R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 6
SEDIMENTARY ROCKS—
THE ARCHIVES OF EARTH HISTORY
OUTLINE
INTRODUCTION
SEDIMENTARY ROCK PROPERTIES
Composition and Texture
Sedimentary Structures
Geometry of Sedimentary Rocks
Fossils—The Biologic Content of Sedimentary Rocks
DEPOSITIONAL ENVIRONMENTS
Continental Environments
Transitional Environments
Marine Environments
PERSPECTIVE Evaporites—What We Know and Don’t Know
INTERPRETING DEPOSITIONAL ENVIRONMENTS
PALEOGEOGRAPHY
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 6:
 Sedimentary rocks have a special place in deciphering Earth history because they
preserve evidence of surface processes responsible for deposition, and many contain
fossils.
 Geologists recognize three broad areas of deposition—continental, transitional, and
marine—each with several specific depositional environments.
 The distinctive attributes of sedimentary rocks result from processes operating in
specific depositional environments.
 Textures such as sorting and rounding as well as sedimentary structures provide
evidence about depositional processes.
 Three-dimensional geometry of sedimentary rock bodies and fossils are also useful
for determining depositional environments.
 Sedimentary structures such as ripple marks, cross-bedding, and mud cracks taken
with other rock features allow geologists to make environmental interpretations with
a high degree of confidence.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 The interpretations of sedimentary rocks in the chapters on geologic history are based
on the considerations reviewed in this chapter.
 Interpretation of how and where rocks formed, especially sedimentary rocks, is the
basis for determining Earth’s ancient geographic features.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the origin and environmental significance of sedimentary rock features
 determination of specific depositional environment through a variety of sedimentary
rock features.
 the importance of observing present-day depositional processes and environments to
aid in the interpretation of ancient processes and environments
 the basic features of deposition in continental, transitional, and marine environments
 the importance of sedimentary structures and fossils in interpreting sedimentary
environments
 the reconstruction of paleogeography through sedimentary rocks
CHAPTER SUMMARY
1. Sedimentary rocks preserve evidence of the surface processes responsible for
deposition, and any included fossils record prehistoric life.
Figure 6.1
Jurassic-age Navajo Sandstone in Zion National Park, Utah
2. The physical and biologic features of sedimentary rocks provide information about
the depositional processes responsible for them. Rounding and sorting are important
features of texture, and give us information about transportation and deposition.
Figure 6.2
Rounding and Sorting in Sediments
3. Sedimentary structures and fossils are the most useful attributes of sedimentary
rocks for environmental analyses, but textures and rock body geometry are also
helpful.
Table 6.1
Summary of Sedimentary Structures
Figure 6.3
Stratification in Sedimentary Rock
Figure 6.4
Turbidity Currents and Graded Bedding
Figure 6.5
Cross-Bedding
Figure 6.6
Current Ripple Marks
Figure 6.7
Wave-Formed Ripple Marks
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Figure 6.8
Mud Cracks Form in Clay-Rich Sediments When They Dry and
Contract
Figure 6.9
Bioturbation Results from Organisms Burrowing Through
Sediments
Figure 6.10 Using Sedimentary Structures to Determine the Relative Ages of
Deformed Sedimentary Rocks
4. Geologists recognize three primary depositional areas—continental, transitional,
and marine—each of which has several specific environments.
Figure 6.11 Depositional Deposits
Table 6.2
Summary Chart of Rocks and Features in Depositional
Environments
5. Fluvial systems may be braided or meandering. Braided streams deposit mostly
sand and gravel, whereas deposits of meandering streams are mostly mud and
subordinate sand bodies with shoestring geometry.
Figure 6.12 Fluvial Deposits of Braided and Meandering Streams
6. An association of alluvial fan, sand dune, and playa lake deposits is typical of desert
depositional environments. Glacial deposits consist mostly of till in moraines and
outwash.
Figure 6.13 Deposits in Deserts
Figure 6.14 Glaciers and Their Deposits
7. The simplest deltas, those in lakes, consist of a threefold sequence of rocks, grading
from finest at the base upward to coarser-grained rocks. Marine deltas dominated by
fluvial processes, waves, or tides are much larger and more complex.
