Chapter 7: Pages of the Earth's Past: Sedimentary Rocks

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You know that feeling when you pick up a handful of sand?

It just seems like, well, sand.

But Rachel Carson had this beautiful quote, in every grain of sand, there is a story of Earth.

That's a great way to put it.

And it's so true for what we're diving into today, sedimentary rocks.

Exactly.

Think of them like Earth's history books, you know, layers upon layers, each one recording some ancient event or environment.

Yeah.

And they often form this kind of cover over the older, harder rocks underneath the igneous and metamorphic stuff geologists sometimes call the basement.

So our mission today in this deep dive is to really explore these sedimentary rocks.

We'll look at how they form the different kinds, the incredible stories they tell about Earth's past.

And also where we find these really thick stacks of them.

Right.

We want to give you a solid grasp of this really fundamental part of geology quickly,

but thoroughly.

Okay.

So these Earth story books, sedimentary rocks, they basically come in four main types based on how they originated.

Yeah.

Four main classes.

The first one is clastic.

Now clastic, that comes from a Greek word meaning broken.

Right.

Exactly.

That tells you a lot.

Clastic rocks are essentially cemented together fragments, bits and pieces of older rocks.

Like that sandstone from Antarctica shown in the chapter.

Perfect example.

Just broken bits glued back together.

Okay.

Type two.

That would be biochemical sedimentary rocks.

Think shells.

Shells.

Like from sea creatures.

Rocks formed from the accumulation of shells grown by organisms.

Limestone is the classic one here.

Got it.

And number three.

Organic sedimentary rocks.

These are carbon rich.

Formed from the remains of plants or other organisms that got buried and preserved.

Coal, for instance.

Coal is the prime example.

Yeah.

And then finally, number four is chemical sedimentary rocks.

Okay.

Chemical.

How does that work?

These form when minerals that were dissolved in water actually precipitate out of the solution.

They crystallize directly from the water.

Like salt forming when water evaporates.

Exactly like that.

Evaporates are a key type we'll discuss.

And geologists also have some descriptive words based on what the rocks are mostly made of.

They do.

If it's mostly quartz, it's called siliceous.

Siliceous.

If it's mostly clay, it's argilicious.

Argilicious.

We've got it.

And if it's rich in carbonate minerals, like calcite or dolomite, we call it a carbonate rock.

Right.

And how common are these different types?

Is one more dominant?

Oh, definitely.

Clastic rocks, the siliceous and argilacious ones make up the vast majority.

Something like 70 to 85 % of all sedimentary rocks.

Wow.

Okay.

So most sedimentary rocks are made of broken bits.

That's right.

The biochemical and chemical ones, most of the carbonates make up the other 15 to 25%, roughly.

Other types are pretty minor.

Okay.

Let's focus on the big category then.

Clastic rocks.

How do these broken pieces actually become solid rock?

It seems like quite a process.

It is.

Yeah.

You can think of it as a five -step process, really.

Five steps.

Okay.

Step one.

Weathering.

You need to break down the original rock first.

Like mountains crumbling over time.

Exactly.

Physical breakdown, chemical breakdown, turning solid rock into loose grains and dissolved ions.

Intralude B in the chapter goes into detail on weathering processes.

Okay.

So weathering creates the raw material.

What's next?

Step two.

Erosion.

You need to get those loose bits moving.

So wind, water, ice.

Right.

Something picks them up.

Precisely.

Gravity pulling stuff downhill, rivers carrying sediment, glaciers scraping the land, wind blowing sand.

That's erosion, the removal of the debris.

And once it's moving, that's transportation.

Step three.

You got it.

And how far and what size of material gets transported depends heavily on the agent doing the transporting.

How so?

Well, think about water.

Really thick, slow moving stuff like honey.

High viscosity, low velocity.

Yeah.

Carry much.

But a fast flowing river.

Low viscosity, high velocity.

It can move big cobbles, even boulders during floods.

