
A. The origin and
development of sedimentary rocks is a major part
of the Earth’s rock cycle. The sedimentary process of the rock cycle
includes weathering,
erosion, transport, deposition, burial, compaction,
and lithification of
sediments.
B. There are Two primary types of sediment that
form sedimentary rocks:
1) Detrital and 2) Chemical
C. The Classification of detrital
sedimentary rocks is based primarily on
texture (sediment size).
D. The Classification of chemical
sedimentary rocks is based mainly by
composition (mineralogy).
E.
The Formation of each type of sedimentary rock is controlled primarily
by their
depositional environment:
F. Sedimentary rocks are characteristically
layered. Sedimentary rocks form
characteristic structures – the most basic units being
stratigraphic “beds”
or “strata”
and “formations”. Places like the Grand
Canyon are amazing
examples of
thick stacks of sedimentary strata, which display a wide
variety of rock types, structures, depositional
settings, and ages.
G.
Layered stacks of “strata” contain sedimentary substructures that
provide
clues to: 1) the depositional environment, 2) the sediment's
mode of
transport, and 3) the age of the rock.
H.
Several groups of important resources that are found in sediments
and
sedimentary rocks.
I. Sediment cover and sedimentary rocks
constitute a small part of
the Earth’s crust (~5%),
but they cover much of the Earth’s surface -
nearly all of the seafloor (~ 95%), and much
of the continents (~75%).
A.
What is a Weathering? -- The physical breakdown
and chemical
alteration
(decomposition) of rocks and minerals at or near Earth’s surface.
v Two Types of Weathering
Processes
· Mechanical
· Chemical
B.
What is Erosion? – The
Removal and Transport of Weathered Material
from the Site
of Origin
v Several Types of Erosional
Agents
·
Simple
Gravity – Mass Wasting
·
Running
Water
·
Glaciers
·
Wind
·
Marine Waves
and Currents
IV. Mechanical Weathering
A.What is Mechanical Weathering?
– The physical breakdown of Earth
material into
ever-smaller pieces that retain the chemical composition of
the parent material.
B. Types of Mechanical Weathering
·
Frost Action (wedging)
·
Pressure Release (jointing, exfoliation)
·
Heating/Cooling (expansion/contraction)
·
Salt Crystal Growth (wedging)
·
Organisms (wedging and grinding)
C.
Conditions Favoring Mechanical
Weathering
· Rugged
Terrain
· Inherently
Weak Rock
· Very
Cold and/or Hot Climates
· Shorelines
· Wet
Climates
· Heavy
Forestation
D.
Good Example Localities of
Mechanical Weathering
· Yosemite
· San
Diego Coastline and Backcountry
· Find
some others
V. Chemical
Weathering
A.
What is Chemical Weathering? – Those processes by
which
rock materials are decomposed by chemical alteration of the
parent material.
B.
Agents of Chemical Weathering
1. Water
2. Atmospheric gases (mainly
oxygen)
3. Acids
4. Organisms
C. Types of Chemical Weathering
1. Solution (simple dissolving
of mineral ions)
2. Oxidation (mineral
“rusting”)
3. Hydrolysis
(water-ion/mineral-ion exchange)
4. Acid Solution (dissolving
minerals with an acid)
· Also
called “Carbonation”
D. Factors that Affect the Rate of Chemical
Weathering
1. Particle Size
2. Mineral/Rock Structure
(cleavage, joints, fractures)
3. Parent Rock Mineralogy
Ř
The Variability of Silicate Mineral Stability
ü
Olivine - V. Unstable at surface conditions
ü
Pyroxene
ü
Amphibole
ü
Ca-Plagioclase
ü
Biotite
ü
Na-Plagioclase
ü
K-Feldspar
ü
Muscovite
ü
Quartz – Very Stable at Surface Conditions
Ř
Carbonate Minerals – Very Susceptible to Acid
4. Climate – A Biggie!
Ř
Hot/Wet = Highest Rate
Ř
Cold/Wet
Ř
Hot/Dry
Ř
Cold/Dry = Slowest Rate
5. Biological Activity
E.
Chemical Weathering Products
1. Clays –
(from weathered silicates)
2. Dissolved
Ions – (from all rock types)
3. Acids –
(from all rock types)
F.
Good Example Localities of Chemical Weathering
1. Amazon and Congo Rainforest
Basins
2. Florida
3. Find some other places
VI.
Sediment Production and Classification
A. All Sedimentary rocks originate from preexisting
rock
that has
undergone some degree of weathering and erosion,
i.e.
sediment.
1)
Define weathering and erosion.
2)
Contrast weathering and erosion.