Figure 6.15 Origin of a Delta
Figure 6.16 Marine Deltas
8. A barrier island complex includes lagoon, beach, and dune subenvironments in
which beach sands grade offshore into finer deposits
Figure 6.17 Barrier Islands and Tidal Flats
Enrichment Topic 1. Investigating Sand.
The International Sand Collectors Society has assembled a collection of sand samples
from various locations around the world. Accompanying slide presentations and
suggestions for classroom activities are included in the kit. Carbonate, quartz, and
gypsum sand are some of the included samples that are discussed with regards to
formation and source rocks. The Sand Collectors Society Kit is available from
http://www.paccd.cc.ca.us/instadmn/physcidv/geol_dp/dndougla/SAND/KIT/home.htm .
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
9. Inner shelf deposits are mostly sand, whereas those of the outer shelf are mostly
mud; both have marine fossils and bioturbation. Much of the sediment from land
crosses the shelves and is deposited on the continental slope and rise as submarine
fans.
Figure 6.18 Depositional Environments on the Continental Shelf, Slope, and
Rise
10. Either pelagic clay or oozes derived from the shells of microscopic floating
organisms cover most of the deep seafloor.
Figure 6.19 Sediments on the Deep Seafloor
Enrichment Topic 2. Microorganisms.
Many students are surprised to discover that some of the intricate microscopic remains were
produced by single-celled organisms. Students can investigate foraminifera, diatoms,
coccolithophores, radiolarians, and ostracodes. Are any of these multicellular organisms?
What are the preferred environments for each organism? Are calcareous species more likely
to be found in arctic or tropical regions? Is there selective preservation of calcareous and
siliceous fossils in some locations? The University of California Museum of Paleontology
offers a brief overview of microfossils
(http://www.ucmp.berkeley.edu/people/klf/About_Microfossils.htm). Several SEM photos
are available on the MicroGallery website:
(http://www.ucmp.berkeley.edu/people/klf/MicroGallery.htm )
11. Most limestone originates in shallow, warm seas where little detrital mud is
present. Just like detrital rocks, carbonate rocks may possess cross-beds, ripple
marks, mud cracks and fossils that provide information about depositional
processes.
Figure 6.20 Carbonate Depositional Environments
12. Evaporites form in several environments, but the most extensive ones were
deposited in marine environments. In all cases, they formed in arid regions with
high evaporation rates.
13. With information from sedimentary rocks, as well as other rocks, geologists
determine the paleogeography of Earth.
Figure 6.21 Lower Silurian Strata in the Eastern United States
Figure 6.22 Middle and Upper Ordovician Strata in Northern Arkansas
Figure 6.22 Paleogeographic Maps
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
LECTURE SUGGESTIONS
The Nature of Sedimentary Rocks and Depositional Environments
1. Some students find “clay” to be a confusing term because it can refer to both size
and composition, depending on the context. The term “mud” is a useful way to
designate size but not composition.
2. Glacial deposits offer a good opportunity to review clast sizes, sorting, and
rounding. Because till is directly deposited by melting ice, the particle sizes range
from small to extremely large. However, there is no opportunity for rounding or
sorting. Outwash, however, is deposited by meltwater. As such, the particle size is
restricted. (Ice is more powerful a geologic agent than running water, and can carry
much larger particles.) Outwash deposits also differ from drift because they are
sorted. (Running water sorts; glacial ice does not.)
3. Discuss the distinction between weathering and erosion, and the role each plays in
producing characteristics of composition, bedding, and other sedimentary rock
characteristics. Weathering by abrasion may occur during transportation, and
particles in transport—with the exception of oolites—never get bigger!
4. Emphasize to students that, unlike detrital sedimentary rocks, the shell and skeletal
particles that occur in biochemical sedimentary rocks are not transported by
streams to depositional sites in marine environments. Instead these biochemical
rocks form from the organisms that lived on, in, and above the sea floor.
5. This chapter provides another opportunity to reinforce the concept of facies.
Specifically, you may try to clarify for students that facies move in relation to one
another and the shoreline as sea level rises and drops, or deltas prograde.