Okay.

What about ice and wind?

Ice is amazing.

It's so viscous.

It can carry anything, regardless of size, even if it's moving slowly.

Huge boulders, fine dust.

It's all embedded.

Wind is more selective.

Strong winds can move sand and dust, but a gentle breeze might only carry the finest dust particles.

So weathering, erosion, transportation.

Yeah.

The sediment is on the move.

Yeah.

Is this stuff somewhere, right?

Absolutely.

That's step four.

Deposition.

The sediment settles out of the transporting medium.

Like when a river flows into a lake and slows down.

Perfect example.

The water loses energy, can't carry its load anymore, so the sand and mud drop out.

Or when a glacier melts, it just dumps everything it was carrying.

And the final step,

turning that pile of loose sand or mud into actual rock.

That's step five.

Lithification.

Turning sediment into stone.

Lithification.

How does that happen?

Two main processes involved here.

First, compaction.

Just squashing it.

Pretty much.

Yeah.

The weight of all the sediment piling up on top squeezes the grains closer together, pushing out water and air.

Makes sense.

And the second process.

Cementation.

Think of it as natural glue.

Yeah.

Groundwater flows through the tiny spaces left between the grains.

And that water often carries dissolved minerals, like quartz or calcite.

As the water flows through, these minerals can precipitate out, crystallizing in the pore spaces and cementing the grains together.

Okay.

So weathering, erosion, transport, deposition, lithification.

That's how we look at it, to describe it, to understand its story.

Ah, good question.

Geologists look at several key characteristics.

The most basic is class size or grain size.

How big the pieces are.

Exactly.

There's a whole scale.

From massive boulders down through cobbles, pebbles, which we collectively call gravel.

Then sand, silt, and finally mud, which includes the tiniest clay particles.

Right.

I remember seeing a table with the actual sizes.

Yes.

Table B .1 in Interlude B lays that out.

So size is fundamental.

What else?

Class composition.

What are the actual bits made of?

Like is it quartz sand or bits of old lava?

Precisely.

Are they individual mineral fragments, like quartz or feldspar?

Or are they little chunks of pre -existing rocks, which we call lithic class or rock fragments?

Often it's a mix.

And the shape of the clasts must matter too.

Yeah.

Are they sharp or rounded?

Absolutely.

We look at angularity, how sharp the edges and corners are.

Freshly broken rock fragments tend to be very angular.

Okay.

And sphericity.

That's how round it is overall.

Yeah.

How close it is to being a sphere versus being, say, long and skinny or flat.

Then there's sorting.

What's that about?

Sorting describes how uniform the grain size is within the rock.

If all the grains are roughly the same size, it's well sorted.

Like beach sand, maybe?

Often, yes.

Wind and waves are good at sorting.

But if you have a jumble of big pebbles mixed with sand and silt, that's poorly sorted.

Okay.

And sometimes you have big bits floating in finer stuff.

Right.

If you have larger clasts surrounded by much finer grains, we call that finer material the matrix.

You also mentioned something called sedimentary maturity.

Sounds like sediments can grow up.

Yeah.

Huh.

In a way, yes.

Sedimentary maturity refers to how much the sediment has changed from its original source rock during transport.

How does it change?

Well, as sediment travels, say down a river, a few things happen.

The weaker, less stable minerals tend to break down or dissolve.

The constant bumping and grinding rounds off the sharp corners so clasts become less angular and more spherical.

And the sorting improves as different sizes get separated.

So a mature sediment would be?

Typically well sorted, well rounded grains composed mostly of resistant minerals like quartz.

Think of sand on a tropical beach that's usually very mature.

An immature sediment like maybe near the base of a cliff might be poorly sorted, angular, and contain lots of different, less stable minerals.

And the type of cement holding it together is also part of the description.

Yes, definitely.

We note the character of the cement is it mostly quartz, mostly calcite, something else.

