Download Chapter 5—The Sedimentary Archives

The Earth Through Time
CHAPTER 5—THE SEDIMENTARY ARCHIVES
CHAPTER OUTLINE FOR TEACHING
I. Tectonic Settings
A. Cratons
1. Continental shield
2. Continental platform
B. Orogenic Belts
C. Depositional basins
II. Environments of Deposition
A. Continental: streams deposits, alluvial fans, lake deposits, glacial deposits, wind deposits
B. Transitional: deltas, barrier islands, lagoons, tidal flats, estuaries
C. Marine: shallow marine (continental shelf), continental slope and rise (turbidity deposits), deep marine
realm
III. Color in Sedimentary Rocks
A. Black and Dark Gray (especially shales): organic carbon and iron sulfides
B. Red and Brown (especially clastic rocks): iron oxides (hematite)
IV.
Textural Interpretation
A. Textural Components: clasts, matrix, and cement
B. Main Observations: grain size, sorting, shape (roundness and sphericity)
C. Orientation of Grains
V. Interpretation of Sedimentary Structures
A. Stratification (layering)
B. Internal Structures (graded bedding, cross-bedding)
C.
Between-bed Structures (mud cracks, ripples, scour and sole marks)
D. Geopetal (sedimentary ‘way-up’) Structures
VI. Interpretation of Sand and Sandstone Types
A. Quartz-rich (mature; quartz sand)
B. Feldspar-rich (immature; arkosic sand)
C. Rock Fragment-rich (immature; lithic sand)
D. Clay-rich (immature; graywacke)
VII.
Interpretation of Carbonates
A. Main Processes
1. Chemical direct precipitates (carbonate mud)
2. Biochemical: organic contribution (shells, etc.)
B. Main Components: mud, shells, oöids, rip-up clasts
VIII. Sedimentary Rock Record
A. Rock (lithostratigraphic) Units: named for geographic site
1. Formation
2. Group
3. Member
B. Time-rock (chronostratigraphic) Units
C. Facies Concept
1. Biofacies
2. Lithofacies
D. Sea-level Change
1. Transgression (onlap)
2. Regression (offlap)
3. Facies shift (Walther's Law)
E. Correlation: determining equivalence between strata, an important component
branch of geology called stratigraphy
1. Physical (lithostratigraphic)
2. Fossil (biostratigraphic)
3. Temporal (chronostratigraphic)
F. Unconformities: breaks in the stratigraphic record
1. Angular unconformity
2. Disconformity
3. Nonconformity
IX.
Depicting the Past
A. Graphics for Correlation
in the
1. Geologic column (local section) and columnar sections
2. Stratigraphic cross-sections (several local sections)
3. Structural cross-sections
B. Mapping Graphics
1.
2.
3.
4.
Geologic map
Paleogeographic map
Isopach map
Lithofacies map
Answers to Discussion Questions
1. (a) Shallow-marine deposits are indicated typically by mature sandstones and dark shales containing shallowmarine fossils. b) Deep-marine (including continental-rise) deposits are indicated typically by dark shaley
sandstones and dark shales containing deep-marine trace and body fossils. c) Transitional (deltaic) environments
are indicated typically by immature sandstones and shales containing mixed shallow-marine and freshwater fossils.
d) Continental (desert) environments are indicated typically by well-sorted sandstones and arkoses that are red
and contain no fossils except continental terrestrial trace fossils.
2. Oxidation in the desert realm causes any iron to produce redness and any original organic material to be removed.
3. Matrix is fine inter-granular material deposited with the coarser grains at deposition or shortly thereafter. Cement is
inter-granular crystalline void fill formed long after deposition (in most instances). Common cements in sandstones
are: quartz; calcite; and hematite. Quartz is the most durable (hardest and most stable) cement.
4. The immature nature of a poorly-sorted sandstone with mud matrix and angular grains suggests an unstable,
tectonically active source where material is being shed rapidly into a terrestrial or marine basin. The mature nature
of a quartz sandstone with well-sorted grains suggests a stable area. Fossil clams (marine organisms) and ripplemarks (shallow-water features) indicate a shallow-marine setting.
5. Textural differences between mature and immature are indicated below.
Mature
well-sorted
Immature
poorly sorted
quartz-rich
low matrix
stable minerals
rounded
quartz-poor
high matrix
unstable minerals
angular
6. The sequence of layers, limestone-shale-sandstone, suggests an increase in bottom
proximity to clastic source, both indicative of progressive shallowing of water.
energy and increasing
7. The Bahama Banks carbonate platform is highly productive today due to warm, clear, shallow (less than 200 m)
sea water that is saturated with calcium carbonate.