6. To illustrate how certain depositional environments are recognized by geologists,
offer a few sets of sedimentary rock types, textures, structures, and types of fossils
(e.g. mud cracks, rain drop prints, and fossil plants or animals) and have students
determine which depositional environments are most likely represented by the
collective evidence.
CONSIDER THIS
1. If a sedimentary facies is deposited during a transgression or regression, is that
facies of the same age everywhere? If so, why? If not, how might age equivalence
within the facies be demonstrated?
2. What types of sedimentary rocks would provide evidence of arid conditions? What
types of sedimentary rocks would provide evidence of tropical climates?
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
3. Has the distribution and abundance of some sedimentary rock types changed over
the duration of Earth history? Throughout the rest of the course, focus on how these
changes are tied to atmosphere and hydrosphere evolution, as well as changes in
plate tectonics.
IMPORTANT TERMS
alluvial fan
barrier island
biogenic sedimentary structure
Bioturbation
braided stream
continental rise
continental shelf
continental slope
cross-bedding
Delta
depositional environment
drift
fluvial
graded bedding
meandering stream
mud crack
outwash
paleogeography
playa lake
progradation
ripple mark
rounding
sand dune
sedimentary structure
sorting
stratification (bedding)
tidal flat
till
varve
SUGGESTED MEDIA
Videos
1. Weathering and Soils, Earth Revealed #15, Annenberg/CPB
2. Sedimentary Rocks: The Key to Past Environments, Earth Revealed #17,
Annenberg/CPB
3. Running Waters I: Rivers, Erosion and Deposition, Earth Revealed #19,
Annenberg/CPB
4. Running Waters II: Landform Evolution, Earth Revealed #20, Annenberg/CPB
5. Water Cycle and Erosion, EME Corporation
6. Coastal Dunes, Films for the Humanities and Sciences
7. Glacial Deposits, Films for the Humanities and Sciences
8. Limestone, Films for the Humanities and Sciences
9. River Channel Forms, Films for the Humanities and Sciences
10. Waves and Beaches, Films for the Humanities and Sciences
11. Geomorphology: Study of the Shape of Earth, Tell ME Why Sales Co.
12. Running Water: How it Erodes and Deposits, Tell ME Why Sales Co.
13. Waves, Coastlines, and Beaches, Tell ME Why Sales Co.
14. Weathering and Erosion, Tell ME Why Sales Co.
15. Living Glaciers and Signs of Ancient Ice Sheets, Educational Images Ltd
16. Sand Dune Ecology and Formation, Educational Images, Ltd.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Software
1. Earth’s Dynamic Surface, Tasa Graphics Arts, Inc.
2. Stream Meanders and Deltas, Educational Images, Ltd.
3. Earth’s Dynamic Surface, RockWare, Inc.
4. Explore Cross Sections, Geological Society of America
5. Stream Meanders and Deltas, Educational Images Ltd
6. Continental Glaciation, Educational Images Ltd
Demonstration Aids
1. Deep-Sea Sediments, slide set, Educational Images, Ltd.
2. Submarine Canyons and Deep Sea Fans slide set, Educational Images, Ltd
3. Sedimentary Rocks Features, JLM Visuals
4. Sedimentary Features and Environments, JLM Visuals
5. Primary Sedimentary structures: Graded and Convoluted Bedding, Bioturbation,
Loading, Rill Marks, Educational Images, Ltd.
6. Primary Sedimentary Structures: Cross Bedding, Ripple Marks, Mud Cracks
7. Sedimentary Rock Collection, Earth Science Educator’s Supply
8. Glaciers and Their Tracts, slide set, Educational Images Ltd
9. Beaches, slide set, Educational Images, Ltd
10. Seabeaches: Their Formation and Erosion, slide set, Educational Images, Ltd
11. Rocks and Topography: Sandstones, slide set, Educational Images, Ltd
CHAPTER 6 - ANSWERS TO QUESTONS IN TEXT
Multiple Choice Review Questions
1.
2.
3.
4.
a
c
e
b
5.
6.
7.
8.
d
d
a
c
9. d
10. d
Short Answer Essay Review Questions
11.