That can tell you about the groundwater chemistry during lithification.

Okay.

So using all those features, size, composition, shape, sorting, maturity cement, we can classify the specific types of plastic rocks.

Exactly.

And table 7 .1 in the chapter summarizes the common ones.

Let's run through a few.

Conglomerate.

That's made of rounded gravel, pebbles, and cobbles cemented together.

Think of the gravel you find in a river bar turned into rock.

Ambrica.

Sounds similar.

Oh, similar size, but the key difference is the clasts are angular, not rounded, like cemented rubble from the bottom of a cliff or a fault zone.

Okay.

Then you mentioned diamictite.

That sounded a bit different.

It is.

Diamictite has large clasts, often of various sizes, floating in a muddy matrix.

It's typically very poorly sorted.

Think of deposits left by glaciers, glacial till, or massive debris flows.

Right.

Moving to smaller grains.

Sandstone.

That seems pretty straightforward.

Well, yes and no.

It's made of sand size grains, but there are important varieties based on composition and maturity.

Quartz sandstone or quartz aronite is the mature one, mostly quartz grains.

Very common in beady and desert dune environments.

Then there's arcos.

It has a lot of feldspar mixed in with the quartz sand.

Feldspar.

That breaks down more easily than quartz, right?

So it's less mature.

Exactly.

Arcosy often forms closer to the source rock, especially if the source was something like granite, which is rich in feldspar, alluvial fans.

Gotcha.

Any others?

Lithic sandstone contains lots of sand sized rock fragments, not just mineral grains.

And then there's whack or gray whack.

Whack.

Yeah.

It's basically a poorly sorted sandstone sand grains and rock fragments mixed up in a clay rich matrix.

Often forms from underwater landslides.

Those turbidity currents we'll talk about.

Okay.

And even finer than sand.

You get into the mud rocks.

Siltstone is made of silt size grains.

Finer still, you have shale and mudstone, both made of clay and very fine silt.

What's the difference between shale and mudstone?

Shale is laminated.

It splits easily into thin sheets.

Mudstone is more massive.

It doesn't split so neatly.

Both form in very quiet water environments like floodplains, lagoons, deltas, or the deep ocean.

So putting it all together, the characteristics of a classic rock, the grain size, rounding, sorting composition, they really are clues, aren't they?

Absolutely.

They tell a story about where the sediment came from, how far it traveled, and the environment where it finally came to rest and became rock.

It answers that question, where does beach sand come from?

It's the end product of that whole weathering, erosion, and transport journey.

Amazing.

Okay.

Let's shift gears completely.

What about rocks formed by life?

Biochemical sedimentary rocks.

Right.

These are fascinating.

They form from the hard parts, usually shells of organisms.

Like seashells piling up.

Exactly.

Many marine organisms extract dissolved ions, like calcium and carbonate, from seawater to build their shells or skeletons.

When they die, these hard parts can accumulate on the seafloor.

If they get buried and lithified, they become biochemical rock.

And the main type here is limestone.

Limestone is the big one, yes.

It's composed primarily of calcium carbonate, the mineral calcite, or its cousin, aragonite.

And where does all that calcium carbonate come from?

A huge variety of organisms, corals, certain types of algae, clams, oysters, snails, even microscopic plankton floating in the water, like coccolithophores and foraminifera.

Their shells pile up.

So does all limestone look the same?

Not at all.

The texture varies a lot depending on the type of organisms and how the material accumulated.

You can have reef limestones built directly by corals.

You can have limestones made of broken shell fragments, like sand on a carbonate beach.

Or you can have very fine grained limestone made from that microscopic plankton mud.

What are some specific limestone types?

Well, if you can clearly see fossils in it, we call it fossiliferous limestone.

If it's made of that superfine carbonate mud, it's called micrite.

And chalk is a specific type made almost entirely of the shells of those microscopic plankton.

Think of the White Cliffs of Dover.

Ah, okay.