8. Features useful for determining the direction of current of the depositing medium (i.e., water and air) are: planar
cross-bedding, trough cross-bedding, and asymmetrical ripples.
9. Gradual and continuous subsidence of a sedimentary basin allows great thicknesses of sedimentary rock to be
deposited in shallow water. The subsidence rate was about equal to rate of deposition on average.
10.
c
11. b
12. Mud cracks mean that the sediment was exposed to drying conditions at the surface. Graded bedding means that
sediments of different sizes settled according to size after they entered standing water. Cross-bedding means that
sediments were moved along by air or water and deposited on the front of a sand dune. Asymmetrical ripple marks
mean that sediment moved by currents that were moving in one main direction.
13.
Angular unconformities show truncation of inclined beds below the unconformity surface. Disconformities show
little or no truncation between essentially parallel beds. Nonconformities show erosional effects on rocks that are
not sedimentary (igneous and/or metamorphic rocks), but sedimentary layers overlie the nonconformity. Angular
unconformities and disconformities demonstrate the effects of erosion on pre-existing sedimentary rocks.
Nonconformities show effects of erosion on non-sedimentary rocks. Angular unconformities develop after the older
layers are tilted, whereas disconformities do not.
14.
An isopach map shows the distribution of rock thickness, whereas a lithofacies shows the distribution of specific
types of sedimentary rock.
CHAPTER ACTIVITIES
Student activities for in-depth learning:
1. Find some images of sedimentary structures on the web at one of many sites that show such illustrations, for
example,
http://www.indiana.edu/~geol105/images/gaia_chapter_5/sedimentary_structures.htm.
Make a sketch of two
sedimentary structures of your choosing and briefly explain how they form and what they mean when found in
sedimentary rocks.
2. Go to the Earth Science World Image Bank (http://www.earthscienceworld.org/images/search/ index.html) and enter
the keyword “unconformity.” Study the images of angular unconformities in particular. Pick out one and make a
sketch of the main features that you see (sedimentary beds, the unconformity surface, etc.). Describe the order of
events in the formation of this angular unconformity as shown in the photograph. Make a note of where the
photograph was taken, the age of the rocks, and any other pertinent information.
Chapter 5—The Sedimentary Archives
CHAPTER OVERVIEW
The title of this chapter emphasizes the historical significance of sedimentary rocks as indicators of past environments.
When interpreting sedimentary rocks, it is important to consider the tectonic elements of a continent as well as
particular areas such as cratons, orogenic belts, shields, and platforms. These tectonic settings determine the type of
depositional environments, for example, marine, continental, or transitional. These listed environments of deposition
are associated with specific types of sandstones, shales, limestones, and other sedimentary rocks. Some positive
general indicators relate to color, grain size, sorting, textural maturity, and sedimentary structure. The field of
stratigraphy is introduced using examples of how it is applied to the geologic column and world wide correlations. This
chapter concludes with a description of the different types of maps used by geologists and the economic resources
found in sedimentary rocks.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an understanding of the following concepts:








Explain how the tectonic setting is reflective of the different kinds of sedimentary rocks.
Describe the different kinds of tectonic settings and how each is reflective of the size and shape of clastic
particles.
Define the principle elements of a continent, including cratons, shields, platforms and mobile belts.
Describe the different kinds of environments of deposition (marine, continental, and transitional) and
characters of each.
Discuss how sedimentary rock color is indicative of some of the different environments.
Discuss the types of sedimentary rocks and how each is interpretative of different environments:
sandstones, shales, and carbonates.
Discuss how the sedimentary rock record is interpretative through rock units, facies, sea level changes,
correlation, and unconformities.
Describe the different types of mapping used in environmental interpretation including geologic maps,
paleogeographic maps, isopoch maps, and lithofacies maps.
CHAPTER OUTLINE
I.
Tectonic Setting is the Biggest Factor in Sediment Deposition
A. Depositional Basins
B. Cratons, Shields, Platforms, and Orogenic Belts
II.
Environments Where Deposition Occurs
A. Marine Depositional Environments
1. Shallow Marine (Continental Shelf) Environment
2. Continental Slope Environment
3. Deep Marine Environment
B. Continental Depositional Environments
1. Stream Deposits
2. Alluvial Fans
3. Lake Deposits (Lacustrine)
4. Glacial Deposits
5. Wind Deposits
C. Transitional Depositional Environments
1. Deltas
2. Barrier Islands, Lagoons, Tidal Flats, and Estuaries
III.
