Review: sedimentary rocks were once sediments. The LITHIFICATION processes change sediments into sedimentary rocks, slowly and gradually.

THE STORIES THAT SEDIMENTARY ROCKS TELL:

Sedimentary Bedrock Story Part 1)

Sedimentary rocks tell us the story of the sediments that became the sedimentary rock, i.e. the environment in which the sediments were deposited.

Sedimentary Bedrock Story Part 2)

Sedimentary rocks tell us the story of how the sediments became consolidated rock.

Part 1 of the Sedimentary Bedrock Story.

WHAT SEDIMENTS BECAME THE SEDIMENTARY ROCK

(What a story! This is earth science at its most exciting ... clues to the past.)

Sedimentary rocks tell us the conditions when and where the sediments that became the sedimentary rock were deposited. REMINDER: Sediments are fragments of bedrock that have been moved.

(b) In order for there to be a sedimentary rock,

(a) there had to be sediments.

Key observations for Sedimentary Bedrock Story Part 1:

What was the environment like here when the sediments that became this rock were deposited?

Key observation #1:

Look at size, shape, and sorting of the non-cement components (clasts) of your sedimentary rock.

Are all the clasts about the same size, for example, all sand-size or all gravelsize? Are all the clasts the same material, for example, all quartz sand or all quartz gravel? Or is it a mixed-bag kind of rock.

Use your knowledge of sedimentary processes to hypothesize about the processes that sorted the clasts.

HINT: What are your choices?

1) Water dominates the erosional and depositional processes:

• streams,
• ocean currents,
• storm waves,
• lakes,
• runoff across hill slopes.

1) Wind erodes and deposits sediments but tends to be overrated as an agent of change.

2) Ice as glaciers is a significant agent of sedimentation but only in some very limited environments.

3) Gravity is a powerful and unrelenting agent of sedimentation such as landslides, rockfalls and other slope processes but its evidence usually doesn't last long enough to be preserved as a rock record. (Gravity drives the other processes as well, water, wind, and ice.)

SO ... think about water first and what environment ... stream, lake, etc.

Key observation #2:

Look for layering.

Running water sorts sediments by size and density. So when the particles are deposited, their sorting by size and shape is far from random. That's why some sediments look layered. The size and materials of a layer may be relatively uniform or may be gradational for example from large-size (coarse) material at the bottom of a layer to fine sediments at the top of a layer.

Think about how the water is sorting the layers. What kind of layering would you expect in a fast-flowing stream versus a slow-flowing stream, versus wind forming sand-dunes, or glaciers bulldozing moraines.

So ... keep thinking about water first and its many environments.

Key observation #3

Look at the color of your sedimentary rock.

As a sweeping generality, let color help you ... but don't depend on it.

CAUTION: the following observations are for sedimentary rocks. They won't hold true for igneous and metamorphic rocks and there are even exceptions for sedimentary rocks.

Red, maroon, blush red, pink, orangish red:

Sedimentary rocks will tend to be red if the sediments were laid down by water, wind, or ice in an alternatingly wet and dry environment such as a stream bed or mud flat along an ocean's edge. Small amounts of iron persist in virtually all sediiments. The iron oxisizes (like rust) and turns the rocks red. Think about the red rocks of the Colorado Plateau. Notice how some of the bedrock of Salt Lake County is red. Red Butte, the reddish mountain just east of the University of Utah and the Red Butte Gardens and Arboretum, is a good example of a series of layer and layer and layer of sedimentary rock that was once sediments laid down in streams, on mud flats, and as sand dunes.

Grey, blue gray, dark gray. tan gray, gray banded:
In contrast, sedimentary rocks will tend to be gray if the sediments were laid down by water, wind or ice in water such as into a lake or ocean.

For example, the northern part of the Oquirrh Mountains has many gray sedimentary rocks. The sediments started out with about as much iron as those of Red Butte, i.e. not much. But because the Oquirrh's sediments were laid down long ago by water in water in an environment similar to today's Gulf of Mexico and not exposed to oxygen they didn't oxidize, (their iron is "reduced"). So the rocks of the Oquirrhs are not red, they are gray. Most sediments deposited in oceans or lakes are gray.

Part 2 of the Sedimentary Bedrock Story:

HOW DID THE SEDIMENTS BECOME SEDIMENTARY BEDROCK?

Key observations for Sedimentary Bedrock Story Part 2:

Key Observation 1)
Look at what is holding the grains of your sedimentary rock together. Do you see gray or white cement filling the voids and holding the grains together? Do you see lots of voids (empty places)? Or do the individual grains look small enough and so compact that there is no need for cement?

