Robledo Mountain Geology
[music 00:00 00:42]
Scott Elrick: Spencer, so I'm looking down
the canyon here, and I'm seeing rocks that
do not look like what I see here.
The most obvious thing that I'm seeing is
this really big crack and rocks that seem
to be bending and different shapes.
What's going on?
Spencer Lucas: Well, we're here along the
eastern edge of the Robledo Mountains, so
what we're looking at is the master fault
of the Rio Grande Rift.
There's an enormous fracture here in the earth's
crust.
On the east side of the fracture, down the
arroyo, the rocks there have dropped down
thousands of feet.
Then, here we are on the west side of the
fault, so we're looking at the older rocks,
the Permian rocks still at the surface.
When you look across the fault, what you see
is a very dramatic change in the geology from
these older rocks, which are here on the up
block of the crust, from the rocks that are
on the down block of the crust.
That, in fact, if you wanted to find these
Permian rocks on the other side of the fault,
you'd have to go thousands of feet down below
the valley surface.
Scott: One of the things that I see here is
that...you have to look carefully, at least
I do anyway.
I can see that some of the rocks, not only
are they sort of turning up, but some are
almost vertical.
What's causing that?
Spencer: What's happening here is, OK, we
have the master fault, which is running more
or less north south, but we also have other
faults around it, and this is one of those
other faults.
We have a fracture here.
A lot of times when you have a fracture and
there's movement, the rocks end up being pulled
up or pushed up against that fracture.
What's interesting here is, these limestones
that are pulled up along the fault, some of
them are fractured.
You can see that.
But some of them are actually literally bent.
The reason they probably got bent is there
was enough heat here associated with the fault.
This was happening under the ground surface
that the rocks became what we would call ductile.
They actually became somewhat plastic, and
they were able to bend without simply cracking.
Scott: OK, we've heard master fault.
I've heard about the Rio Grande Rift, and
I see there's the Rio Grande River just over
there.
What's the connection?
Spencer: Well, the Rio Grande Rift is this
series of down drop blocks to the earth crust
that runs all the way from Alamos to Colorado
down through El Paso.
That process of the crust dropping began about
25 million years ago, plus or minus.
As it dropped, it created all these depressions.
But eventually, by about two or three million
years ago, a single river began to flow down
through all those valleys or all those basins.
That is the Rio Grande River that drains now
all of the basins of the Rio Grande Rift,
and, of course, it flows from Colorado all
the way down to the Gulf of Mexico.
Scott: OK, so then, if I understand this right,
all the other mountains that are bounding
the Rio Grande River, do they have a similar
master fault that you could trace as you would
go north?
Spencer: Most of them do.
Well, even if the Organ Mountains, which are
just east of Las Cruces, they're on the other
side of the rift.
There would be an enormous fault there that
separates the Organs from the basin fill that
the City of Las Cruces is built on.
As you move north, say when you get to Albuquerque,
for example, the Sandia Mountains are also
just east of the rift.
So yeah, along the sides of the rift, both
sides, there are big faults, and usually there
are mountain ranges popped up along.
Robledos is one of these blocks of crusts
that's popped up along the western margin
of the rift.
Scott: So these mountains are part of a larger
system that really defines, in a way, the
big scale geology of New Mexico.
Spencer: The big scale geology of New Mexico
and, actually, part of a whole system that
geographers, geologists refer to as the North
American Basin and Range.
We're in the basin and range of western North
America.
That, of course, extends out into Arizona,
up through parts of Utah and Nevada.
Where, basically there was a stretching of
the crust, an opening up of the crust if you
will, during the last 25 or so million years.
A lot of blocks dropped; a lot of blocks popped
up.
Hence, you get the basins where it's down
drop; the ranges where it's popped up.
That is the dominant geologic structure of
much of the western United States.
Scott: That means that this Monument is part
of that larger system and part of the larger
picture.
Spencer: Right.
It is exactly that, and so there's another
level of geology you can learn about here.
You can learn about the Permian rocks and
the fossils, but you also have this, geologically
speaking, young history of the Rio Grande
Rift, of part of the basin and range that
you can learn about here in the monument.
Scott: All from this one outcrop.
Spencer: All from this one outcrop.
Scott: [chuckles] Thank you.
Spencer: OK, one of the interesting things
you see when you look at the geology here,
when you look at the layers of rocks, is we
can pull out what we think of as four levels
of cycles, a sort of hierarchy of events that's
occurring in terms of how the rock is formed.
The first level is the difference between
marine and non marine.
We have the marine limestone, the sea.
Then we have the red beds, which represent
on land.