What Rock Color Tells Us
A. Shades of Black
B. Hues of Red and Other Colors
IV.
What Rock Texture Tells Us
A. Grain Size and Sorting
B. The Shape of Grains
C. Orientation of Grains
V.
What Sedimentary Structures Tell Us
VI.
What Four Sandstone Types Reveal About Tectonic Settings
A. Maturity
B. Quartz Sandstone
C. Arkose
D. Graywacke
E. Lithic Sandstone
VII.
Limestones and How They Form
VIII.
Organizing Strata to Solve Geologic Problems
A. Rock Units (Formally Called Lithostratigraphic Units)
B. How We Name Rock Units: Geography + Rock Type
C. What Are Facies
IX.
Sea-Level Change Means Dramatic Environmental Change
X.
Stratigraphy and the Correlating of Rock Bodies
A. Correlating Rock Bodies
B. Correlation Methods
C. Using Lithostratigraphic Correlation
D. Using Chronostratigraphic Correlation
XI.
Unconformities: Something is Missing
XII.
Depicting the Past
A. Geologic Columns and Cross-Sections
B. Geologic Maps
C. Paleogeographic Maps
D. Lithofacies Maps
KEY TERMS (pages are given in parentheses)
angular unconformity (110): An unconformity below which older strata dip at a different angle (usually steeper) than
the overlying strata.
alluvial fan (86): The result of abrupt deposition when stream-transported materials accumulate quickly where a
rapidly flowing river emerges from a mountainous area onto a flat plain.
arkose (99): Sandstones containing 25% or more of feldspar (derived from erosion of a granitic source area).
barrier island (89): A transitional (between marine and non-marine) environment builds islands parallel to shorelines
by wave and current action.
biofacies (104): When a facies can be distinguished because of biologic (rather than lithologic) differences, it can be
designated a biofacies. Fossil assemblages differ allowing for the recognition of different environments.
biostratigraphic correlation (108): The correlation of rock units by the use of its fossil content.
cement (in clastic sedimentary rock) (91): A chemical precipitate that crystallizes in the voids between grains
following deposition. Some cements include silica (SiO2), and calcium carbonates (CaCO3).
chronostratigraphic correlation (108): Correlation that links units by age equivalences as determined by fossils or
radioactive dating methods.
clast (82): An individual grain, fragment, or constituent of detrital sediment or sedimentary rock produced by physical
breakdown of a larger mass.
continental shelf (84): Oceanic topographic regions included in the shallow marine realm. They are nearly flat,
smooth surfaces that fringe the continents in widths that range from only a few kilometers to about 300 km at depths
that ranges from low tide to about 100 meters. They resemble continents in their structure and composition.
continental slope (85): Those areas of the ocean floor that extend from the seaward edge of the continental shelves
down to the ocean depths. The surface is inclined from 3 to 6 degrees.
correlation (107): Determination of the equivalence of rock bodies in different locations.
craton (82): The long-stable region of a continent, commonly with Precambrian rock, either at the surface or only
thinly covered with younger sedimentary rocks.
cross-bedding (94, 96): An arrangement of laminae or thin beds traverse to the planes of stratification. The inclined
laminae are usually at inclinations of less than 30 degrees and may be either straight or concave.
delta (82): Transitional environments (between marine and non-marine) form deltas when streams enter bodies of
standing water, undergo an abrupt loss of velocity, and drop their load of sediment.
discharge (of stream) (86): The rate of stream flow at a given instant expressed in terms of volume per unit of time.
disconformity (110): A variety of unconformity in which bedding planes above and below the plane of erosion or nondeposition are parallel.
eolian environment (88): Environment in which wind is an important agent of sediment transport and deposition.
epicontinental sea (107): In the geologic past, there have been repeated advances of the sea into low-lying regions
of the interiors of continents. The resulting inland sea is termed epeiric, meaning “a sea over a continent.”
estuary (89): The seaward mouth of rivers that are engulfed by the sea. Freshwater from streams and saltwater from
the ocean are brought together. Mixing may occur, and typically, the lighter freshwater will flow seaward above the
denser saltwater.
facies (104): A particular aspect of sedimentary rocks that is a direct consequence of sedimentation in a particular
depositional environment. It allows identification of the environment of deposition.
ferric (91): The combination of two iron atoms with three oxygen atoms. Ferric minerals include hematite and tend to
color the rock red, brown, or purple.
ferrous (91): Occurs when oxygen is in short supply such as stagnant lake or ocean where the iron atoms have lost
only two electrons.
