The branch of geology
called stratigraphy deals
with the study of
Earth's history
as recorded in layers of rock.
Earth records time
by forming rock.
Here at the Grand
Canyon, that time
is laid out like the pages
of an open history book
through the exposure of
different layers of rock
of different ages.
But how do we read that history?
It's not written in words.
Instead, it's written
in the language of rock.
We need to be able to
speak that language
to interpret the history
being presented here.
In speaking the
language of rock,
we need to think about the
different principles that
guide our interpretation of
stratigraphy, how we interpret
the relative ages of rock
layers, which are older,
which are younger.
There are several guiding
principles to stratigraphy.
The first is simple.
It's called original
horizontality.
This means that sedimentary
rock layers are initially
formed in layers that are
approximately horizontal.
This reflects the
deposition of sediment
on some surface, namely lake bed
or seafloor or some other earth
surface.
That surface is
approximately horizontal,
and so the layers
start out horizontal.
Any change in position from
that original horizontality
has to occur after
those sedimentary rock
layers were formed.
Next principle is superposition.
Superposition simply means
that if the layers are not
disturbed, they're still
going to be in order.
They're deposited in
order from oldest layer
at the bottom to youngest
layer at the top.
This is because there's no
way to get younger sediment
beneath older sediment.
Sediment accumulates
on the surface.
The youngest sedimentary layer
is the one that's on top,
and layers get progressively
older as you move down
that sequence of layers.
The next principle applies
not to sedimentary rocks
but to other types of
rocks like igneous rocks.
You can get igneous rocks
beneath older rocks.
This occurs when magma
intrudes into the crust
and then crystallizes,
creating what's called
a cross-cutting relationship.
Our igneous intrusion
would be younger
than any rock it intrudes
and older than any rock
it does not.
In other words, for
igneous rock to intrude,
there has to be some rock there
already to be intruded into.
The same cross-cutting
relationships
can be applied to
faults as well.
A fault is younger
than any rock layer
it cuts across and older than
any rock layer it does not.
In speaking the
language of rocks,
we also need to be able to
recognize where there are gaps,
where there are missing
pages, where the time was not
recorded because rock
was not being formed.
These are called unconformities.
Think of an
unconformity as a gap
in the stratigraphic record.
There are three types of
unconformities that form
in slightly different ways.
In other words, the
presence of an unconformity
indicates not only missing
time but a specific set
of geologic events that occurred
to form that unconformity
but were not recorded by
the formation of rock.
The first type is
a nonconformity.
This occurs when sedimentary
rock layers directly
form on or deposited
on overlying igneous
or metamorphic rocks.
This indicates that
an erosional surface
formed in those igneous
and metamorphic rocks
before new sedimentary
layers were deposited.
In other words,
there's missing time
between the sedimentary rock
and the igneous and metamorphic
rock beneath it.
Otherwise, our
sedimentary rock would
be crosscut by that
intrusion, which
shows signs of metamorphism
because it would already
have been there when the
igneous or metamorphic activity
occurred.
The next type of
unconformity is probably
the easiest one to recognize.
It's called an
angular unconformity.
This occurs when sedimentary
rock layers that are still
in their original
horizontal position directly
overlie rock layers
that are tilted.
How does an angular
unconformity form?
You deposit layers in
horizontal position.
Then you disturb those layers
through a folding or uplift
event.
Those layers are now no
longer in horizontal position.
Then, new layers that still
remain in horizontal position
are deposited on top of them.
That contact between older
layers no longer horizontal
and younger layers
still horizontal
is the angular unconformity.
So again, the
angular unconformity
represents not just missing
time when no rock was formed,
but also specific series
of events that took place
during that missing time.
Uplift and folding
of older rock layers,
erosion of those older deformed
layers, and then finally time
starts being recorded again
when new rock layers deposit
on that erosional surface.
The last type of unconformity
is called a disconformity.
This occurs when you have
a period of nondeposition,
no time recorded by
formation of rock,
between sedimentary layers
that are still horizontal.
How does this form?
Again, we have a sequence of
sedimentary layers deposited,
and then deposition stops.
We stop forming rock, and
we start eroding it away.
