4.B.5 Primary Sedimentary Structures and Sedimentary Basins

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来自 University of Illinois at Urbana-Champaign 的课程

Planet Earth...and You!

25 个评分

University of Illinois at Urbana-Champaign

Planet Earth...and You!

25 个评分

Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions—all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today. Planet Earth presents an overview of several aspects of our home, from a geological perspective. We begin with earthquakes—what they are, what causes them, what effects they have, and what we can do about them. We will emphasize that plate tectonics—the grand unifying theory of geology—explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity—including a variety of devastating natural disasters such as earthquakes—occur where they do. We consider volcanoes, types of eruptions, and typical rocks found there. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future.

从本节课中

Week 4: Rocks and Mineral Resources

As part of the Week 4 Assignment, you will take a close look at your daily surroundings to identify Earth resources. The video lectures for the week examine various aspects of finding, extracting, and using resources such as metals and stones. For the lab, you will utilize Google Earth to examine several mining sites around the world. In the discussion, you will weigh the pros and cons of mining operations, as many communities have had to do already. This week also includes peer grading discussions, as explained on the How Graded Discussions Work page. Finally, we provide an optional assignment for those who would like to identify some common minerals.

4.B.1 Introducing Classes of Rocks5:42

4.B.2 The Nature of Igneous Rocks9:25

4.B.3 Different Types of Sedimentary Rocks4:46

4.B.4 Formation of Clastic Sedimentary Rocks6:53

4.B.5 Primary Sedimentary Structures and Sedimentary Basins6:26

4.B.6 Carbonate Sedimentary Rocks6:11

与讲师见面

Dr. Stephen Marshak
Professor and Director of the School of Earth, Society, and Environment
Department of Geology
Dr. Eileen Herrstrom
Lecturer
Geology

[MUSIC]

If you think about the conditions under which sedimentary rocks are deposited,

you can imagine that there are going to be certain kinds of features that develop on

the surfaces of beds or

sometimes inside of beds that reflect the conditions of deposition.

For example, let's look at a sand dune that's being deposited in the desert.

On this particular day when the photograph was taken, the wind was blowing very,

very strongly.

The wind was blowing from the left to the right across the photograph,

bringing sand up the back side of this sand dune until it accumulated and

then slipped down the face of the sand dune.

Overall, beds tend to be horizontal.

They can't be deposited on a slope, because if they were, gravity would

eventually pull the sediment down to the floor of the basin, further offshore.

So in this case, sand overall is being deposited in a horizontal layer.

But within the sand, there are these successions of slopes developing.

The overall layering is called the main bedding.

The inclined layer, with inside the main bed, is called a cross bed.

We're seeing them form here in these modern sand dunes.

Here, we're seeing the relict of cross bed preserved

in cliffs of sandstone that formed hundreds of millions of years ago

in a place that's now exposed in Zion National Park in Utah.

These are Jurassic sand dunes.

You can see the main beds as the boundaries between the individual layers

and the cross beds as the inclined slopes within the individual layers.

At the time of deposition, keeping in mind the image we had in our last slide,

the wind was blowing from left to right.

Subsequent to deposition, these rocks have been tilted a little bit, so

the layering is now no longer perfectly horizontal.

Another feature, or another type of what we call sedimentary structure that forms

during deposition, are ripple marks.

If you go to a beach or you look at the floor of a stream, you'll

see that the surface of the sediment typically is not perfectly planar, but

is rippled into individual bumps and valleys, which have elongate crests.

These features are called ripple marks.

Here we're seeing an example formed along the coast of Cape Cod in

Eastern United States exposed at low tide.

Now, look at that image and compare that to this image.

They look almost identical.

But the rocks that we're seeing in this image,

first of all, the beds are no longer horizontal,

because mountain building processes tilted them to an angle of almost 45 degrees.

But also, these rocks were deposited

originally at the time the dinosaurs lived.

In fact, just around the corner from this exposure,

there are dinosaur footprints superimposed on the ripple marks.

We've said that sedimentary rocks form in a variety of stages.

First, there's production of the sediment.

Second, there's the transportation of the sediment.

Third, there's the deposition of the sediment.

And fourth, there's burial and the process of turning the sediment into rock.

Let's look at these final stages.

There are certain places in the earth where the crust over time may start at,

at one elevation.

And if we look in cross-section here, over time,

the crust gradually warps down as so.

As I'm showing here.

Now, this process by which the crest gradually sinks is called subsidence.

The depression and its contents is called a sedimentary basin.

In a sedimentary basin,

the accumulation of sedimentary beds is much thicker than its surrounding regions.

So you can imagine all these layers down here are composed

of layers of sediment that have accumulated.

Now, there are various tectonic reasons for why sedimentary basins form.

Some of them are a consequence of loading of the crest by a weight,

like a mountain belt that pushes an area down and

creates a depression that then fills with sediment.

Some are formed along the margins of continents, where the boundary

between the continent and the ocean basin gradually sinks as it cools.

There are various reasons, and we don't really have time to go into those now.

But basically, this process of subsidence can take millions,

tens of millions, even hundreds of millions of years,

creating a low area that accumulates sediment over geologic time.

Now, if you come down to some depth within the sedimentary basin and

enlarge that area, here are the layers of sediments.

You can imagine that there's a compression in the vertical,

in the vertical direction,

we're squeezing those sediments because of the weight of the overlying material.

As that squeezing process takes place, water

that was once in the grains, or once between the grains will escape.

And the grains will pack closer together.

Later on, we may have waters coming through that carry dissolved minerals.

And out of these waters

we precipitate water carrying

dissolved minerals.

That water can stick the grains together by cement.

So basically, the process of lithification,

the process of turning sediment into rock, involves two phenomena.

First, the compaction of sediment, the squeezing it together,

to remove pore space and to push the grains closer together.

And secondly, the process of cementation,

producing mineral cements that hold the grains together.

Once that process is complete, we no longer call the material sediment.

We call it a sedimentary rock.

[MUSIC]

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