Trust your observations.
The Sedimentary Bedrock Story Part 2 is pretty straight-forward. A combination of chemical and physical processes turns the loose unconsolidated sediments into consolidated, firm, coherent, continuous bedrock. Compaction and cementation are the two major physical and chemical processes of lithification. Fluids greatly affect the rate and nature of the processes. Some cementation can take place right at the earth's surface, but cementation and compaction usually take place as sediments are buried by more sediments, and more sediments. Pressure of the overlying mass of sediments helps make sediments firm and coherent sedimentary bedrock. Groundwater precipitates chemicals in void spaces as it moves through the sediments. The chemically-deposited minerals such as calcium carbonate (lime) fill the pore spaces and bind neighboring pieces of sediment. Heat increases the deeper the sediment is buried and accelerates the process of lithification.

Key Observation 2)
How "firm and coherent" is your rock? Can you break it with your hands? How difficult is it to chip of a chunk of it with a rock hammer? This tells how far along the lithification process has progressed

The demarcation between sediments and sedimentary rocks is vague although the two end-members are easily distinguished. Some geologists would argue that you can dig the material with a front-end loader, call it sediment, but if you have to blast it, call it sedimentary rock (from Charlie Hunt, retired USGS geologist, resident of West Valley City). Others would have different definitions. Can you draw some inferences about the lithification process by the hardness of the rock?

COMMON SEDIMENTARY ROCKS:

Remember ... the most important teaching concept is to tell the story of the sedimentary rock. However ... names are helpful handles of communication and if your students want to learn a few names, here are the names of three common rocks of Salt Lake County.

Sandstone:

• consolidated from loose sand, usually cemented by some lime
• environments of deposition ... a beach, or a sand-dune, or a slow-moving river.

Conglomerate:

• consolidated from loose gravels and sands
• environments of deposition ... streams, or high-energy beaches

Shale:

• consolidated mud
• environments of deposition ... lake sediments, ocean muds, mud flats.

THREE EASY STEPS TO TELL THE STORY OF A SEDIMENTARY ROCK. ...

Imagine, you have just been handed a piece of sedimentary rock.

Bedrock Story Step 1)
The rock is a piece of sediment. What bedrock did it come from? You recognize the rock is a sedimentary rock by its layering and because of its constituents of sand grains, or gravel, or gray layering.
"What an interesting rock ... look how it used to be just a lot of loose pebbles and now it's cemented all together. It takes a long time for the little bit of mineral in ground water to cement all these pebbles together into a rock."

You examine the rock with the help of students.
You notice what holds the rock together and comment on how the sediments became rock (cemented? compacted? both?).
"Notice how cement is holding all the little sand grains together. That's what makes this rock a rock and not just a handful of sand. »
-or-
"Notice how the grains just barely touch each other and how much open space there is in this solid rock. Water can actually flow through this rock, like a rock sponge."

You comment on the non-cement components (clasts) of the rock, their size and how well or poorly the clasts have been sorted (all the same stuff or not).
"Look at all the gravel in this rock and how they are all the same size."

You look for details of layering
"This cross-pattern looks really interesting. "

You take note of the color of the rock. .. reddish or grayish.
"Gray rocks that split easily are some of my favorites because they sometimes have fossils. "

Bedrock Story Step 2)
What kind of environment deposited the sediments of this sedimentary rock?

Based primarily on the size of the clasts (gravel, sand, clay) and their sorting you consider the most likely way it was deposited.
"This is just the type of sand you get on a beach. I bet your rock tells the story of an ocean beach like what we see in California today, but it also looks like the sand of a sand dune. Which do you think it is ... sandybeach or desert sand? how come? I wonder what it was like back then."
-or-
"This rock reminds me of what I saw along a river ... muds and sand and small pebbles all in one area but sorted by layers. Look how rounded each of the pebbles is and so smooth. I wonder where this river was going and where it came from because there's no river like that here now. I wonder what it was like back then. "

Step 3)
Where is the rock going (on the rock cycle)?

Just about any rock a child gives you is being broken up and dissolved into smaller bits of rock and carried by water downhill.

In much of the country, the bits of rock are headed toward the ocean unless they are thwarted and buried as sediment before they get there. In Salt Lake County, the bits and pieces of sediment are headed toward the lowest part of the basin, Great Salt Lake, unless they get thwarted and buried as sediments ... or made into concrete ... or added to a rock collection. But what will eventually happen to the rock collection? And eventually what will happen to the sediments buried in Salt Lake Valley? And then what may happen?

STORIES ABOUT METAMORPHIC ROCKS >>