But sometimes limestone looks really solid, not like obvious shells.

Why is that?

Good point.

That's because things can happen after the sediment is deposited, during diagenesis.

Burrowing creatures can mix things up.

Water flowing through can dissolve some carbonate and re -precipitate it, causing crystals to grow larger or change form like aragonite turning into more stable calcite.

This can obscure the original shell structures and make the rock look more massive and crystalline.

Interesting.

Okay, the other biochemical rock you mentioned was chert.

What's that made of?

Chert is made of silica, silicon dioxide, SiO2.

Specifically, it forms from the accumulation of silica shells produced by certain microscopic plankton, like radiolaria and diatoms.

So, similar process to chalk, but silica shells instead of carbonate.

Exactly.

These tiny silica shells rain down on the deep ocean floor, forming a silica -rich ooze, often mixed with clay.

Over time during burial and diagenesis, the silica dissolves and re -precipitates as extremely fine -grained quartz, forming solid chert.

And it often forms in layers.

Yes, that's why it's often called bedded chert.

The color can vary to impurities, like iron oxide can make it red, which we call jasper.

Flint is another common name, often for the darker varieties.

It's very hard and breaks with sharp edges, which is why it was so important for early human tools.

Right.

Okay, so that's life -building rocks from minerals in the water.

What about rocks made from the organic matter itself?

Organic, sedimentary rocks.

Yes, these form from the accumulation and alteration of organic material, the actual carbon -based tissues of plants and other organisms like cellulose, fats, proteins.

How does that stuff not just decay away?

The key is deposition in an environment with very little oxygen.

Think swamps, stagnant lakes, lagoons.

Without oxygen, the microbes that normally cause rapid decay can't thrive.

So, the organic matter gets buried along with other sediment and preserved.

And these are important because they contain energy.

Hugely important, especially since the industrial revolution.

Because they're rich in organic chemicals, they can burn.

Coal is the classic example.

Tell us more about coal.

Coal is black, combustible, and mostly carbon, anywhere from 40 to over 90 percent.

The carbon is bound up in complex organic molecules called macerals.

It forms specifically from buried plant remains.

So, ancient swamps turning into rock.

Essentially, yes.

As the plant debris gets buried deeper and pressure and temperature increase.

This compacts the material, drives off water and other volatile molecules like methane and CO2, and concentrates the carbon, transforming it into peat, then lignite, then betuminous coal, and finally, under enough heat and pressure, anthracite coal.

Okay.

And the other organic rock was oil shale.

Right.

Oil shale is a bit different.

It's a fine -grain clastic rock like shale or mudstone, but it contains a significant amount, maybe 15 to 30 percent, of organic material called kerogen.

Kerogen.

Where does that come from?

Kerogen in oil shale typically comes from the fats and proteins of microscopic organisms like plankton and algae that lived in lakes or quiet marine environments.

Again, low oxygen allows their remains to accumulate in the mud without being fully decomposed or eaten.

Burial and lithification transform this organic gunk into the waxy black kerogen.

And this kerogen can be turned into oil?

It can.

If you heat the rock up sufficiently, that's why it's called oil shale.

Fascinating.

Okay.

That leaves one major category.

Chemical sedimentary rocks.

These form directly from water.

Yes.

These precipitate directly from surface water solutions when the water becomes super saturated with certain dissolved minerals.

They often have a crystalline texture because they form by crystal growth.

And the first type was evaporates, related to evaporation.

Exactly.

As the name suggests, they form when salt water evaporates, leaving the dissolved salts behind as solid minerals.

Where does this happen?

Think of environments like desert lakes with no outlet or restricted seas where evaporation rates are high and water inflow is limited.

To get really thick deposits, you need large volumes of water to evaporate over long periods.

This can happen, for example, when plate tectonics partly closes off an ocean basin or during continental rifting.

The Bonneville salt flats in Utah are a spectacular modern example of a dried up salt lake.