formation (103): A mapable, lithologically distinct body of rock having recognizable contacts with adjacent rock units.
geologic map (103): Map showing the distribution of rocks of different kinds and ages that lie directly beneath the
loose rocks and soil covering most areas of the surface.
graded bedding (95): Consists of repeated beds, each of which has the coarsest grains at the base and successively
finer grains nearer the top.
graywacke (99): Immature sandstone consisting of significant quantities of dark, very fine-grained material (usually
clay, chlorite, micas, and silt). There is little or no cement, and the sand-sized grains are not in close contact because
they are separated by the finer matrix particles. The matrix constitutes about 30% of the rock, with the remaining
coarser grains consisting of quartz, feldspar, and rock particles.
group (104): The combination of formations into larger units.
hematite (90): Ferric mineral that tends to color the rock red, brown, or purple.
isopach map (113): A map depicting the thickness of a sedimentary unit.
lacustrine deposit (86): Pertaining to lakes, as in lacustrine sediments (lake sediments). Sediments that are
deposited in lakes.
lagoon (89): Water-filled area found between the mainland and a barrier island. Represents a transitional environment
(between marine and non-marine).
lithic sandstone (99): A rock that has a more transitional composition and texture is termed a lithic sandstone.
Feldspars are relatively scarce, whereas quartz, muscovite, chert, and rock fragments are abundant. Fine-grained
detrital matrix does not exceed 15% and the remaining voids are filled with mineral cement or clay. Quartz grains are
better rounded and more abundant, the sorting is better, and the quantity of matrix is lower than in greywacke.
lithofacies (104): When the differentiating characteristics are lithologic (rather than biologic), facies can be designated
as lithofacies.
lithostratigrapic correlation (115): Bodies of rock identified by their distinctive lithologic and structural features
without regard to time boundaries. Features used include texture, grain size, clastic or crystalline, color, composition,
thickness, type of bedding, nature of organic remains, and appearance of the unit in surface exposure.
matrix (in clastic sedimentary rock) (98): A category of bonding material that consists of finer clastic particles (often
clay) that were deposited at the same time as the larger grains and that fill the crevices between them.
maturity (of sandstone) (98): Aggregates of well-rounded, well-sorted, highly stable minerals (such as quartz).
member (104): Distinctive smaller units within formations.
mud cracks (94, 97): Indicate drying after deposition. Developed by shrinkage of mud or clay on drying and are most
abundant in the subaerial environment.
nonconformity (111): An unconformity developed between sedimentary rocks and older plutons or massive
metamorphic rocks that had been exposed to erosion before the overlying sedimentary rocks were deposited.
offlap sequence (104): A sequence of sediments resulting from a marine regression and characterized by an upward
progression from offshore marine sediments (often limestone) to shales and finally sandstones (above which will follow
an unconformity).
onlap sequence (104): A sequence of sediments resulting from a marine transgression. Normally, the sequence
begins with a conglomerate or sandstone deposited over an erosional unconformity and followed upward in the vertical
section by progressively more offshore sediments.
orogenic belt (83): Great linear tracts of deformed rocks, primarily developed near continental margins by
compressional forces accompanying mountain building.
paleogeographic map (113): A map showing the geography of a region or area at some specific time in the geologic
past.
paraconformity (111): Recognized as a type of unconformity only by missing fossil zones.
platform (83): That part of a craton covered thinly by layered sedimentary rocks and characterized by relatively stable
tectonic conditions.
quartz sandstone (98): Characterized by the dominance of quartz with little or no feldspar, mica, or fine clastic matrix.
Quartz grains are well sorted and well rounded. Commonly held together by such cements as calcite and silica.
red beds (90): Prevailing red, usually clastic sedimentary deposits.
ripple marks (95): Commonly seen sedimentary features that develop along the surface of bedding planes.
Symmetric ripple marks are formed by the oscillatory motion of water beneath waves. Asymmetric ripple marks are
formed by air or water currents and are useful in indicating the direction of movement of currents.
rock unit (= lithostratigraphic unit) (103): Bodies of rock identified by their distinctive lithologic and structural
features without regard to time boundaries. Features used include texture, grain size, clastic or crystalline, color,
composition, thickness, type of bedding, nature of organic remains, and appearance of the unit in surface exposure.
rounding (of sedimentary particles) (93): A particle becomes rounded by having sharp corners and edges removed
by impact with other particles. The roundness can be used to infer the history of abrasion. It reflects the distance the
particles had traveled, the transporting medium, and the rigor of transport. Wind is the best transport medium.
