The
Crestone Eagle, August 2007:
If you find fault, don’t dwell on
it!
Summary of CFI/CSC geology field trip on July 14
story by Thomas Cleary
illustrations by James McCalpin
 On
a brilliant and hot Saturday last month about 30 folks from
the Valley and beyond participated in a field trip called
“Finding Fault in the San Luis Valley” hosted
by the Colorado Field Institute’s Jerrilyn Wueste and
Crestone Science Center’s Dr. Jim McCalpin. The trip
went from views and exposures of the Sangre de Cristo fault
from within the Baca Grande Subdivision, to Valley View Hot
Springs to examine a trench through a spur of the fault, to
Joyful Journey Hot Springs to talk about underlying basin
structure. Discussion focused on the faults and accompanying
geothermal springs and included Lexam’s mineral exploration,
geothermal development potentials, and geohazard evaluation,
among other topics.
A brief geologic history of Colorado and the Valley
Geology is cyclic, according to the scientific method, the
theory of plate tectonics and radiometric dating. For example,
the current Rocky Mountains rose up from under a large interior
seaway covering much of what we now call the western United
States about 65 million years ago. They have (mostly) been
eroding away ever since. Before them were the Ancestral Rocky;
Mountains; these were completely eroded and down warped to
create the basin for the seaway. It was during this time 220
million years ago (mya) that our famous Crestone Conglomerate
was formed from stream borne sediments deposited near the
foot of a mountain. Before those mountains were the long eroded,
so-called Pre-Ancestral Rockies—and so on.
Under the San Luis Valley are faults that are associated
with each of these mountain building events (orogenies), and
these faults remain active today. Faults are breaks in the
surface (or crust) of the Earth at which movement occurs during
stress releasing events (earthquakes).
During the Laramide Orogeny, 65 mya, our region was rising
but not necessarily mountainous. There is a gap in the rock
record for this period representing a time of erosion or at
least no deposition. It is known that during this time the
region was under great lateral pressure and old (1.2 billion
ya) granites were thrust up and over younger rocks including,
in part, Crestone Conglomerate. But it was not until 19 mya
that the Sangre de Cristos were uplifted, granites, conglomerates,
and all, as a result of the formation of the Rio Grande Rift.
The great rift valleys such as the East African, Baikal,
and Rio Grande are thought to originate due to major upwelling
or convective forces from below the crust (such as a hot spot
within the mantle); this initially creates an uplift of the
region as a whole. In our region this has been dated by the
non-faulted, flat lying, continuous nature of the Sante Fe
formation consisting of volcanic ash and exploded debris dated
at 19.5 mya. This also denotes the last time the Arkansas
River flowed south into the Rio Grande.
As this uplift continued, the region experienced a pulling
apart (tension) and thinning of the crust, breaking it up
into block along the reactivated fault planes. As the crust
spreads and thins due to the uplift, some block continue to
rise (horsts), while others drop downward (grabens), and others
bend or tip in response to the uplift and subsidence.
Meanwhile the horsts erode off sediments into the graben
regions. This describes the substructure of our area: the
massive uplifted block of the Sangres, bound by Wet Valley
(east), Pleasantville (northeast) and San Luis Valley (west)
grabens, and moving further west across the Valley, the subsurface
Alamosa Horst (exposed in the San Luis Hills south of Alamosa),
then the Monte Vista Graben hinged off the San Juan mountains.
Most of this structure is covered by 5-10,000 feet of loose
(non-lithified) gravel, sand, silt, and clay. These sediments
were carried down from the mountains in streams and during
glacial periods as recent as 15,000 ya.
These faults are still active as we saw evidence of this
by looking at places where steep faces cut through young sediments
(fault scarps). By trenching through the surface expression
of these deep faults, and dating layers within them, it seems
that these faults move in 3 to 6 foot jolts every 7.5-10 thousand
years. The last event was 7,000 ya, plus or minus 1000 years!
This fault runs directly through Chalets I and II in the Baca
subdivision!
We’re in hot water now
The faults that are instrumental in the structure of the Valley
are also involved in hot water production. Surface (meteoric)
water soaks into the sediments and down fractures and continues
deep into the subsurface. Most meteoric water resides in the
upper aquifer (unconfined), but some makes it deep into the
lower aquifers bound by impermeable clay layers (confined).
In the SLV, one of the impermeable layers was formed of 100
feet of clay deposited in a lake that existed in the northern
Valley when sediments dropped out of the Rio Grande blocking
the flow of its own tributary from the northern Valley, chiefly
Saguache creek.
The San Luis Lakes are remnants of this once great lake 300,000
ya. The deeper the water goes, the hotter it gets due to the
closer proximity to the mantle. As substances heat up they
expand. We all know that hot air rises; so does hot water.
And if that hot water finds the loose and fractured rock associated
with a fault zone, it will take that path of least resistance
up to the surface. It is no wonder that Valley View Hot Springs
is located on the main fault at the base of the Sangres. Joyful
Journey is above a fault along the Alamosa Horst. What about
the Sand Dunes (Hooper) pool? It is fed by an exploratory
oil well that is thought to have encountered one of the subsurface
faults.
There has been talk of geothermal developments for Elk Park
or elsewhere in the Valley. While current understanding of
the subsurface faults would increase the odds of hitting hot
water, it is still a risky proposition. And water law requires
that water from the confined aquifer must be reinjected back
to the same aquifer, requiring a second well. Hot water solar
panels may be a more feasible alterative.
Lastly, Lexam and oil and gas exploration
For an oil well to be productive it must have (1) a source
rock from which the oil was made within a narrow range of
heat and pressure, (2) a reservoir rock that has the correct
amount of pore space to store the oil but also release it
to a pumping well, and (3) a structural trap (often in the
shape of an elongated umbrella) where the gas, which floats
on top of oil, or oil, which floats on top of water, could
be trapped. Lexam thinks it found this combination as stated
in a published report of June, 2006 www.lexamexplorations.com/energy_oil_gas_reports.php.
And according to a recent press release, it thinks it confirmed
them with this spring’s seismic testing. We will see
if they are confident enough to spend the 10+ million dollars
per hole to see if they are right. Let’s hope for the
best.
This information was taken from field notes from the trip,
prior knowledge from my Environmental Geology Bachelors in
Science from CSU, and a related article by Dr James McCalpin
that can be found at www.nps.gov/archive/grsa/resources/docs/Trip2021.pdf.
That said, this paper has not been peer reviewed, nor extensively
fact checked, and the information herein may be analogous
to the saying “lightning is static electricity caused
by clouds rubbing together,” that is, simplified to
the point of being wrong. But I think it is pretty close to
the current truth!
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