And different salts form depending on how much water evaporates.

That's right.

As seawater evaporates, different minerals precipitate out in a sequence.

Typically gypsum forms first when about 80 % of the water is gone.

Then halite common salt precipitates when about 90 % has evaporated.

If it dries up completely, you get mostly halite and gypsum plus smaller amounts of other salts and carbonates.

Okay.

What about travertine, chemical limestone?

Yes.

While most limestone is biochemical, travertine is calcium carbonate that precipitates directly from groundwater, usually where it emerges at the surface in springs either hot or cold or within caves.

What makes it precipitate?

Often it's due to degassing.

Groundwater can hold more dissolved CO2 under pressure.

When it reaches the surface, the pressure drops, CO2 bubbles out, and this makes the water less acidic, causing calcium carbonate to precipitate.

Evaporation at the surface can also concentrate the ions.

Sometimes microbes play a role too.

And this forms those cool terraces at hot springs.

Exactly.

Like at mammoth hot springs in Yellowstone.

It also forms cave decorations like stalactites and stalagmites.

It's often beautifully banded due to variations in the water chemistry or environment over time, and it's sometimes used as a decorative building stone.

Tufa is a porous variety often formed in lakes.

Then there was dolostone.

What's that?

Dolostone is a carbonate rock, but instead of being made of calcite, calcium carbonate, it's composed mainly of the mineral dolomite, which is calcium magnesium carbonate.

Does it precipitate directly?

Rarely.

Most dolostone forms when magnesium -rich groundwater reacts with pre -existing limestone, replacing some of the calcium with magnesium.

This can happen in certain shallow marine environments or after the limestone is Okay.

And lastly, chemically precipitated chert.

We already had biochemical chert.

Right.

Chert can also form chemically, primarily through replacement.

Microscopic quartz crystals can gradually replace other minerals in a pre -existing rock.

Like replacing limestone.

Yes, that's common.

You can get nodules or layers of chert forming within limestone long after the limestone was deposited.

The Onondaga chert in New York is a good example.

This replacement chert, like Flint, was also prized for toolmaking.

Can it replace other things?

Oh, yes.

Silica -rich groundwater can replace buried volcanic ash or even wood.

That's how we get petrified wood.

Silica replaces the original wood cells, preserving the structure, even the growth rings.

Wow.

And another variety is agate.

That's chert that precipitates, often in concentric, colorful bands, filling cavities or hollows within rocks.

The banding comes from impurities varying as the silica precipitates layer by layer.

Amazing variety.

Now, when we look at outcrops of these rocks, we often see patterns within the layers themselves, right?

Sedimentary structures.

Yes, and these structures are incredibly informative.

They are features formed during deposition or very shortly after, and they give us direct clues about the conditions in the depositional environment.

Like the layering itself.

Yeah.

Bedding.

Exactly.

The most basic structure is bedding or stratification.

A single layer is called a bed.

The boundary between beds is a bedding plane.

A sequence of beds is called strata.

And these layers can look different.

Absolutely.

Beds can vary in thickness, color, grain size, composition.

These changes reflect changes in the depositional conditions over time.

Maybe the current velocity changed or the sediment source shifted or sea level went up or down.

Think of the distinct bands of rock you see in the Grand Canyon.

Each band represents a different set of conditions.

Geologists group these related layers together.

Yes.

A distinct sequence of strata that can be traced over a region is called a stratigraphic formation.

Formations are often grouped into larger units called groups.

Geologic maps show the distribution of these formations at the earth's surface.

But sometimes the layers get messed up.

They can be, yeah.

If organisms burrow through the sediment after it's deposited, they can disturb or destroy the original layering.

That process is called bioturbation.

Okay.

What about features like ripple marks?

I see those on beaches sometimes.

Ripple marks, dunes, and the cross bedding they create are all consequences of deposition in a current wind or water.

They are bed forms, shapes created on the surface of the sediment.

How do ripples form?

