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07lab sedimentary structures, Study Guides, Projects, Research of Geology

Sedimentary geology

Typology: Study Guides, Projects, Research

2015/2016

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ESS 210 Lab 7: Sedimentary Structures
Name: _______________________________
Lab 7: Sedimentary Structures
Sedimentary rocks account for a negligibly small fraction of Earth’s mass, yet they are
commonly encountered because the processes that form them are ubiquitous in the near-surface
environment. Thus, they preserve the history of that portion of the planet that is most familiar.
Sedimentary rocks indicate paleoenvironment, i.e. ancient climates and ecology. Sometimes
they provide the only remaining evidence of former mountain ranges or shallow seas.
Sedimentary rocks are also essentially the only type of rock that contains fossils, which not only
are indicative of previous environments, but also are crucial in dating and correlating rock units.
Sedimentary rocks also provide a record of previous geologic hazards such as seismic events,
volcanic eruptions (ash deposits), storms, and fluctuations in climate. Furthermore, key
economic natural resources involve sedimentary rocks. Resources such as coal, oil, natural gas,
gypsum, aggregate (sand and gravel), and salt are all found within sediments.
Sedimentary Structures
Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and
mudcracks can be preserved in sedimentary rocks. These structures provide important
information about depositional environments such as flow direction, climate (arid, semi-arid, or
humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which
direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so
establishing the original orientation is not always easy, but is often useful.
Bedding
Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly
important sedimentary structure called bedding. Bedding layers can range in thickness from
millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in
orientation; tilted bedding indicates that the rock has been deformed in some way.
Lithification
Sedimentary rocks start out as loose sediment. To become a sedimentary rock, the sediments
must be lithified, which involves compaction and cementation. Compaction occurs through
pressure via deep burial. Water is removed and the grains are packed tightly together. During
cementation, minerals such as quartz, calcite, or hematite precipitate out of water and fill the
spaces between the clasts, locking them together. The term friable describes a poorly cemented
rock that falls apart easily. Note: rocks can become friable either because they were never
cemented thoroughly, or because the cement has been re-dissolved and removed.
Sorting
Sorting describes sediment in terms of the variability in the size of its particles. Sediment having
a wide range of particle size, as in a jumble of gravel, sand, and finer particles deposited by a
flood, is poorly sorted. If the size range is small, as in a uniform gravel, the sediment is well
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Name: _______________________________

Lab 7: Sedimentary Structures

Sedimentary rocks account for a negligibly small fraction of Earth’s mass, yet they are commonly encountered because the processes that form them are ubiquitous in the near-surface environment. Thus, they preserve the history of that portion of the planet that is most familiar. Sedimentary rocks indicate paleoenvironment, i.e. ancient climates and ecology. Sometimes they provide the only remaining evidence of former mountain ranges or shallow seas. Sedimentary rocks are also essentially the only type of rock that contains fossils, which not only are indicative of previous environments, but also are crucial in dating and correlating rock units. Sedimentary rocks also provide a record of previous geologic hazards such as seismic events, volcanic eruptions (ash deposits), storms, and fluctuations in climate. Furthermore, key economic natural resources involve sedimentary rocks. Resources such as coal, oil, natural gas, gypsum, aggregate (sand and gravel), and salt are all found within sediments.

Sedimentary Structures Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and mudcracks can be preserved in sedimentary rocks. These structures provide important information about depositional environments such as flow direction, climate (arid, semi-arid, or humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so establishing the original orientation is not always easy, but is often useful.

Bedding Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly important sedimentary structure called bedding. Bedding layers can range in thickness from millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in orientation; tilted bedding indicates that the rock has been deformed in some way.

Lithification Sedimentary rocks start out as loose sediment. To become a sedimentary rock, the sediments must be lithified, which involves compaction and cementation. Compaction occurs through pressure via deep burial. Water is removed and the grains are packed tightly together. During cementation, minerals such as quartz, calcite, or hematite precipitate out of water and fill the spaces between the clasts, locking them together. The term friable describes a poorly cemented rock that falls apart easily. Note: rocks can become friable either because they were never cemented thoroughly, or because the cement has been re-dissolved and removed.

Sorting Sorting describes sediment in terms of the variability in the size of its particles. Sediment having a wide range of particle size, as in a jumble of gravel, sand, and finer particles deposited by a flood, is poorly sorted. If the size range is small, as in a uniform gravel, the sediment is well

sorted. Changes in grain size typically result from fluctuations in the velocity of the transporting agent, such as water or wind.

Depositional Environments A sedimentary environment is a geographic location characterized by a particular combination of geologic processes and environmental conditions. Geologic processes include the currents that transport and deposit sediments (water, wind, or ice) and the plate tectonic settings that affect sedimentation. For example, the geologic processes of a beach environment include the dynamics of waves crashing against the shore, shoreline currents, and the distribution of sediments on the beach. Environmental conditions include the kind and amount of water (ocean, lake, river, arid land), the landscape (lowland, mountain, coastal plain, shallow ocean, deep ocean), and biological activity.

Figure 7.2 Depositional Environments

Station 1

  1. Describe samples 1a, 1b, and 1c.

Structure Grain size Sorting Flow direction (e. g. R to L)

Depositional Environment

1a

1b

1c

  1. Examine samples 1d and 1e.

a. What is the flow direction in 1d? (R to L, L to R, or indeterminate?)

b. Which side is the top of sample 1d (A or B)?

  1. What sedimentary structure is common to all of these samples?

Station 3

  1. Describe samples 3a, 3b, and 3c.

Grain size Sorting Structures 3a

3b

3c

  1. Samples 3a and 3b formed in a different type of environment than 3c. Was the environment for 3a and 3b much higher energy, much lower energy, or did the two environments have a similar energy? What indicates this?

Station 4

  1. Describe samples 4a and 4b.

Grain size Sorting Structure Depositional Environment 4a

4b

  1. Does sample 4a have a structure that you would expect to be commonly preserved? Why or why not?