We form an erosional surface.
Finally, we deposit
new sedimentary rock
on top of that surface.
The result is an
irregular contact
between our younger
and older rock.
Both players are still in
original horizontal position,
but there is an irregular
contact between them.
You may also find
fragments of the older
rock within the
younger rock, pointing
to a period of erosion, a
period of nondeposition,
time not being recorded
within that sequence.
Finally, in thinking about
the language of rock,
we need to also be thinking
about the processes that
form different types of rock.
In other words, in
different environments,
we lay down sediment
and form rock
with different characteristics.
This is because rock
and sediment reflects
the processes of formation.
Using our knowledge of
the type of sediment
that accumulates in
different environments
and recognizing that
type of sediment
in different kinds
of sedimentary rock,
we can infer the conditions that
must have existed at the time
that layer was being formed.
For example, the
sediment deposited
by glaciers is very
different than the sediment
that's deposited
in deserts, which
is very different than the
sediment being deposited
on tidal mudflats, and very
different than the sediment
deposited out in the open ocean.
Recognizing those
differences and how
they reflect different
environmental conditions allows
us to infer what the region
must have looked like,
what environmental conditions
existed at the time
a particular rock
layer was being formed.
So let's apply our knowledge of
stratigraphy and unconformities
and different
depositional environments
to the sequence of
rock layers that
has been exposed for our viewing
by the erosion of the Grand
Canyon.
This figure shows a
schematic illustration
of the different rock types
exposed in the canyon walls
and also the nature
of their contacts,
allowing us to infer
the different types
of unconformities that
exist in this sequence.
Using superposition and
cross-cutting relationships,
we see that the
oldest rock layer here
is the Vishnu Schist,
a metamorphic rock.
That Vishnu Schist was intruded
by the Zoroaster Granite,
because the granite
cuts across the schist
but does not cut across
the younger rock layers
of the Grand Canyon Supergroup.
So the first rock
layer to be formed here
is the schist, followed by
intrusion of the granite.
What tectonic setting
is represented
by metamorphic rocks like schist
and high silica felsic igneous
rocks like granite?
Well, those kinds
of rocks form in
a convergent tectonic setting.
You make high silicon
magmas like granite
in subduction zones.
You form schist and other
high-pressure metamorphic rocks
by mountain building and
continental collision.
So the Vishnu Schist and
the Zoroaster Granite
represent a time when the
Grand Canyon region was
at a convergent plate boundary.
Subduction followed
by collision,
forming high silica
igneous rocks
and also resulting
in metamorphism
of those rocks, whatever rock
existed there previously,
into metamorphic
rock such as schist.
Where can we find a
similar tectonic setting
to the one represented
by the oldest rock
layers exposed at the
bottom of the Grand Canyon?
Well, we identified that the
schist and granite represent
a convergent plate boundary,
subduction and continental
collision, forming high silica
igneous and metamorphic rocks.
We see the same types of
rocks being formed today
in the North Cascades.
Subduction is occurring, high
silica magmas are being made.
At the same time, thick
blocks of buoyant crust
are being accreted to,
colliding with North America,
forming metamorphic rocks in a
compressional mountain range.
So at the time the Vishnu
Schist and Zoroaster Granite
were being formed,
the tectonic setting
of the Grand Canyon
region was similar to that
of the Cascades.
The same tectonic setting
collisional plate boundary
also existed earlier
in the Appalachians,
where similar types
of rocks can be found.
What's the next rock unit to
form here at the Grand Canyon?
Well, superposition tells
us the next layer to form
are the rock layers of the
Grand Canyon Supergroup,
consisting of a series
of related rock types,
sandstone layers
that are very coarse,
some shale layers, and also some
intermixed mafic or basalt lava
flows.
We need to be thinking about
what tectonic setting is
represented by the formation
of the rocks that belong
to the Grand Canyon Supergroup.
Well, basalt lava flows,
coarse sandstones and shales,
this is inferred to represent
a continental rift zone.
Ancient North America collides
with another continent
during the time that the
Vishnu Schist and Zoroaster
Granite were formed, a
mountain range results.
A mountain range
gradually erodes,
and eventually the continent
breaks apart again,
breaks apart by the process
of continental rifting.