They're small ridges formed perpendicular to the current flow.

If the current flows mainly in one direction, like a river, the ripples are asymmetric with a gentle slope upstream and a steeper slope downstream.

And if the flow goes back and forth, like waves on a beach.

Then you get symmetric ripples with sharp crests and rounded troughs.

And yes, these features can be preserved in ancient rocks.

Dunes are just bigger versions.

Essentially, yes.

Much larger bed forms, common in deserts and some river or coastal settings.

And cross bedding is related to these.

Yes.

Cross beds are the internal layers within a ripple or dune.

As the ripple or dune migrates in the current, sediment gets eroded from the upstream side and deposited on the steeper downstream or downwind slip face.

This creates layers that are inclined relative to the main bedding surfaces.

And the direction they slope tells you the current direction.

Exactly.

But looking at the orientation of cross beds in ancient sandstones, like the huge ones in Zion National Park, which represent ancient desert dunes, we can figure out which way the wind or water was flowing billions of years ago.

Incredible.

You also mentioned turbidity currents and graded beds.

Right.

A turbidity current is a dense underwater avalanche of sediment mixed with water that flows rapidly down a submarine slope, often triggered by earthquakes or storms.

Like a muddy underwater landslide.

Pretty much.

As this dense flow moves downslope, it starts to slow down.

When it slows, the sediment begins to settle out.

But the coarsest, heaviest particles settle first, followed by progressively finer particles.

So it sorts itself by size as it settles.

Exactly.

This creates a distinctive layer called a graded bed, which finds upwards coarse at the bottom, fine at the top.

These deposits, called turbidites, are common in deep marine environments.

Are there other markings found on the bed surfaces?

Yes.

Several useful ones.

Mud cracks form when wet mud is exposed to the air and dries out, shrinking and cracking into characteristic polygonal patterns.

If these get buried quickly, they can be preserved.

I've seen pictures of those.

Scour marks are small troughs eroded into a sediment bed by the current, often parallel to the flow direction.

Fossils, of course, are crucial bed surface markings, shells, bones, footprints, burrows.

And sometimes you can even find paleosols preserved.

Paleosols.

Ancient soils.

Exactly.

A buried soil horizon, which can look different in texture and color from the rock layers above and below it.

So why is studying all these structures so important?

Because they were direct evidence of the depositional environment.

Ripples and cross beds tell us about currents.

Mud cracks tell us the sediment was exposed to air.

Graded beds indicate turbidity currents.

Fossils tell us about the life forms present.

They are absolutely key to interpreting Earth's history recorded in the rocks.

Which leads us to the next big question.

How do geologists actually figure out the specific depositional environment where a rock sequence formed?

It's like being a detective.

You use all the clues together.

The rock type, sandstone, shale, limestone, the grain size, sorting, rounding, the sedimentary structures we just talked about, the types of fossils.

Putting it all together to reconstruct the ancient landscape.

Was it a glacier?

A desert?

A river floodplain?

A beach?

Deep ocean.

And these environments fall into two broad categories.

Generally, yes.

Terrestrial, meaning on land, non -marine, and marine, meaning in the sea.

One common feature in terrestrial settings is the formation of red beds, sediment stained red by iron oxides, indicating exposure to oxygen.

Okay, let's tour some terrestrial environments.

Glaciers.

Glacial environments are dominated by ice.

Glaciers transport sediment of all sizes, completely unsorted.

When the ice melts, it dumps this mixture as glacial till, which lithifies into diamictite.

Mountain streams, fast water, big rocks.

Exactly.

Steep, turbulent streams carry coarse gravel, cobbles, even boulders during floods.

These deposits often lithify into conglomerate or breccia.

Alluvial fans, where streams come out of mountains onto plains.

Right.

Especially in arid regions.

The stream suddenly loses energy and deposits a wedge -shaped fan of sediment, often coarse sand and gravel.

If the source rock was granite, you might get Arcos forming here.

