Hi folks, I'm Dr. Chapman. Let's explore
Grand Canyon National Park to learn
about unconformities
and how sometimes the clues to unlocking
the history of our planet
are not in the rock record, but in the
absence of it.
The Grand Canyon is one of the best
places to understand and experience
stratigraphy.
It beautifully illustrates the principle
of Original Horizontality -
the concept that sedimentary rocks are
originally deposited in flat lying
horizontal layers.
It also demonstrates the Law of
Superposition, which states that
sedimentary rock layers are deposited in
a sequence with the oldest on the bottom
and the youngest on the top.
The oldest rocks in the Grand Canyon are
1.7 billion year old Proterozoic
metamorphic
rocks like the Vishnu schist, which is
exposed along the Colorado River.
The youngest rocks in the Grand Canyon
are 270 million year old Permian
limestones,
belonging to the Kaibab Formation that
are exposed at the rim of the canyon.
If you drive north from the Grand Canyon
to Zion National Park you encounter
an even younger part of the stratigraphy.
At Zion, the oldest exposed rocks are
part of the Kaibab Formation
and the youngest are 120 million year
old Early Cretaceous sandstone and
conglomerate -
part of the Cedar Mountain Formation.
It's exposed along the very highest
locations
within the park. All of the rocks exposed
at Zion National Park were also
deposited
as flat lying layers above the
present-day Grand Canyon area,
before there was a canyon. But those
layers have all been eroded away.
In arid climates, limestone is relatively
resistant to erosion.
This is why the Kaibab Formation, which is mainly
limestone, forms a vast plain above the
Grand Canyon
and also forms a cliff at the rim of the
Grand Canyon.
Other cliff faces in the Grand Canyon
are made of erosionally resistant layers
like the Mississippian-age Redwall limestone. Layers that form
gentle slopes or benches in the Grand
Canyon
are made of less resistant rock layers
like the Cambrian Bright Angel shale
and the Pennsylvanian-age Supai Group.
Although the Grand Canyon contains a
beautiful pile of sedimentary rocks,
there are time gaps in the rock record
that are missing called unconformities.
There are three main flavors of
unconformities
and the Grand Canyon has all of them.
The first type is called a
nonconformity and forms when
sedimentary rocks are deposited on top
of older
metamorphic or igneous rocks. Metamorphic
rocks and
intrusive igneous rocks form underground
deep within the Earth whereas
sedimentary rocks are deposited
at the surface of the Earth. So
geologists can deduce that there was
some
event that eroded down or otherwise
brought up the deep metamorphic and
igneous rocks up to the surface of the
Earth.
Such events usually take a lot of time -
millions of years -
and that time is what is recorded by the
nonconformity.
In the Grand
Canyon the boundary between the 1.7
billion year old metamorphic rocks, like
the Vishnu schist,
and 1.2 billion year old sedimentary
rocks in the Grand Canyon
Supergroup is a nonconformity. This
nonconformity represents half a billion
years of missing time in the rock record.
The second type of unconformity is
called a disconformity and this is the
most boring and most difficult type to
recognize.
A disconformity represents a period of
erosion or non-deposition
between parallel layers of sedimentary
rocks.
In the Grand Canyon there's a
disconformity between the Cambrian Muav limestone
and the Mississippian Redwall limestone
where more than 150 million years of
time are missing.
But it's hard to tell because both
limestones look very similar.
One way to tell them apart would be to
identify age diagnostic fossils in the
rocks.
For example, trilobites are common in the
Cambrian period,
but uncommon in younger periods.
Conversely, bryozoans are not present in
the Cambrian, but common in younger rocks.
Trilobites are common in the Muav
limestone, but are rare in the Redwall
limestone.
And bryozoans can be found in the Redwall
limestone, but not in the Muav limestone.
This is an example of using
biostratigraphy to date the layers of
rock.
Another way to recognize the Muav-
Redwall disconformity
is to hike along and closely examine the
contact between the two formations.
