Hi folks, Dr. Chapman here. Today we'll
visit Mesa Verde National Park and
discuss depositional environments and
foreland basins.
Mesa Verde National Park in southwest
Colorado is home to over 600 cliff
dwellings that were built by the ancestral Pueblo
people around the year 1200 C.E.
The majority of these cliff dwellings
were built into the Cliffhouse Sandstone,
which is the uppermost formation of the
Mesaverde Group.
Formations are the primary unit used
when formally subdividing stratigraphy
or making a geologic map.
They can be anywhere from a few meters
to a few kilometers thick
and they are usually named after a nearby
geographic location.
It is somewhat outdated now, but
historically formations were given a
rock name when they were composed of a dominant lithology.
For example, the Cliffhouse Sandstone is
the same thing as the Cliffhouse
Formation and it was named after the cliff
dwellings in Mesa Verde National Park.
Non-geologists often mistakenly, but
innocently, call outcrops formations,
as in, "that formation over there looks
like a hedgehog."
Formations are lumped into groups and occasionally groups lumped
into supergroups.
Groups and super groups often cover very
large geographic areas
and the formations and formation names
within them change from location to
location.
The Mesaverde Group, named after Mesa
Verde National Park,
extends from Montana to New Mexico.
Whereas the Cliffhouse sandstone
nomenclature is only used in the four
corners region.
Besides the Cliffhouse Sandstone, the
Mesaverde Group at Mesa Verde National
Park contains
the Menefee Formation and the Point
Lookout Sandstone.  Beneath the Mesaverde
Group is the Mancos Shale or Mancos Group.
Anytime sedimentary rocks are present,
geologists ask themselves a series of
questions.
One question is, "What
depositional environment do these rocks
represent?
Interpretations of depositional
environment are based on the principle
of Uniformitarianism.
This principle presumes that the
geologic processes operating today are
the same as they were in the past - that
is they've been uniform throughout time.
For example, if geologists find a sandstone with similar
composition,
texture, and sedimentary structures to
modern beach sand -
we infer that the sandstone was
deposited in a beach environment in the past.
Geologists often refer
to rocks deposited in a specific
environment as a facies.
So we might refer to a collection of
strata as a beach facies.
There are hundreds if not thousands of
possible depositional environments and
facies, but a good introduction to them is to
imagine a river making its way from the
mountains to the sea.
Within or close to the mountains, streams
or rivers transport a mixture of
sediment including coarse sediment like cobbles
or boulders.
These might get deposited during a large
flood and the resulting rock type would
be called a conglomerate.
As sediment is transported farther down
the river, the clasts are worn down,
broken into smaller pieces, and sorted by
size.
As the river approaches the shoreline,
sediment may be deposited in coastal
features
like marshes, estuaries, deltas, lagoons,
and swamps.
These depositional environments usually
contain lots of organic material, like
dead plants,
and is where rocks like coal originally
form.
When the river reaches the sea the water
velocity drops precipitously
and most of the sediment gets dumped
near the shore.
Waves rework sediment near the coastline
to form beaches or barrier islands
and finer grain sediments can be carried
farther away from the shore by currents
and waves.
Only very fine grain sediment like clay
or mud is transported very far from the
shore.
It too eventually settles
out of suspension and ends up forming
rocks like shale.
At any given point in time sedimentary
rocks are being deposited in all of
these depositional environments
more or less at once and they're
collectively called a depositional
system.
In Mesa Verde National Park the Mancos
Shale was deposited offshore in a deep
water marine environment.
Overlying and gradational with the Mancos Shale is the Point Lookout Sandstone
that was deposited as a beach and as
nearshore sands in a barrier island
system.
Above the Point Lookout Sandstone, the
Menefee Formation was deposited in a
coastal marsh or swamp environment
and contains thin coal layers. This
pattern of transitioning from offshore
to beach to marsh facies
as we go up stratigraphic section tells
us that the relative sea level was
dropping -
called a regression.  The Cliff House
Sandstone was deposited in a shallow
near shore environment
so the transition from the coastal
Menefee Group to the near shore Cliff
House Sandstone
records a relative sea level rise - called
a transgression.
Outside of the National Park, even
younger stratigraphic layers are exposed
and we know that the Cliff House
transgression
was short-lived and the overall pattern
was one of regression and relative sea
level lowering.
We call it relative sea level because
the actual sea level elevation may not
be changing at all.
It could just be the land surface moving
up or down.
If you follow the Mancos Shale to the
west into central Utah,
you'll eventually encounter age-equivalent shoreline beach deposits.
As the shoreline moved from central Utah
to southwest Colorado,
it was depositing beach sandstone all
along the way -
resulting in a regionally extensive
sheet or layer of sandstone.
When the shoreline reached Mesa Verde
National Park, it deposited the Point
Lookout Sandstone.
As the shoreline migrated past Mesa
Verde, it deposited the Menefee Formation.
The vertical succession of sedimentary
facies tells us what kind of depositional
environment
existed at that specific spot in the
geologic past.
It also tells us what kind of facies were
present laterally
tens to hundreds of kilometers away.  This concept, using
a vertical sequence of facies to
understand lateral changes,
is called Walther's Law and it's a
really powerful tool for interpreting
depositional environments.
Another major question geologists ask
about sedimentary rocks
is, "Why did they accumulate in this
location?"
Sediments don't pile up for no reason.
There has to be a sedimentary basin to
collect and hold them.
The type of basin the sedimentary rocks
exposed in Mesa Verde National Park
were deposited in
is called a foreland basin. Foreland
basins form when the lithosphere is
flexed downward by the weight of a
mountain range -
like standing on the edge of a diving
board. The mountain range that
formed the forland basin is the Sevier
Thrust Belt, which extends from southern
California to
northern Yukon Territory in Canada. A
flexural depression formed in front of
the thrust belt sort of like a moat.
Sediment, chiefly eroded from the thrust
belt itself, began to fill this
depression or basin.
Initially the downward movement of the
land surface, called
subsidence, was occurring much faster
than the sediment coming in could fill
it up.
As a result, a huge inland sea formed
during the Late Cretaceous - 
called the Western Interior Seaway. At
its maximum extent, the Western Interior
Seaway
connected the Arctic Ocean to the Gulf
of Mexico.
The period of rapid subsidence when the
seaway was filled with water is recorded
by the deep water
Mancos Shale. As the thrust belt grew and
the basin filled up with sediment the
coastline closed in
and the seaway eventually disappeared.
This regression and basing filling event
is recorded by the Mesaverde Group.
Mesa Verde National Park is a national
treasure because it tells a story of how
an ancient civilization
adapted and interacted with the land.
It's part of the cultural heritage that
binds the modern Pueblo people
and Pueblo nations in New Mexico and
Arizona. The park also tells an even
older story  - a geologic story
in which the artifacts are sandstone and
shale.  The great
migrations and movements are shorelines marching across the continent -
and it connects us not just to the land
but to the Earth itself.
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Take care.