When that occurs, we
have low silica magmas
being formed by
decompression, erupting
on the surface as basalt, and
then in that continental rift
zone where we have
divergent tectonic stresses,
we have subsiding basins, basins
subsiding along normal faults.
It's in those basins that
sandstones and shales
may be deposited.
The sandstones and shales,
well, the sandstone
is coarse because it's close to
the sediment source, the rising
fault block mountains
on the other side
of those normal faults.
And at times, those basins
may support lakes in which
shale is going to be deposited.
A similar tectonic
setting to the one that
existed in the
Grand Canyon region
at the time the Grand
Canyon Supergroup was formed
can be found today in the
Basin and Range Province
in Rio Grande Rift
of the Western US.
Here, divergent
tectonic stresses
are producing fault
block mountain ranges,
subsidence along normal faults.
Coarse sands are
accumulating because we're
close to the sediment
source, the uplifted fault
blocks on the other side
of those normal faults.
In some places, large
pull-apart basins or grabens
are formed, which
occasionally support
lakes in which shale and other
fine-grained sediment made
of clay is being deposited.
And of course, the stretching
and thinning of the crust
is accompanied by low
silica basalt lava flows.
Again, using our knowledge
of where these different rock
types form and why they
might be found together
in a set of related
units allows us
to infer what the
tectonic setting
and geologic environment
of the Grand Canyon region
was at the time the Supergroup
units were being formed.
It was a continental rift zone.
The remaining part
of the sequence,
the upper part of
the Grand Canyon,
consists of flat line
sedimentary rock layers.
Superposition dictates that
the layers on the bottom
are the oldest, the layer
on top is the youngest.
In other words, as you move up
towards the rim of the canyon,
you're encountering
younger rock.
Conversely, as you hike into
the canyon from the rim,
you're hiking back through time.
You're encountering
older and older rock
as you descend deeper
into the canyon.
What is a tectonic
setting represented
by the flat line sedimentary
rock layers of the Upper Grand
Canyon?
Well, what does a continental
rift eventually grow into?
Well, if the rifting
process continues,
a new divergent plate boundary
develops a new mid-ocean ridge,
an ocean ridge that grows
by sea floor spreading.
What happens to the trailing
edges of the continental crust,
the continental crust that broke
apart during the rift stage?
It eventually cools and
subsides and becomes
what's known as a passive
continental margin
and thereby becomes a
place for thick layers,
for thick sequence of
sedimentary rock layers
to accumulate on
that passive margin.
So the rock layers of
the Upper Grand Canyon
represent sedimentary
rock layers
accumulated on a subsiding
passive margin as our ocean
basin grows, first by rifting,
represented by the Grand Canyon
Supergroup, and
then by sea floor
spreading while the former
rifted edges of the continent
become a passive
continental margin.
As you can see,
there's a variety
of different types
of sedimentary rock
layers represented in
the sequence deposited
on the passive margin.
We have sandstones, we have
shales, we have limestones,
different types of
sedimentary rocks
forming in different types
of depositional environments.
So what are the
depositional environments
represented by each
of these rock types?
What kind of environment
does sandstone form in?
What does the size
of the cross-bedding
tell us about the environment
in which that sandstone formed?
What kind of environment
does shale form in?
What kind of environment
does limestone form in?
By recognizing the different
environments represented
by these different
rocks, we can infer
how the environmental and
geologic conditions changed
in the Grand Canyon region as
this sequence of rock layers
was deposited on
the passive margin.
Well, the oldest to the
passive margin layers
is the one directly above the
supergroup called the Tapeat
Sandstone.
This is what the Tapeat
Sandstone looks like.
You can see that it has
these angled features
within otherwise flat-line
sedimentary layers.
Those angled features
are called cross beds.
They also tell us
something about how
that sand was deposited to
eventually become sandstone.
Cross beds--
small-scale cross beds
form when a ripple migrates
across a bedding surface.
Sand grains are pushed
up and over that ripple
and then deposited again.
In other words,
the ripple migrates
in the down current direction,
forming these angled features
called cross beds.