3)
Examples of this process occurring in nature
4) See
Fig 6.3, pg 159
B. What is Sediment? Defined: Solid particles derived by
chemical and mechanical weathering
of preexisting
earth material.
1) Three types of Sediment - Detrital, Chemical, and
Biochemical
C. The Major Types of Sediment - (determined by origin)
1) Detrital - derived directly from preexisting rock via
both,
mechanical weathering (grains of ground-up original
composition source material); and chemical weathering
(altered rock-derived clays).
Examples: river gravel, quartz beach sand and
lagoonal clays
2) Chemical
- derived secondarily from the precipitation of new
minerals from ion-rich aqueous solutions, e.g.
seawater, (ions
originated from weathered/dissolved rock material).
Examples: calcareous and siliceous sea-bottom
oozes
3) Biochemical
- derived from organisms precipitating new
minerals from aqueous solutions, e.g., seawater,
(ions originated
from
weathered/dissolved rock material).
Examples:
shells, skeletons, and reefs
D. Classifying Detrital Sediment
1) Classification primarily
based on particle size.
2) Secondary classification
based on composition.
3) Sediment Size and Names
--- See Table 6.1
>2mm = gravels --- large
2mm - 1/16mm
= sand --- medium
1/16mm - 1/256mm
= silt --- small
< 1/256mm
= clay --- very small
E. General Relationship between sediment size and
particle composition:
·
Boulders, cobbles, and gravels consist of rock fragments.
·
Sand and silt consist mostly of mineral fragments.
·
Clay-sized particles consist mainly of clay minerals.
VII. Sediment Transport and Refinement
A. Sediment transport and deposition follow/overlap with
weathering and erosion in the rock cycle (see rock cycle
diagram).
B. What is Sediment Transport? Defined: The movement of
detrital and chemical
sediments by natural agents and
processes.
C. What is Sediment Refinement? Defined: The physical
changes in size, shape, and
size distribution of detrital
sediment during transport by the processes of
abrasion and
changing transport agent energies over time.
· Size Reduction -----
abrasion
· Rounding ------ abrasion
· Sorting ----- changes in transport agent energy.
D. Agents of Sediment Transportation
1) Running/Moving Water
·
Sheet flow, streams, rivers, waves, currents
·
By far the dominant transport agent on the planet.
·
Best at reducing (abrading), rounding and sorting
·
Dominant agent of marine and fluvial environments.
2)
Wind
·
A dominant agent in deserts
·
Good at reducing (abrading), rounding, and sorting
·
Removes clay/silt, pushes sand, trouble with gravel
·
Agent of dune formation
3) Ice (glaciers)
·
Active agent in high mountains and polar regions
·
Excellent reducer (abrading and milling)
·
Poor sorting agent
4) Gravity (mass wasting
movements)
Local agent in areas of
slope instability +/- flooding
Poor at reducing,
rounding, and sorting
5) Volcanic eruptions (blast
effect)
Move tremendous amounts
of material in short time
VIII. Sediment Depositional Environments
A. What is a Sediment Depositional
Environment?
Defined: Any geographic area in which sediment is
deposited.
B. Variations in Depositional Environments
1) The Earth has a wide variety of depositional environments.
2)
Each depositional environment has a unique combination
of active transport agents, surface topography, physical,
chemical,
and biological processes, that together, impart
distinctive characteristics
to the deposited sediment.
3) Depositional settings are classified according to
prevailing
geographic conditions.
·
continental vs. marine,
·
high energy vs. low energy,
·
detrital-rich vs. detrital-starved,
·
wind vs. water vs ice vs. gravity-deposited.
C.
Classification of Depositional Environments
1) Three major depositional settings (see Fig. 6.5, pg. 161)
§ Continental
§ Transitional
§ Marine
2) Continental - Interior/Non-marine
·
Glacial
·
Lake
·
Aluvial
·
Desert dunes
·
Streams, rivers, floodplains (fluvial)
3) Transitional - Shorelines
§ Deltas
§ Lagoons/Estuaries
§ Beaches
4) Marine - Offshore/Interior Marine
·
Barrier islands
·
Continental Shelf
·
Offshore reefs
·
Submarine fans
·
Deep-sea basins
IX. Classic Present-day Depositional Sites
of North America
·
Gulf of California
·
Gulf of Mexico
·
Eastern Seaboard
·
Basin and Range Province
X. Classic Ancient Depositional Sites of
North America
·
West Central United States
·
Rocky Mountains/Colorado Plateau
·
Southeastern California and Nevada
XI. The Lithification Process - Turning
Sediments Into Rock
A. What is Lithification? Defined: A series of processes,
including burial, compaction and cementation,
whereby
sediment is converted to solid rock.