scour marks (97): A mark produced by the cutting or scouring action of a current flowing over the bottom of a river or
body of water.
shield (82): Large areas of exposed ancient crystalline rocks; a component of cratons.
sorting (91): A measure of the uniformity of the sizes of particles in a sediment or sedimentary rock.
sphericity (of sedimentary particles) (93): A descriptive term regarding shape of a particle and how closely the grain
approaches the shape of a sphere.
stratigraphy (107): The science of rock strata; concerned with all characters and attributes of rock as strata, and their
interpretation in terms of mode of origin and geologic history.
stream deposits (86): Sand, silt, and clay along the banks, bars, and floodplains of streams. Discharge will alter water
velocity and type of sediment being transported.
structural cross-section (113): A cross-section showing the features transected by a vertical plane. It is a level line
that is parallel to sea level.
tectonics (82): The structural behavior of a region of the Earth’s crust.
tidal flat (89): Represents a transitional environment (between marine and non-marine). Mud-covered flats that are
alternately inundated and drained of water by tides.
turbidite (86): Sediment deposited from a turbidity current and characterized by graded bedding and moderate to poor
sorting.
turbidity current (86): A mass of moving water that is denser than surrounding water because of its content of
suspended sediment and that flows along slopes of the sea floor as a result of that higher density.
unconformity (110): An erosional gap or hiatus in a rock record. A surface separating an overlying younger rock
formation from underlying formations and representing an episode of erosion or nondeposition.
Walther’s Principle (106): Walther recognized onlap and offlap patterns of sedimentations. He observed that the
succession of facies occurring laterally is also seen in vertical succession of facies. This relationship, in which the
vertical succession of facies corresponds to the lateral succession, is known as Walther’s Principle.
CHAPTER 5—THE SEDIMENTARY ARCHIVES
CHAPTER OUTLINE FOR TEACHING
I. Tectonic Settings
A. Cratons
1. Continental shield
2. Continental platform
B. Orogenic Belts
C. Depositional basins
II. Environments of Deposition
A. Continental: streams deposits, alluvial fans, lake deposits, glacial deposits, wind deposits
B. Transitional: deltas, barrier islands, lagoons, tidal flats, estuaries
C. Marine: shallow marine (continental shelf), continental slope and rise (turbidity deposits), deep marine
realm
III. Color in Sedimentary Rocks
A. Black and Dark Gray (especially shales): organic carbon and iron sulfides
B. Red and Brown (especially clastic rocks): iron oxides (hematite)
IV.
Textural Interpretation
A. Textural Components: clasts, matrix, and cement
B. Main Observations: grain size, sorting, shape (roundness and sphericity)
C. Orientation of Grains
V. Interpretation of Sedimentary Structures
A. Stratification (layering)
B. Internal Structures (graded bedding, cross-bedding)
C.
Between-bed Structures (mud cracks, ripples, scour and sole marks)
D. Geopetal (sedimentary ‘way-up’) Structures
VI. Interpretation of Sand and Sandstone Types
A. Quartz-rich (mature; quartz sand)
B. Feldspar-rich (immature; arkosic sand)
C. Rock Fragment-rich (immature; lithic sand)
D. Clay-rich (immature; graywacke)
VII.
Interpretation of Carbonates
A. Main Processes
1. Chemical direct precipitates (carbonate mud)
2. Biochemical: organic contribution (shells, etc.)
B. Main Components: mud, shells, oöids, rip-up clasts
VIII. Sedimentary Rock Record
A. Rock (lithostratigraphic) Units: named for geographic site
1. Formation
2. Group
3. Member
B. Time-rock (chronostratigraphic) Units
C. Facies Concept
1. Biofacies
2. Lithofacies
D. Sea-level Change
1. Transgression (onlap)
2. Regression (offlap)
3. Facies shift (Walther's Law)
E. Correlation: determining equivalence between strata, an important component
branch of geology called stratigraphy
1. Physical (lithostratigraphic)
2. Fossil (biostratigraphic)
3. Temporal (chronostratigraphic)
F. Unconformities: breaks in the stratigraphic record
1. Angular unconformity
2. Disconformity
3. Nonconformity
IX.
Depicting the Past
A. Graphics for Correlation
1. Geologic column (local section) and columnar sections
2. Stratigraphic cross-sections (several local sections)
3. Structural cross-sections
B. Mapping Graphics
in the
1.
2.
3.
4.
