Sedimentary Rocks

We rock. We come in rock form and sediments. We're a huge part of geology, in fossils, in dating. We are essentially the most important kind of rock there is and we are everywhere.

[what-do-i-know-about-geolog-blog]

What are some of the ways we can observe a rock's history today?

First, we would have to determine the rock’s age. Did you know there’s two different types of aging? Relative and absolute?

What’s the difference? Well, relative would be like saying, this rock is older than this rock. Or this tree is the oldest relative to those other three trees.

While absolute aging, is saying that rock is 150 millions year old. And that tree is 23 years old. Get the picture?

So, to observe a rock’s history, we would need to determine it’s absolute age first, which can be done somewhat accurately through radiometric dating.

Keep in mind, that radiometric dating, generally only works with metamorphic or igneous rocks. Not sedimentary.

[the-sediments-blog]

What are you talking about? Sediments are awesome, we work everywhere.

[the-sediments-blog]

Hey.

Know where else sediments are awesome?

Fossils. We make the best fossils. Paleontologists love us.

[the-sediments-blog]

Know where else sediments are great? Glaciers.

Continental and alpine, sediments help so much with glaciers.

[the-sediments-blog]

How are sediments useful?

Sediments give us clues to past environments, with fossils and layers. They can tell us the direction of the water or wind during that time period, and with graded bedding, they could tell us just how fast or slow the water/wind was moving. They can tell us just what kind of life lived in this environment, how many there were, whether they were land or marine creatures.

[what-do-i-know-about-geolog-blog]

You should probably explain what graded bedding is…

[the-sediments-blog]

Graded bedding is basically when the sediments are layered by size. 

So, you can see the larger sediments are at the bottom. That’s because they’re heavier so they fell faster that the other sediments. The next largest would fall next and so on, which creates this gradation of sediments. Obviously, that means the finest particles, like silt and clay, are at the top.

This picture is a good example of graded bedding, especially since it has very distinct different depositional periods. If it were more angled, it could be called crossbedding, where the beds are deposited on an angle, according to water/wind currents. However, with crossbedding, it doesn’t need to be layered by size.

[the-sediments-blog]

How are sediments useful?

Sediments give us clues to past environments, with fossils and layers. They can tell us the direction of the water or wind during that time period, and with graded bedding, they could tell us just how fast or slow the water/wind was moving. They can tell us just what kind of life lived in this environment, how many there were, whether they were land or marine creatures.

[the-sediments-blog]

Types of Sediment

(WARNING: SOME ARE TASTIER THAN OTHERS)

Sediments are formed from weathering and erosion of rocks. They come in a multitude of sizes, from very very fine (silt and clay) to very coarse (gravel, pebbles, boulders)

[he-who-knows-all-geo-blog]

To be more specific, the order goes 

silt/clay

fine sand

sand

fine gravel/fine pebbles

gravel/pebbles

boulders

(smallest to the largest)

However, this is only for sediment size.  It doesn’t include the actual types of sedimentary rocks. 

[the-sediments-blog]

Yes, so sedimentary rocks, actually go one more step past sediments. After being eroded and transported, they get deposited and layers begin to form. And once enough pressure from the layers on top is obtained, lithification (compaction and cementation) takes place to form rocks.

Three types, with a bunch of types within those types..:

Clastic

conglomerate: well rounded, coarse grained

breccia: clasts more angular

tillite: from glaciers

sandstone: sand sediments, well rounded

siltstone/claystone: mudstone

shale: easy cleaves along bedding

Chemical

limestone: fizzes with acid, precipitate of carbon carbonate

chert: precipitate of silica, diatoms

evaporites: crystals left behind evaporation  (halite)

Organic/Biochemical

limestone

chert

coal: plant/animal remains

gypsum: desert rose

Sedimentary rocks form usually in areas of still waters, seeing as the sediments need to be deposited beforehand. And of course, that means that something had to have been weathered to create those sediments. Rivers are an excellent means of weathering and sorting sediments. Where the river ends/enters a larger body of water is where most sedimentary rocks are.