B. Burial ( > P and >T) + Compaction (reduced of pore space)
C.
Cementation
= Chemical precipitation of new mineral crystals
material
between the sediment particles.
1) Cements include calcite,
silica, and iron oxide.
2) Cement material comes from
ions in the pore fluids
and/or circulating groundwater.
XII. Types of Sedimentary Rocks
A. What is a Sedimentary Rock?
Defined: Rock consisting of cemented
detrital and/or chemical
sediment derived from
one or more processes below:
1) The direct chemical and
mechanical weathering of
preexisting rock (detritus);
2) From the direct
precipitation of minerals from
solution (chemical);
3) Minerals secreted by organisms (bio-chemical).
B.
Sedimentary rocks are classified according to the
type(s) of sediment they contain.
1) Detrital Sedimentary Rocks
§ Cemented aggregates of
detrital (clastic) particles
(See Figs. 6.6 and 6.8a - pgs 162-163)
§ Exhibit "clastic" textures
§ Classification based on
particle sizes
§ Examples include
conglomerate, sandstone, & shale
2)
Chemical/Biochemical Sedimentary Rocks
§ Cemented aggregates of new, chemically-/ bio-
chemically precipitated
(crystalline) particles.
§ Exhibit
"crystalline" textures
(See Fig. 6.8b - pg 163)
§ Classification based primarily on composition
§ Examples include limestone and chert
3)
Sedimentary Rock Classification Chart -- Table 6.2
C.
Detrital or "Clastic" Sedimentary Rocks
1) Form by a multi-step process of
deposition, burial,
compaction, and cementation
of detrital sediments.
2) Exhibits a "clastic" texture is defined as an aggregate
of
distinct angular to
rounded particles that are held
together by
a matrix cement. (see Fig. 6.8a; pg
163)
3) Classification based on size of clasts. (see Table 6.2)
4) There are five general detrital
sedimentary rock types:
·
Breccias --- Angular gravels with a sandy matrix
·
Conglomerates -- Rounded
gravels with a sandy matrix
·
Sandstones -- Dominantly Sand
·
Siltstones -- Dominantly Silt
·
Mudstones -- Mix of Silt and Clay
·
Shales -- Dominantly Clay
5) Cements can be calcite, silica, and iron oxide -
depends on the pore
fluid
chemistry in the depositional/burial environment.
6) Quartz is the dominant mineral in the
conglomerates and
sandstones. WHY?
7) Clay minerals dominate the mudstones
and shales. WHY?
8) Note that 40% of all detrital rocks
are shales and siltstones.
9) Rock type is dependent on
depositional environment.
·
Breccias --- High energy; very close to sediment source.
·
Conglomerates -- High energy; further from
source.
·
Sandstones -- Medium energy; water/wind transport.
·
Siltstones -- Low energy; water/wind transport
·
Mudstone/Shales - V. low energy; water/wind transport.
10) Detrital (Clastic) Sedimentary rocks form in a
wide variety of
depositional settings. (See
Figs 6.5 and 6.16)
D.
Chemical and Biochemical Sedimentary Rocks
1) Form by a multi-step process
starting with chemical
precipitation of new minerals out of solution, which
form thick
accumulations (layers) of the loose chemical
sediments, that in
turn, become cemented together with cements derived
from local
pore fluids
or compounds found in circulating fluids.
2) Exhibits a "crystalline"
texture
is defined as an interlocking
mosaic of mineral crystals
that are held together, in part, by a
matrix cement. (see Fig. 6.8b; pg 163)
3) Classification based on Composition. (see Table 6.2, pg 164)
4) Four general chemical/biochemical
sedimentary rock types:
Limestones
and Dolostones - The Carbonates
· Calcite - CaCO3
· Dolomite - (Ca, Mg)CO3
· Biochemical source-dominant
· Fossil-rich
Chert - Silica
· Microcrystalline quartz -
SiO4
· Silica-producing organisms
· Varieties include flint and
jasper
Evaporites - Precipitated Salts
· Halide - NaCl (table salt)
· Sylvite - KCl (table salt substitute)
· Salt residuals of dried-up
lakes and tidal flats
Coal
- Compressed,
altered land plant remains
· Carbon-rich, organic
compounds
· Forms in bogs and swamps
with no free oxygen
· Increasing P+T (burial) coal
changes form from:
peat to
lignite to bituminous to anthracite.
XIII. Sedimentary Structures
A.
Layered stacks of “strata” contain sedimentary
features (structures) that provide clues to its:
1) depositional environment,
2) sediment mode of transport, and
3) (rock) age.