Geologic map
Paleogeographic map
Isopach map
Lithofacies map
Answers to Discussion Questions
1. (a) Shallow-marine deposits are indicated typically by mature sandstones and dark shales containing shallowmarine fossils. b) Deep-marine (including continental-rise) deposits are indicated typically by dark shaley
sandstones and dark shales containing deep-marine trace and body fossils. c) Transitional (deltaic) environments
are indicated typically by immature sandstones and shales containing mixed shallow-marine and freshwater fossils.
d) Continental (desert) environments are indicated typically by well-sorted sandstones and arkoses that are red
and contain no fossils except continental terrestrial trace fossils.
2. Oxidation in the desert realm causes any iron to produce redness and any original organic material to be removed.
3. Matrix is fine inter-granular material deposited with the coarser grains at deposition or shortly thereafter. Cement is
inter-granular crystalline void fill formed long after deposition (in most instances). Common cements in sandstones
are: quartz; calcite; and hematite. Quartz is the most durable (hardest and most stable) cement.
4. The immature nature of a poorly-sorted sandstone with mud matrix and angular grains suggests an unstable,
tectonically active source where material is being shed rapidly into a terrestrial or marine basin. The mature nature
of a quartz sandstone with well-sorted grains suggests a stable area. Fossil clams (marine organisms) and ripplemarks (shallow-water features) indicate a shallow-marine setting.
5. Textural differences between mature and immature are indicated below.
Mature
well-sorted
quartz-rich
low matrix
stable minerals
rounded
Immature
poorly sorted
quartz-poor
high matrix
unstable minerals
angular
6. The sequence of layers, limestone-shale-sandstone, suggests an increase in bottom
proximity to clastic source, both indicative of progressive shallowing of water.
energy and increasing
7. The Bahama Banks carbonate platform is highly productive today due to warm, clear, shallow (less than 200 m)
sea water that is saturated with calcium carbonate.
8. Features useful for determining the direction of current of the depositing medium (i.e., water and air) are: planar
cross-bedding, trough cross-bedding, and asymmetrical ripples.
9. Gradual and continuous subsidence of a sedimentary basin allows great thicknesses of sedimentary rock to be
deposited in shallow water. The subsidence rate was about equal to rate of deposition on average.
10.
c
11. b
12. Mud cracks mean that the sediment was exposed to drying conditions at the surface. Graded bedding means that
sediments of different sizes settled according to size after they entered standing water. Cross-bedding means that
sediments were moved along by air or water and deposited on the front of a sand dune. Asymmetrical ripple marks
mean that sediment moved by currents that were moving in one main direction.
13.
Angular unconformities show truncation of inclined beds below the unconformity surface. Disconformities show
little or no truncation between essentially parallel beds. Nonconformities show erosional effects on rocks that are
not sedimentary (igneous and/or metamorphic rocks), but sedimentary layers overlie the nonconformity. Angular
unconformities and disconformities demonstrate the effects of erosion on pre-existing sedimentary rocks.
Nonconformities show effects of erosion on non-sedimentary rocks. Angular unconformities develop after the older
layers are tilted, whereas disconformities do not.
14.
An isopach map shows the distribution of rock thickness, whereas a lithofacies shows the distribution of specific
types of sedimentary rock.
CHAPTER ACTIVITIES
Student activities for in-depth learning:
1. Find some images of sedimentary structures on the web at one of many sites that show such illustrations, for
example,
http://www.indiana.edu/~geol105/images/gaia_chapter_5/sedimentary_structures.htm.
Make a sketch of two
sedimentary structures of your choosing and briefly explain how they form and what they mean when found in
sedimentary rocks.
2. Go to the Earth Science World Image Bank (http://www.earthscienceworld.org/images/search/ index.html) and enter
the keyword “unconformity.” Study the images of angular unconformities in particular. Pick out one and make a
sketch of the main features that you see (sedimentary beds, the unconformity surface, etc.). Describe the order of
events in the formation of this angular unconformity as shown in the photograph. Make a note of where the
photograph was taken, the age of the rocks, and any other pertinent information.
FACTORS AFFECTING SEDIMENTARY CHARACTERISTICS
1.Tectonic setting
2.Physical, chemical, and biological processes in the
depositional environment
3.Method of sediment transport
3
4.Rocks in the source area from which the sediment is
derived
5.Climate (and its effect on weathering)
6.Post-depositional processes of lithification
(cementation, compaction)
7.Time
FACTORS AFFECTING SEDIMENTARY CHARACTERISTICS 4
TECTONICS
Tectonics: The forces controlling deformation or structural behavior of a large area of the Earth's crust over a long period of
time. 5
STRUCTURAL BEHAVIOR
Tectonically stable—mid-western U.S.
