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Sedimentary geology
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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
Structure Grain size Sorting Flow direction (e. g. R to L)
Depositional Environment
1a
1b
1c
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)?
Grain size Sorting Structures 3a
3b
3c
Grain size Sorting Structure Depositional Environment 4a
4b