B. Distinct Sedimentary Structures
1) graded bedding
2) sorted and unsorted bedding
3) cross-stratification
4) ripple marks
5) fossils
XIV. Sedimentary Facies -- Understanding
Variations In
Depositional Environments Though Time
& Space
A. What is a Sedimentary Facies? Defined: Bodies of sediment
that possess distinctive physical,
chemical, and biological
attributes, that are reflective of the
agents and processes that
were active at the site of deposition.
·
Each Sedimentary Facies Reflects its
Respective
Depositional Environment
B.
Depositional environments form an irregular, yet
interconnecting continuum across Earth’s surface. In many
cases one depositional
environment grades laterally into
another. Therefore, the
sedimentary facies that represent one
depositional environment will grade (change) laterally into
another sedimentary facies across the surface of the Earth.
(See Fig 6.16 p. 169.)
·
A very important geologic observation is
that, like everything else
our dynamic planet, depositional environments
change over time.
·
Applying the well-tested principle of uniformatarianism,
regional
scale depositional environments change
gradually and this change
occurs laterally through time across the
Earth’s surface.
·
So, two things are happening to a
depositional basin through time:
1)
Sediment piles up forming a column (stack) that can be
upwards of thousands of meters thick. Each layer (bed)
of sediment has a specific facies.
2)
Sediment facies shift laterally (with their
distinctly matched depositional
environment).
C. Gradual
changes of a depositional environment over
time in a given geographic location, are
due to systematic
change in the physical, chemical, and
biological processes
and transport agents that occur at that
location. The major
causes for these gradual changes in
depositional conditions
are:
1)
Changes in sea level
2)
Changes in climate
3)
Changes in relief/topography
4)
Changes in tectonics
D. Earth's most extensive
depositional setting is the
marine setting
·
Open Marine (Continental Shelves) and
Interior
(Continental
Seaways)
·
This is the final resting place for most of
the detrital
material
being shed off the continents.
·
Parallel belts of depositional marine
environments
ü
Near shore - Sand-dominated
ü
Near offshore - Silts and mud-dominated
ü
Far offshore - Fine clay and
carbonate-dominated
·
See Fig. 6.16
1)
The greatest long-term influence on shifting marine
depositional environments is changes
in sea level.
2) Causes of Sea Level
Changes
·
Rising Seas = "marine transgression"
ü
more water in ocean basins (ice caps melt)
ü
continent sinking
Falling
Seas = "marine regression"
ü
less water in ocean basins (ice caps grow)
ü
continental uplift
E.
Marine Transgressions and Regressions
1)
Systematic Changes in Rising and Falling Sea Level
Reflected
in the Rock Record
2)
Marine Transgression = Sea Level Rise
ü
Shorelines shifts (moves) inland
ü
Depositional environments parallel to
shoreline shift
Inland.
ü
Sedimentary facies parallel to shoreline
shift inland.
ü
Offshore sedimentary facies will start
depositing on
top of near short deposits over
time.
ü
The resultant sedimentary column will have
the near
shore facies on bottom, the
near/offshore facies
in the middle, and topped by far offshore
facies.
ü
This called a transgressive sequence.
ü
See Figure 6.17a-d, pg 170
3)
Marine Regression = Sea Level Drop/Fall
ü
Exact Opposite of Transgression (see above)
ü
This is called a regressive sequence
ü
See Figure 617e-h, pg 170
XV. Recognizing
Marine Transgressions and Regressions
in the Rock Record
A. Distinctive
Sedimentary Facies Sequences in the rock record indicative
either, a marine transgression or
regression.
B. The Grand Canyon
Sequence is a Beautiful example.
XVI. Sedimentary-related Mineral Resources
1)
sand and gravel (construction),
2)
clay (ceramics),
3)
limestone chalk (cement and steel production),
4)
silica (glass),
5)
placer gold, diamonds, tin, and uranium (fuel and weapons),
6)
petroleum and natural gas, oil shale, tar sands, and coal
7)
banded iron formations.
XVII. Important Vocabulary Words
abrasion/abrading
bed (bedding)
biochemical sed' rock
breccia
burial
carbonate rock
cementation
chemical sed' rock
clastic
clasts
clastic sed' rock
clay (clay-size)
compaction
conglomerate
cross-bedding
depositional environment
detritus
detrital sed' rock
dolostone
evaporite
fossil
graded bedding
gravel
limestone
lithification
marine transgression
marine regression
mud cracks
ripple marks
rounding
sand (sand-size)
sediment
sedimentary facies
sedimentary rock
sedimentary structure
silt (silt-size)
sorting
strata (stratification)