In the case of the
Tapeat Sandstone,
cross beds are angled
the opposite way,
indicating the
current direction.
In this picture
was right to left.
You can see the same
type of environment
if you visit the beach.
Lots of sand, a relatively
high-energy environment
close to the shoreline.
High-energy environment needed
to deposit sand-sized grains.
And the high energy is also
reflected in the formation
of ripples and cross beds.
So the Tapeat Sandstone
is interpreted
as representing a sandstone
layer deposited either
at the beach or very
close to the shoreline
on that passive margin.
The next layer to be deposited
on the passive margin sequence
is called the
Bright Angel Shale.
Shale is a fine-grained
clay-sized sedimentary rock.
You can see that
those clay grains
are being deposited in
very thin layers that
are more or less horizontal.
To get these fine clay-sized
grains to deposit,
you need a low-energy
environment.
The water has to be deeper, you
need to be farther off shore.
That indicates that sea
level rose at this location.
As sea level rose, the
Grand Canyon region
found itself further
from the shoreline.
Sea level rise moves
the shoreline away
from the Grand Canyon region
and puts the Grand Canyon
region in deeper water
farther offshore.
The environment has
changed from nearshore
to offshore, high
energy to low energy.
Sediment being deposited
changes from sand to clay.
That shows up as a change
from sandstone to shale.
In other words, whenever
the depositional environment
changes, the type of
sediment and sedimentary
rock that we form changes.
In this case, the sea
level rise at this location
has moved the region away
from the shoreline, put it
into deeper water, changing
the type of sediment from sand
to clay.
Sandstone to shale
is now being formed.
The next layers to be deposited
are the Mauv and Redwall
Limestones.
Limestone is a chemical
sediment made of the mineral
calcite, calcium carbonate.
In what environment does
calcium carbonate get deposited?
Well, it deposits in
warmer subtropical water.
So the presence of limestone
layers in the Grand Canyon
region is telling
us that at the time
those limestone
layers were formed,
this area was closer to the
equator than it is today.
The environment would have been
similar to that of the Bahamas
today, shallow tropical sea.
Calcium carbonate is less
soluble in warm water
so it's being deposited
as a chemical sediment.
Limestone is being formed.
Due to continental drift,
the Grand Canyon region
later moved northward,
away from the equator.
But during the time it
was a passive margin
and forming these
layers of limestone,
we were closer to the
equator at this location.
One of the younger layers in
the passive margin sequence
is the Coconino Sandstone.
Coconino Sandstone is a little
bit different than the Tapeat
Sandstone, farther
down and older,
because it has large cross
beds, cross beds that are
on the order of meters in size.
Those large cross beds indicate
sand deposited as dunes.
In other words, in an aeolian
or wind-dominated environment.
Dunes are depositional
features formed by wind action.
And just like form ripples,
just like forming cross beds
and ripples, you also form
cross bedding in dunes.
Only the scale of that cross
bedding is much larger.
Here we see modern dunes.
Here we see dunes that
have been cemented
by groundwater flow,
which has laid down
a chemical cement between
those sand grains,
preserving the cross beds.
And then on the right, we
see the Coconino Sandstone
with its preserved cross
beds, large cross beds
indicating deposition
of that sand as dunes.
Those might have
been beach dunes
or they might have been dunes
in an arid or desert-like
environment further inland.
Either way, Coconino Sandstone
represents sea level fell.
Remember, we have been
making shale and limestone.
Water off shore, sea level fall
means the Grand Canyon region
is now above sea
level, depositing sand
in aerial or aeolian
environment or wind
rather than water is the
primary driver of sand movement
and deposition.
In reading this history told by
the layers exposed by erosion
of the Grand Canyon, we also
have to watch for time periods
when rock was not
formed, periods
when time was not recorded
because no rock was being made.
Those are the unconformities.
And the Grand Canyon
sequence of layers
represents examples or contains
examples of all different types
of unconformities.
There's a wide range in ages of
rock seen at the Grand Canyon
from about 1.7 billion
years old to about 300
million years in age.
In other words, Permian
to precambrian in age.
Permian, precambrian referring
to intervals of geologic time.
