Historical Geology
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
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.
¨ 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.
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
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
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 .
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
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.
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?
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.
|
alluvial fan |
depositional environment |
ripple mark |
|
barrier island |
drift |
rounding |
|
biogenic sedimentary structure |
fluvial |
sand dune |
|
Bioturbation |
graded bedding |
sedimentary structure |
|
braided stream |
meandering stream |
sorting |
|
continental rise |
mud crack |
stratification (bedding) |
|
continental shelf |
outwash |
tidal flat |
|
continental slope |
paleogeography |
till |
|
cross-bedding |
playa lake |
varve |
|
Delta |
progradation |
|
Demonstration Aids
|
1. a |
5. d |
9. d |
|
2. c |
6. d |
10. d |
|
3. e |
7. a |
|
|
4. b |
8. c |
|
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.
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 land-dwelling 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 well-rounded. 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 well-rounded & sorted.
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,
cross-bedding, 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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