Subsiding (sinking) —New Orleans or Mexico City
Rising gently—New England and parts of Canada after glacial retreat
Rising actively to produce mountains and plateaus—parts of Oregon in the Cascade mountains
6
•Craton
- Shield
- Platform
•Orogenic belt
PRINCIPLE
TECTONIC ELEMENTS OF A CONTINENT
7 FIGURE 5-1 The tectonic parts of a continent.
CRATON
Craton—the stable interior of a continent.
Shields—Large areas of exposed crystalline rocks.
Platforms—Ancient crystalline rocks covered by flat-lying or gently warped sedimentary rocks.
8 FIGURE 5-1 The tectonic parts of a continent.
OROGENIC BELTS
Orogenic belts—Elongated regions bordering the craton which have been deformed by compression since Precambrian.
Orogenic belts are mountain belts.
9
FIGURE 5-1 The tectonic parts of a continent.
DEPOSITIONAL ENVIRONMENTS
All of the physical, chemical, biological and geographic conditions under which sediments are deposited.
By comparing modern sedimentary deposits with ancient sedimentary rocks, the depositional conditions can be interpreted.
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DEPOSITIONAL ENVIRONMENTS
Sediments and sedimentary rocks may be:
Extrabasinal in origin—formed from the weathering of pre-existing rocks outside the basin, and transported to the
environment of deposition.
Intrabasinal in origin—formed inside the basin; includes chemical precipitates, most carbonate rocks, and coal.
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DEPOSITIONAL ENVIRONMENTS
There are three broad categories of depositional environments:
Continental environments (on land)
Transitional environments (along contact between ocean and land)
Marine environments (ocean)
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FIGURE 5-2 Marine, transitional, and continental environments of deposition.
MARINE DEPOSITIONAL ENVIRONMENTS
1.Continental shelf
2.Continental slope
3.Continental rise
4.Abyssal plain
13 FIGURE 5-5 Submarine fan built of land-derived sediment emerges from a
submarine canyon.
CONTINENTAL SHELF
The flooded edge of the continent. Flooding occurred when the glaciers melted about
10,000 years ago.
a.Relatively flat (slope < 0.1o)
b.Shallow water (less than 200 m deep)
c.May be up to 300 km wide (averages 80 km wide)
d.Exposed to waves, tides, and currents
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e.Covered by sand, silt, and clay
f.Larger sedimentary grains are deposited closer to shore.
g.Locally cut by submarine canyons (eroded by rivers during Ice Age low sea level
stand)
h.Coral reefs and carbonate sediments may accumulate in tropical areas
CONTINENTAL SHELF—CONT'D 15
CONTINENTAL SLOPE
The steeper slope at edge of the continent.
a.Located seaward of the continental shelf
b.Boundary between continental & oceanic crust
c.May be about 20 km wide
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CONTINENTAL SLOPE—CONT'D
d.Deeper water
e.More steeply inclined (3–6o)
f.Rapid sediment transport down the slope by dense, muddy turbidity currents
g.Passes seaward into the continental rise
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CONTINENTAL RISE
At the base of the continental slope:
a.More gradual slope
b.May be hundreds of km wide
c.Water depths of 1400 to 3200 m
d.Submarine fans form off submarine canyons
e.Turbidity currents transport sediment downslope from continental shelf (turbidites)
f.Passes seaward into the abyssal plain
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DEEP MARINE REALM
The deep ocean floor
a.Nearly flat
b.Water depths of 3 to 5 km + (2–3 miles +)
c.Covered by very fine-grained sediment and shells of microscopic organisms
Clay
Volcanic ash
Foraminifera (calcareous)
Radiolarians (siliceous)
Diatoms (siliceous)
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TRANSITIONAL DEPOSITIONAL ENVIRONMENTS
Environments at or near the transition between the land and the sea.
1.Deltas
2.Beaches and barrier Islands
3.Lagoons
4.Tidal flats
5.Estuaries
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DELTAS
a.Fan-shaped accumulations of sediment
b.Formed where a river flows into a standing body of water, such as a lake or the sea
c.Coarser sediment (sand) tends to be deposited near the mouth of the river; finer
sediment is carried seaward and deposited in deeper water.
d.The delta builds seaward (or progrades) as sediment is deposited at the river
mouth.
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DELTAS
Mississippi River delta Niger River delta 22 FIGURE 5-9 Tale of two deltas: the
Mississippi River (A) and Niger River (B) deltas.