Evaporites are mostly rock gypsum and rock salt, formed in saline lakes, playa
lakes, and some marginal marine environments. Evaporites form in arid
environments with high evaporation rates.
12.
Inner shelf deposits are mostly sand, contain marine fossils, are bioturbated, and
are commonly cross-bedded. Bedding planes are marked by wave-formed ripple
marks. This is the high energy area that is periodically stirred up by waves and tidal
currents. Outer shelf deposits are typically mud with marine fossils. This the low
energy part of the shelf.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
13. Desert environments are commonly inferred from an association of features found
in sand dune, alluvial fan, and playa deposits. Alluvial fans form along the margins
of desert basins and form a triangular deposit of sand and gravel. Wind-blown
dunes are typically composed of well-sorted, well-rounded sand with cross-beds
that are meters to tens of meters high. They may have associated fossils of landdwelling plants and animals. The more central part of a desert basin might be a
playa lake in which laminated mud and evaporites accumulate.
14. Braided streams have multiple broad, shallow channels in which mostly sheets of
gravel and cross-bedded sand are deposited and mud is nearly absent. Meandering
stream deposits have mostly fine-grained sediments on floodplains but do have
cross-bedded sand bodies, each with a shoestring geometry. Meandering streams
have point bar deposits, consisting of a sand body overlying an erosion surface that
developed on the convex side of a meander loop.
15. Both are formed by moving water. Current ripple marks are small and have an
asymmetrical profile. They form where water flows in one direction over sand.
Wave formed ripple marks are symmetrical and small. They form where waves
move to and fro. Since current ripple marks show flow from one direction, they can
be used for paleoenvironmental analysis.
16. Current ripple marks can be used to determine ancient current directions because
they show flow from one direction. Cross-bedding can also indicate ancient currents
because individual beds are inclined downward in the direction of the flow.
17. Bottom to top: fine-grained flat-lying bottomset beds with marine fossils; foreset
beds; flat-lying topset beds with fossils of land plants and animals.
18. Graded bedding is usually the result of turbidity currents. These sediment-laden
currents move along the bottom of the seafloor. As the velocity diminishes, they
deposit large particles followed by progressively smaller ones, forming graded
bedding.
19. As the water level rises and falls with the tides, ripples will stack on top of each
other in opposite directions producing cross-bedding in opposite directions.
20. Both rounding and sorting are useful in environmental analysis. Rounding indicates
transportation, because gravel tends to become rounded quickly as particles collide
with one another. Smaller particles carried in suspension are usually not so wellrounded. If a deposit is well-sorted, the particles are all about the same size.
Glaciers are powerful and transport all sizes of particles, with deposition upon
melting. Wind has limited capacity to transport, so its deposits tend to be wellrounded & sorted.
64
R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Apply Your Knowledge
1. Fossils in sedimentary rocks will give indications about the climate, geography, and
geologic processes in the area when the organism lived. The types of rock deposits
(rock types, and associations) can also be used to determine whether the ancient
environment was continental (glacial, lake, desert, river), transitional (beach, delta,
barrier island, tidal flat) or marine (shelf, rise, deep seafloor). Textures of
sedimentary rocks, including rounding and sorting, provide clues about ancient
depositional processes. Sedimentary structures such as ripple marks, crossbedding, and mud cracks allow geologists to make environmental interpretations.
2. The dinosaur fossils within the sandstone and mudstone indicate that these deposits
were terrestrial, and sands and mud could be deposited within a floodplain. After
the death of the dinosaurs (or their exit from the area), a transgression took place,
as indicated by the sandstone-shale-limestone sequence. The sandstone appears to
be marine; the limestone is marine, as indicated by the fossils within in.
3. The “V” of the mudcrack opens toward the surface, indicating that the inner
sandstone lens to the left is younger. This is confirmed by the ripple marks, which
also indicate that the bottom of the sandstone layer is toward the outer edge of the
U-shaped fold. Graded bedding also indicates that the bottom of this bed is toward
the upper right, and lower right corners. Therefore, the youngest rocks are in the
middle of the fold (syncline).
4. The bedding in the image appears to be (large-scale) cross-bedding. This most
probably formed in a wind-blown environment, with the ancient current direction
originating in the left, and flowing to the right.
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