Deserts, dominated by wind.

Largely, yes.

Wind sorts sediment very well, carrying away dust and leaving behind well -sorted sand that accumulates in large dunes with prominent crossbedding.

You can also get evaporates forming in temporary desert lakes.

Playas.

Rivers.

Fluvial environments.

Rivers transport sediment in channels, often sand and gravel, leaving crossbeds, and deposit finer silt and mud on adjacent flood plains during floods, forming shales and silt stones, maybe with mud cracks.

River deposits often show lens -shaped channel sandstones cutting through finer floodplain mud stones.

And lakes.

Quieter water.

Generally, much quieter.

Coarse sediment gets dumped near the shore where streams enter.

Fine clay settles out in the deeper parts, forming laminated shales.

You might get small deltas forming where rivers into the lake.

Okay, now let's head to the coast and into the ocean marine environments.

Right.

Starting near the coast, you have marine deltas, where large rivers enter the sea.

These are complex environments with swamps, channels, and underwater slopes, resulting in a mix of sandstones, silt stones, shales, and sometimes coal.

Coastal beaches.

Lots of wave action.

Yes.

Waves sort the sand well, round the grains, and often create ripple marks.

Beach deposits typically become well -sorted quartz sandstone.

Moving offshore into shallow water.

Shallow marine environments below the influence of normal waves tend to accumulate finer sediment, well -sorted silt and mud, which become siltstone and mudstone.

If the water is warm, clear, and free of clastic sediment input.

Then you get carbonate environments.

Exactly.

Shallow marine carbonate environments are where limestones form.

Organisms like corals build reefs.

Shells accumulate on beaches and in lagoons.

You get a whole range of limestone types, depending on the specific setting.

Reef rock, shelly sands, carbonate muds.

And finally, way out in the deep ocean.

Deep marine deposits consist mainly of fine clay settling from the water column, forming shale or mudstone, and the microscopic shells of plankton.

If the plankton have calcite shells, you get chalk or fine limestone.

If they have silica shells, you get bedded chert.

Turbidity currents also bring coarser sediment down submarine canyons, depositing graded beds, turbidites on the abyssal plain or continental rise.

It's amazing how many different environments leave their mark.

Now, why do we find these incredibly thick sequences, sometimes kilometers thick, only in certain places?

They don't cover the whole planet evenly.

That's a crucial point.

You only get really thick accumulations of sedimentary rock in specific regions called sedimentary basins.

Basins, like big bowls on the Earth's crest.

Sort of, but they form gradually.

A sedimentary basin is a region where the lithosphere sinks over long periods, a process called subsidence.

The sinking creates space accommodation space for sediment to pile up.

They generally fill as they sink, rather than being deep empty holes initially.

And what makes the lithosphere sink?

Plate tectonics is usually the driver.

There are several types of basins related to different tectonic settings.

Okay, what are the main types?

One type is rift basins.

These form where a continent is stretching and pulling part.

The crust thins and subsides, often with faults dropping blocks down, creating troughs that fill with sediment, sometimes including lake deposits and evaporates.

Makes sense.

What else?

Passive margin basins form along the edges of continents that aren't active plate boundaries.

These margins were often formed by rifting in the past, and the thinned crust continues to cool and subside for a very long time, allowing enormous thicknesses up to 15 or 20 kilometers of sediment from land and shallow seas to accumulate.

The Atlantic coast of North America is an example.

Wow, 20 kilometers.

Any others?

Intracontinental basins form within the interiors of continents, sometimes over old failed rifts.

They can subside slowly over long periods.

The Michigan basin is a classic example, filled with shallow marine sediments, carbonates, evaporates, and even coal.

And the last type?

Foreland basins.

These develop adjacent to mountain belts formed by plate collisions.

The immense weight of the mountains and the slices of rock pushed onto the continent actually weighs down the lithosphere in front of the mountains, creating a trough that fills with sediment eroded from the rising mountains.