Wide range of ages but only
a small fraction of time
is actually recorded
by rock, again
because some of these
unconformities last hundreds
of millions of years,
representing times when
rock was not being deposited.
Because of a change in tectonic
setting during that time,
Vishnu Schist,
Zoroaster Granite,
convergent plate boundary,
mountain range was formed.
It took a long time to
erode those mountains away
till we could start
making sediment again.
By that time, we now have
a divergence setting,
a continental rift zone.
The Grand Canyon
Supergroup is formed.
Then there's another
unconformity that follows.
Why?
Because the rifting
process leaves
the area elevated as the new
ocean grows between the now
fragments of a continent.
That area has to cool and
subside, drop below sea level
before it can start
forming rock again,
before sedimentary rocks
can start being deposited.
Once that happens, the area
has completed its transition
from continental drift
to passive margin,
and we start making
sedimentary rock layers.
But sea level change
on that passive margin
means that sometimes the
area was above sea level,
no rock was formed
because we were
above sea level at that time.
Other times, it was
below sea level,
and rock was being formed.
But what are the different
types of unconformities
present in the sequence?
Well, schist and granite are
igneous and metamorphic rocks.
The Grand Canyon Supergroup
contains basalt lava
and igneous rock,
but it also contains
sandstone and shale,
sedimentary rocks.
So we have a nonconformity
present between the schist
and granite and then the
rock layers belonging
to the Grand Canyon Supergroup.
That nonconformity,
this particular one,
is sometimes called
the Great Unconformity,
because it represents a
very long period of time,
nearly a billion
years, representing
the time it took for this area
to go from a collisional plate
boundary with a mountain
range to a divergent plate
boundary with pull-apart basins
from continental rifting.
Between the Grand
Canyon Supergroup,
whose layers are
tilted because they're
formed by sinking of a
fault, subsidence of basins
along normal faults, we
have an angular unconformity
between the tilted
Grand Canyon Supergroup
and the flat layers
of the passive margin.
And then within the flat-line
layers of the passive margin,
we find examples
of disconformities,
periods of nondeposition
due to low sea level.
In other words, at times
the Grand Canyon region
was flooded by high sea
level causing sediment
to be deposited
at this location.
Other times, the
sea would retreat,
the area would find
itself above sea level,
and rock would generally not
be formed as a result. Instead,
erosion would occur when the
area was above sea level,
resulting in erosional
surfaces or disconformities
within the passive
margin sequence.
Here's a closer look
at that nonconformity.
Here we have metamorphic
and igneous rocks
with flat-lined sedimentary
rocks directly on top of them.
That contact, the
nonconformity, represents
a period of missing time during
which erosion was occurring.
The mountain ranges
that were formed
by the metamorphic
and igneous activity
were being eroded away,
leveled by erosion,
which took a long
period of time,
approaching a billion years.
Once the region was
reduced to near sea level,
sedimentary rocks could
start forming again.
Here's a closer look at
the angular unconformity.
Below it, the rock layers of
the Grand Canyon Supergroup
representing sedimentary
rocks in basalt
formed in a
continental rift zone.
Above it are the
flat-line layers
deposited on the passive margin.
The contact between the two,
between our tilted and flat
layers, is an
angular unconformity.
That unconformity
represents a period of time
when our elevated trailing
edges of the continent cooled
and subsided below sea level
and became a passive margin.
Not until they fell
below sea level
could sedimentary rock
layers start forming again.
During the rift stage,
space to accumulate sediment
occurred as a result
of subsidence,
subsidence in these basins
along normal faults.
Once the faults die out, the
basins fill with sediment,
then we stop making
sediment, stop forming rock.
During that time, the
whole area finds itself
farther and farther
from the ocean ridge,
which is where the plate
boundary is now, gradually
cooling and subsiding to
form a passive margin,
dropping below sea
level and allowing
the deposition of
sediment to resume,
forming the flat-line layers
on the passive margin.
And within our flat line
layers of the passive margin,
we find several irregular
contacts or erosional surfaces
representing periods of
time of low sea level,
area was well above
sea level as a result,
and no rock was being formed.
Instead, we have erosion
of already existing rocks
occurring.