That's our first level, the highest level
of cycle, and this repeats in some pattern.
Then we have another cycle within the red
beds where we have these thinly bedded sandstones
that have all the footprints in them.
In between we have levels of clay or mudstone,
and in that mud we see kind of green layers
that represent plant roots.
So here we're seeing the cycles between sheet
flooding and the formation of what we would
call flood plain deposits or more stable landscapes.
That's the second level of cyclicity.
Then the next level is, let's go into the
pile of sand itself, the sheet flood deposit.
If we look at that we see thicker beds and
thinner beds.
The thicker beds are probably single events,
and the thinner beds represent other events.
That's another level of cyclicity.
Then the fourth level, the lowest level of
all if you will, is every individual bed itself
in the sandstone.
That represents some sort of cycle.
I think all geologists would look at this
hill and agree there's a hierarchy here of
events.
Then the real question becomes, "What is driving
these events," and that's the question of,
"What caused these rocks to really form?"
Well, that's a hard question to answer, or
a hard question for all of us geologists to
agree on.
One of the things we know was happening, and
we've talked about this before, is we know
there were ice ages in the Southern Hemisphere.
Some would believe that we have high sea level;
that's why we have the marine.
We have low sea level; that's why we have
the non marine.
You could argue that just the glacial interglacial
cycle is driving the highest level of cyclicity
here.
When you get within the red bed, though, you
have to have something else to drive that.
I'm not sure so much what drives so much the
flood plain to sheet flood, but within the
sheet flood I think we can make a strong argument
that the thicker bed thinner bedded thing
is probably being driven by monsoonal rains.
We know that the climate of Pangea, the supercontinent,
was that of a true monsoon, which means that
there was just a very long wet season when
it rained a lot, and there was a very long
dry season.
I would probably argue that those thick beds
within the sheet flooding represent the runoff
of, say, the wet season.
If that were the case, that would mean each
one of those thick beds came down in about
six months.
So you would get a sort of annual cyclicity
within that pile of rock.
Beyond that, it's hard to speculate.
What would each, then, single bed mean?
There are dozens of those, or hundreds of
those within the pile of rock.
Those could have been daily or weekly events
that brought sediment down onto the flats.
This is a problem we're still working on.
But if we can finally get to a convincing
answer, it may give us a way of knowing how
much absolute time, how many million years
the rocks really represent here.
But that's the way we're going to try to get
at it, by looking at these four cycles and
trying to figure out what's driving sedimentation,
what's causing the formation of the rocks.
[audio trips]
The basic thing we study, or one of the basic
tools of our trade is to create a rock column.
What we did here...
This is this rock section here, we measured.
We go through, we measure the thickness of
every layer, and then we describe the layers.
We're actually standing on that layer.
This is the sandstone with ripples.
This doesn't have a legend.
The diagram's not finished; it's a work in
progress.
What we're looking at then...and these are
meters, so from here to the top of that pyramid
is 117 meters of rock as we measured it.
That's what?
That's almost 400 feet.
That's well over 300 feet that we measured.
We go through and we record every layer.
We sometimes sample the rocks in order to
better describe them in the laboratory.
Then we try to place the fossil localities
into this column, because this gives us time
on the understanding that the oldest layers
are at the bottom of the rock pile.
What we try to do and one of the things we've
been working on, and its complex, is to put
into time order all the fossil localities
that are in the monument.
The way to do that is, you start out with
a place like this where you have the whole
rock column.
Then you go around to the other canyons that
we've been in and you try to figure out, "Where
do those blocks fit into this?"
At the end, what we're going to have...and
we already have this; it's a work in progress,
but by sometime in later 2011, we should have
made up our mind about where everything is.
Then what we get is the possibility of studying
how the fossils may have changed through time.
And, of course, that's evolution.
So that's how we get time in here.
Now, when you sit there and you'll say to
me, we said the other day, "How much time
does that represent?"
The answer is, "I don't really know."
I don't think, geologically, that represents
a lot of time.
I think it might be a million years or less.
That's something we can't easily get at.
But what we can do, not easily but with some
work, is we can put all the fossils into time
succession.
So at least we can say this was living before
this, was living before that.
Then if there are changes, whether it's plants,
tracks, any shells, whatever, we have that
fourth dimension of time that we're able to
try to... and that's how we study evolution,
with fossils.
Evolution at its simplest is just change through
time.
We can see the changes by looking at the different
kinds of fossils, but we need a time clock
in here, and that's what we do.
That's why this is a really key place to doing
this kind of work.
[music 11:58 till end]
Transcription by CastingWords