In some places there are thin lenses of
Devonian-age sedimentary rocks
called the Temple Butte Formation. These
lenses were deposited in
ancient channels that were eroded or
carved into the Muav limestone
during the missing time period.
This gives us a clue that
perhaps streams and rivers
eroded into the landscape during the
Devonian to create the disconformity.
The third and last type of unconformity
is an angular unconformity
and it is the photogenic darling of
geoscientists everywhere.
An angular unconformity is where older
sedimentary rock layers were
tilted or folded and eroded and then new
flat lying sediments were deposited on
top.
The Great Unconformity in the Grand
Canyon is an angular unconformity
and it is arguably the most famous
angular unconformity on Earth.
It separates the Cambrian Tapeats
sandstone from the Grand Canyon Supergroup
and Vishnu schist.  Depending on the exact
location, the Great Unconformity
represents up to a billion years of
missing time.
The Great Unconformity is not just
present in the Grand Canyon, but can be
observed
all over the world and is one of the most
widely recognized and distinctive
stratigraphic surfaces.
Geoscientists debate what caused the
Great Unconformity.
The leading hypotheses are either
regional tectonic uplift and erosion
during the formation of the
supercontinent Rodinia,
which is similar to the supercontinent
Pangaea, but formed
500 million years earlier. Or the second
hypothesis is a global glaciation.
Continental ice sheets and glaciers are
effective agents of erosion and they can
also cause sea level to drop as water is
removed from the ocean
and added to glaciers on land. Today the
opposite is happening as glaciers
on land are melting due to global
warming and sea level is rising.
As sea level drops during a global
glaciation, areas of the continents that
were once
covered by water, and protected by the
oceans, are now exposed to the air
and subject to erosion.
Geologists have theorized that the
extended period of erosion and
continental denudation associated with
the Great Unconformity
may have caused some pretty strange
events - like the formation
of a Snowball Earth.  If you've watched
the video on Joshua Tree National Park,
you'll know that weathering and erosion
of rocks
breaks down minerals like feldspar into
more basic elements
and releases cations like calcium into
streams and rivers.
The dissolved calcium makes its way into
the groundwater and eventually into the
oceans.
Calcium likes to bond with carbonate,
which forms when carbon dioxide is
dissolved in water.
The mineral produced is called calcium
carbonate or calcite,
which is the main mineral that limestone
is made of. If you've ever seen a
stalactite hanging from a cave ceiling -
that is calcite precipitating out of the
water.
So long story short - weathering and
erosion releases cations that help
to suck carbon dioxide out of the
atmosphere and lock it away
in a rock form.
As you know, carbon dioxide is the most
important greenhouse gas.
The erosion that created the Great
Unconformity was so widespread and
intense
that it may have pulled enough carbon
dioxide out of the atmosphere to have
caused the entire Earth to be covered
in ice - a condition affectionately called
Snowball Earth.
It sounds bizarre, but geologists believe
that a Snowball Earth -
entirely covered by glaciers - existed
multiple times in Earth's history.
Another fantastical event in Earth
history that may be linked to the Great
Unconformity
is the Cambrian Explosion.
About 540 million years ago practically
all major animal
evolutionary lineages suddenly appeared
in the fossil record.
Before the Cambrian Explosion, organisms
were simple single-celled organisms or
basic cellular colonies and were mainly soft-bodies
After the explosion
skeletonized animals appear - 
not internal skeletons yet - animals with
shells.
So why did animals with shells suddenly
appear on the evolutionary scene?
Well, these shells are mainly made of
calcium carbonate.
It's possible that during the extreme
weathering and erosion associated with the Great Unconformity
that the amount of calcium in the oceans
increased dramatically,
which may have hastened the evolution of
biomineralization
and adaptations like shells and hard body
parts.
Humans, and our hot body parts - our
skeletons, are the direct ancestors of
the animals that appear during the
Cambrian Explosion.
And perhaps you and everyone you know
owes their very existence to the Great
Unconformity.
Hey, thanks for watching. Check out some
more videos and share them with friends
and family.
Take care.
