4.B.4 Formation of Clastic Sedimentary Rocks

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From the course by University of Illinois at Urbana-Champaign

Planet Earth...and You!

29 ratings

University of Illinois at Urbana-Champaign

Planet Earth...and You!

29 ratings

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.

From the lesson

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

Meet the Instructors

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

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So how do clastic sedimentary rocks form?

There are actually several stages involved,

and we'll go through them in succession.

The process begins by the production of sediment.

Typically, this happens where you have a slope or a cliff of exposed bedrock

because bedrock tends to contain natural joints.

Joints are simply natural cracks that occur in rocks.

Now, blocks will tend to fall off cliffs at the joints, and

tumble down and break up into blocks.

So here we have an example of a small cliff, grass on the top, in Canada.

And you can see the broken blocks at the bottom.

This other photograph shows a very tall cliff,

probably a hundred meters high on the seashore of Western Ireland.

You can see at the top of the cliff, that there are horizontal beds

defined by the somewhat horizontal stripes on the outcrop.

And, there are vertical lines, and the vertical lines are joints.

And notice how individual blocks have broken off the cliff, at the joints and

beds, and are now collecting as a pile of debris at the bottom of the cliff.

Over time, large accumulations of this debris may form on slopes or

at the base of slopes.

Sometimes we call these piles talus piles.

And here's an example, on the side of a cliff in the Uinta mountains of Utah.

Now, as time passes, streams, glaciers,

other moving fluids can pick up these clasts and transport them.

So the second stage during the development of a clastic sedimentary rock,

is the process of transportation.

Now during transportation, the grains bash against each other,

they may undergo weathering and gradually break apart.

But basically the process leads to the gradual decrease in size

of the individual grains, and generally, in the rounding of the individual grains.

Here's a video showing an example of

stream gravels flowing along the bottom of a river.

It may look like not much is happening when you're cruising along the top of

the stream, but underwater there's really quite a bit of sediment transport.

Some within the water itself and

some bouncing along the bottom of the stream bed.

Each time one of the grains or clasts bangs into another, it chips off a piece,

so gradually the grains become progressively rounder.

So, when you go to a place like this beach on the coast of Cyprus, or

this stream valley in the mountains of California, the grains are not as

angular as those joint-bounded blocks that we saw on the base of cliffs.

Rather, they're beginning to get rounded.

The further they've transported, generally the more they've broken down until

eventually you end up with grains of various sizes,

some as fine as clay going up to some that are pebbles or cobbles.

Here's an example of a desert stream that only flows during floods.

And you can see that its floor is littered with clasts.

If the deposits of the stream were to be buried, and cemented, and

turned into rock, we would see layers of conglomerate, and sandstone.

We mentioned already, that in sedimentary rocks, we typically see bedding.

How does bedding form?

Well if you think about what a bed is, a bed is a distinctive layer, usually

it's different from what's above and below by a slight difference in composition or

a major difference in composition.

It may be that one bed is shale, and one bed is sandstone,

and one bed is conglomerate.

In some cases the differences are more subtle.

In some cases there are little soil horizons that had formed between

the deposition of one layer and the next,

and those are preserved as features called paleosols.

But in any case,

we very often will see very distinctive layering in sedimentary rocks.

Let's look at this particular example.

Imagine that the sediment that is ultimately going to form the sedimentary

rock was deposited a the stream.

Well, the ability of a moving fluid, like water or

like air, to carry sediment depends on the velocity of flow.

If the wind blows fast, it can carry larger particles, sometimes roofs and

boards and debris.

But if it blows slowly, it can only carry dust.

Similarly, if a stream is moving very slowly, it can carry clay and

perhaps a little silt.

But if it's moving fast, it can carry sand, or gravel, or even large boulders.

Let's take this example here of the succession of deposition in a stream,

an imaginary stream.

Starts out during a time when the flow of the stream is fairly slow.

So it's only able to carry fine grained material like mud,

maybe a little silt, deposit some layers of fine grain sediment

that will turn into layers of shale and siltstone.

Later on, there's a storm upstream or nearby and

the flow increases dramatically.

The flowing water is able to pick up coarser grains, carry them down, and

deposit them as a layer of sand and gravel on top of the siltstone.

Later, the flow decreases again and we have a repetition of the process.

A layer of clay and silt gets deposited.

Ultimately, if this whole succession gets buried and

turned into rock, we may end up with a succession of beds,

such as what we're seeing in this picture here on a road cut in Utah.

Alternating sandstones, and siltstones, and shales.

The particular type of sedimentary rock that forms

depends on the depositional environment.

That means, the conditions where the sediment is accumulating.

For example, if you have a beach where waves are washing back and

forth all the fine grained stuff will be carried by the moving fluids

out into deeper water leaving behind only the coarser sand.

So you end up sandstone if that sediment gets buried.

If you're out in a quiet-water lagoon, there won't be any sand,

you'll just get mud depositing.

And if that material gets buried and turned into rock, you get shale.

If you're in a stream bed where the flow is fast enough to carry away even sand,

you'll be left with just coarser grains, pebbles and cobbles.

And that material, if it gets cemented, will turn into conglomerate.

So, if we look at an environment where we have, say, mountains in the distance,

stream in the foreground, lakes, sand dunes, and so forth,

we see the different kinds of sedimentary material get deposited in different areas.

And as a consequence, we get a variety of different kinds of sedimentary rocks.

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