And this shows up as
an irregular contact
between those rock
layers and also fragments
of the older rock layer
incorporated within the younger
rock layer.
So looking for signs
of a disconformity
require close inspection
of the contact
between sedimentary rock layers
and the nature of that contact.
Was it flat, in which case
deposition was continuous,
or is irregular, in
which case there's
a period of missing time here.
Let's sum up what
we've learned so far.
Let's read the sequence recorded
in the history of these rock
layers exposed by the
erosion of the Grand Canyon.
We start with the Vishnu
Schist, the oldest rock layer
present here.
Metamorphic rock formed at
a convergent plate boundary,
intruded by a somewhat
younger but still very old
Zoroaster Granite, a
high silica igneous rock,
pointing to the fact
this convergent plate
boundary was a subduction zone.
Then we have a long
period of nondeposition,
a long unconformity,
a nonconformity,
representing the period
of time it took--
representing the
period of time it
took to erode away the
mountain ranges produced
on that convergent
plate boundary.
By this time, the area
becomes divergent,
a rift zone develops,
forming the related rock
units of the Grand
Canyon Supergroup,
including basalts, coarse
sandstones, and shales.
The basalts come
from the low silica
magma that accompanies
continental rifting,
divergence at the surface
decompresses the mantle,
allowing low silica magmas to
be formed and reach the surface.
Sandstones are coarse,
reflecting the fact
that they're deposited close
to the source of the sediment.
Remember, during the
rift stage, you're
breaking the crust up
along normal faults.
Some blocks go up.
Those become mountain ranges.
Other blocks go down.
They become basins.
The basins are where
sediment is being deposited.
Alluvial fans, coarse
sediments like sandstone
and conglomerate, and
at times the basins
would support lakes where
shale may be deposited.
Similar environments
can be found today
in the Rio Grande Rift
of New Mexico and Texas
and also in the
East African Rift.
Then another long
period of nondeposition
follows, another unconformity.
This is an angular unconformity.
Because the supergroup
layers are tilted,
the younger layers
above them remain flat.
This angular unconformity
represents the time
it took for elevated
sections of the continent
to cool and subside
to move far enough
away from the new divergent
plate boundary to cool,
subside, drop below sea level
and become a passive margin.
And on that passive
margin, a variety
of depositional environments
existed at different times,
depending on relative sea
level at this location.
When we're close
to the shoreline,
we're depositing a
coarse sandstone.
We're depositing a coarse
sedimentary rock sandstone
in a relatively high-energy
environment, shallow water,
wave action, and ripples
forming the Tapeat Sandstone
with its small cross beds.
Sea level rises, the environment
becomes further off shore,
deeper water.
Deeper water, and we now
deposit the Bright Angel Shale.
Presence of the limestone layers
represent shallow tropical seas
where calcium carbonate
is being deposited.
The Supai group contains
shale and sandstones,
represents an environment where
once again relatively close
to the shoreline, at
times depositing shale,
at other times
depositing sandstone.
Hermit Shale, deep
water environment
further offshore
produced by, again,
sea level rise, moving
the coastline away
from the Grand Canyon region,
depositing that shale layer.
Coconino Sandstone,
sea level has fallen.
We're now depositing sandstone
coarse grain sediment again.
Has large cross beds
representing deposition
as dunes in a subaerial
or aeolian environment
dominated by wind
rather than water.
Finally, the youngest layers
are the Toroweap and Kaibab
Limestones, representing
one final return
of the sea, deposition
of calcium carbonate
in a shallow warm
water environment.
So the rock layers of
the Grand Canyon region
represent deposition in
a variety of settings.
Ancient rock of the crust, older
rift valley strata, and then
the younger passive
margin layers.
This entire sequence of
rock has been uplifted
in recent geologic time.
And that provided the
Colorado River the power
to erode through those layers.
As erosion occurred,
the river is
cutting into progressively
older and older rock,
exposing them for us to
study and piece together
the history of this region as
told by the rock layers exposed
in the walls of
the Grand Canyon.
So in summary, Grand Canyon is
not just a hole in the ground.
Rather, it's a window.
It's a window allowing
us to look back
through time at progressively
older rock layers
as we move deeper
into the canyon.