[the-sediments-blog]

Sedimentary rocks...

sediments: Are the best kind of rocks.

igneous: Liar!

metamorphic: I agree.

sediments: You're all just jealous. We get to travel everywhere.

igneous: So do we!

sediments: Not as much as we do! We see the top of mountains to the bottom of the ocean.

igneous: We do too! It's called plutons! Beside, we have more crystals than you.

metamorphic: *cough, cough* Not as many as we do!

sediments: Who needs crystals, when you've got clay?

igneous: Shut up, metamorphic! No one can ever find you!

metamorphic: You shut it! You'll be just like me one day!

sediments: why would anyone want to be metamorphic, when they can be sediments instead?

[what-do-i-know-about-geolog-blog]

Speaking of...

Let’s get back on track. Where were we?

Ah, yes, weathering. They shape our landscape by tearing away at rocks and wearing it down to a sand. One way to do that is with water. A lot of it, and moving really fast. Like in rivers.

River processes are quite extensive. 

First thing to remember is that, rivers are measured by their competence (max size particle it can transport) and their capacity (total volume of sediment that can be transported)

They range from old to young, but it’s fairly easy to tell which is which.

Young rivers have:

rapids

waterfalls

vertical erosion

no flood plain

V-shaped valley

Whilst older rivers have:

meandering streams

horizontal erosion

[he-who-knows-all-geo-blog]

Younger rivers also move faster than older ones. With younger rivers, they don’t wander side to side but simply cut right down the middle, fairly easily with their speed and size. Most erosion is by abrasion or hydraulic action. If the rock is incredibly hard, then the river will actually cut right through it, while the sides stay fairly steep, thus V-shaped valley.

Older rivers, or as rivers age, develop meandering streams which means to have a single channel with a sinuous pattern. As they grow older, the meandering streams become more bradided, more complex, sometimes even reaching point of creating an oxbow lake (free-standing body of water, created when a meander cuts off a part of a river)

[the-sediments-blog]

*coughs* Is this where we come in?

[he-who-knows-all-geo-blog]

You forgot:

Mass wasting can also be affected by weathering. A lot of geology is affected by weathering. 

Weathering is everywhere.

[what-do-i-know-about-geolog-blog]

Ah, right! Weathering, a very big part of geology, the part that ruins most geology and geological features. Weathering actually causes many problems in determining ages but it has created the landscape that we live in today.

[ignorant-existence-blog]

How would would the weather have any effect on our landscape? It’s just rain and snow and stuff right?

[the-sediments-blog]

You ignorant primate. You believe weather has no effect on your environment?! It allows mountains to rise and grow, it creates rivers! the weather affects your very lives!

[the-sediments-blog]

Magma: I'm too hot

Igneous: Hot damn

Magma: ...

Igneous: Sorry mom.

[the-sediments-blog]

Types of Sediment

(WARNING: SOME ARE TASTIER THAN OTHERS)

Sediments are formed from weathering and erosion of rocks. They come in a multitude of sizes, from very very fine (silt and clay) to very coarse (gravel, pebbles, boulders)

[what-do-i-know-about-geolog-blog]

Plates! Not kitchen plates! Tectonic plates! They are on top of the Earth’s crust, literally the land we live on. The very earth we walk and stand upon! See?! Look at them! Those arrows indicate which way they are moving. Understand?! 

[ignorant-existence-blog]

They can’t move! They’re rocks! You just said that it’s land. Land doesn’t move!

[the-sediments-blog]

This is going to take a while...

[what-do-i-know-about-geolog-blog]

The most basic thing you will ever need to know about geology: the Rock Cycle!

[ignorant-existence-blog]

a rock-cycle?

[what-do-i-know-about-geolog-blog]

*face-palm* Not even close…

[the-metamorphic-blog]

How can you not know the Rock Cycle?

[the-sediments-blog]

My entire life is the Rock cycle.