So the formation of mountains creates a bainton right next door to catch the debris.

Now within these basin fills, the types of sediment often change vertically, indicating changes in environment.

You mentioned transgression and regression.

Sea level changes.

Yes.

Sea level relative to the land is constantly changing, due to global factors like ice ages or local factors like tectonic uplift or subsidence.

When relative sea level rises, the shoreline moves inland.

That's a transgression.

And deeper water environments shift landward over shallower ones.

Precisely.

So you might get deep water shales deposited on top of what used to be beach sand.

When relative sea level falls, the shoreline moves seaward.

That's a regression.

Terrestrial environments might advance over former marine ones.

These cycles of transgression and regression leave characteristic vertical sequences of rock types.

And once all this sediment is deposited and buried deep within a basin,

it undergoes further changes called diagenesis.

Yes.

Diagenesis includes all the physical, chemical, and biological changes that happen to sediment after deposition, transforming it into rock, lithification is part of diagenesis, and altering the rock afterwards.

What kinds of changes happen during deep burial?

As sediments get buried deeper, pressure and temperature increase.

They interact with warm groundwater.

This can cause minerals to recrystallize, new minerals to form like clay minerals changing, cements to dissolve in some places and precipitate in others.

The rock continues to evolve.

Is there a limit to diagenesis?

Eventually, yes.

At temperatures somewhere around 150 to 300 degrees Celsius, the change had become so significant forming entirely new minerals developing textures like foliation that we consider it metamorphism, which is the topic for another day.

We have really covered a huge amount of ground tracing sediment from its source rock all the way to deep burial and transformation.

Can we quickly recap the absolute key takeaways?

Sure.

We saw there are four main classes of sedimentary rocks.

Clastic broken bits, biochemical shells,

organic plant animal remains, and chemical precipitated from water.

We learned the five steps for clastic rocks.

Weathering, erosion, transportation, deposition, and lithification.

And that clastic rocks are classified by features like grain size, composition, sorting, and rounding, which tell us about their history.

Sedimentary structures like bedding, crossbedding, ripple marks, and mud cracks are vital clues to the depositional environment.

Biochemical rocks like limestone and chert are built by organisms, while organic rocks like coal form from preserved organic matter.

Chemical rocks like evaporates and trabertine precipitate directly from water solutions.

We explored a whole range of depositional environments, from glaciers and deserts on land, to beaches, reefs, and the deep sea, each producing characteristic sedimentary deposits.

We learned that thick piles of sediment accumulate only in sedimentary basins, where the lithosphere subsides due to tectonic forces.

And that changes in sea level cause transgressions and regressions recorded in the vertical stacking of sedimentary layers.

Finally, diagenesis covers all the processes that turn sediment into rock, and alter it after burial, eventually grating into metamorphism.

That sums it up pretty well.

And for listeners who want to explore further, the chapter mentions a Geotour's worksheet and some specific Google Earth sites, places like the Grand Canyon, Death Valley, Zion, where you can actually see these features and environments.

Yeah, those are great resources for visualizing these concepts.

And the review questions in the chapter are always good for checking your understanding.

The chapter also poses some thought -provoking questions, thinking about sedimentary rocks on Mars, or interpreting the geology of the Gulf Coast or the Bahamas.

It shows how geologists apply this knowledge.

Absolutely.

It highlights how these rocks really are the Earth's memory, recording its history page by page.

So next time you see layered rock in a road cut, or even notice the patterns in a stone building, hopefully you'll have a new appreciation for the journey that material took and the ancient world it represents.

Definitely.

Think about a landscape.

You know what sedimentary stories might be hidden beneath your feet, what ancient deserts, seas, or rivers might be recorded down there.

It's a fantastic perspective.

Well, thank you for taking this deep dive with us into the world of sedimentary rocks.

It's been truly enlightening.

My pleasure.

It's a fascinating part of our planet's story.