BEACHES AND BARRIER ISLANDS
a.Shoreline deposits
b.Exposed to wave energy
c.Dominated by sand
d.Marine fauna
e.A few km or less in width but may be more than 100 km long
f.Separated from the mainland by a lagoon (or salt marsh)
g.May be associated with tidal flat deposits
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LAGOONS
a.Bodies of water on the landward side of barrier islands
b.Protected from the pounding of the ocean waves by barrier islands
c.Contain finer sediment than the beaches (usually silt and clay)
d.Lagoons are also present behind reefs, or in the center of atolls.
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TIDAL FLATS
a.Nearly flat, low relief areas that border lagoons, shorelines, and estuaries
b.Periodically flooded and exposed by tides (usually twice each day)
c.May be cut by meandering tidal channels
d.May be marshy, muddy, sandy or mixed sediment types (terrigenous or carbonate)
e.Laminations and ripples are common
f.Sediments are intensely burrowed
g.Stromatolites may be present (if conditions are appropriate)
ESTUARIES
a.Mouth of a river drowned by the sea
b.Brackish water (mixture of fresh and salt)
c.May trap large volumes of sediment
d.Sand, silt, and clay may be deposited depending on energy level
e.Many estuaries formed due to sea level rise as glaciers melted at end of last Ice
Age
f.Some formed due to tectonic subsidence, allowing sea water to migrate upstream
CONTINENTAL ENVIRONMENTS
1.Rivers or fluvial environments
2.Alluvial fans
3.Lakes (or lacustrine environments)
4.Glacial environments
5.Eolian environments
FLUVIAL ENVIRONMENTS
a.Braided and meandering river and stream systems
b.River channels, bars, levees, and floodplains are sub-environments
c.Channel deposits are coarse, rounded gravel, and sand
d.Bars are sand or gravel
e.Levees are fine sand or silt
f.Floodplains are covered by silt and clay
ALLUVIAL FANS
a.Fan-shaped deposits at base of mountains.
b.Most common in arid and semi-arid regions with rapid erosion.
c.Sediment is coarse, poorly- sorted gravel and sand.
LACUSTRINE ENVIRONMENTS (LAKES)
a.May be large or small
b.May be shallow or deep
c.Filled with terrigenous, carbonate, or evaporitic sediments
d.Sediments are typically fine grained but may be coarse near the edges
e.Fine sediment and organic matter settling in some lakes produced laminated oil
shales
f.Playa lakes are shallow, temporary lakes that form in arid regions They periodically
dry up as a result of evaporation
GLACIAL ENVIRONMENTS
a.Sediment is eroded, transported, and deposited by ice (glaciers)
b.Glacial deposits called till contain large volumes of unsorted mixtures of boulders,
gravel, sand and clay
Ashley Cooper/Alamy 31
EOLIAN ENVIRONMENTS
a.Wind is the agent of sediment transport and deposition
b.Dominated by sand and silt
c.Common in many desert regions
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COLOR OF SEDIMENTARY ROCKS
Black and dark gray coloration in sedimentary rocks generally indicates the
presence of organic carbon and/or iron.
Organic carbon in sedimentary requires anoxic environmental conditions.
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COLOR OF SEDIMENTARY ROCKS
Red coloration in sedimentary rocks indicates the presence of iron oxides.
Red beds typically indicate deposition in well-oxygenated continental sedimentary
environments. May also be transitional or marine.
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COLOR OF SEDIMENTARY ROCKS
Green and gray coloration in sedimentary rocks indicates the presence of iron, but
in a reduced (rather than an oxidized) state.
Ferrous iron (Fe+2) generally occurs in oxygen-deficient environments.
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TEXTURAL INTERPRETATION OF CLASTIC SEDIMENTARY ROCKS
Texture = size, shape, sorting, and arrangement of grains in a sedimentary rock.
The texture of a sedimentary rock can provide clues to the depositional environment.
Fine-grained textures typically indicate deposition in quiet water.
In general, it takes higher energy to transport larger grains.
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THREE "TEXTURAL COMPONENTS" TO MOST CLASTIC SEDIMENTARY ROCKS:
1.Clasts—the larger grains in the rock (gravel, sand, silt)
2.Matrix—the fine-grained material surrounding clasts (often clay)
3.Cement—the "glue" that holds the rocks together
a.Silica (quartz, SiO2)
b.Calcite (CaCO3)
c.Iron oxide
d.Other minerals
GRAIN SIZE
Sedimentary grains are
categorized according to size
using the Wentworth Scale.
Gravel
Sand
Silt
Clay
> 2 mm
1/16 - 2 mm
1/256 - 1/16 mm
< 1/256 mm