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Valles Caldera Scientific Drilling
Fraser Goff and Jeffrey M. Heikoop

In the heart of the Jemez volcanic field in northern New Mexico lies the 22-kilometer Valles caldera — a beautiful example of a resurgent caldera. Now owned by the U.S. government and known as the Valles Caldera National Preserve, this collapsed volcanic crater is about 1.25 million years old and hosts a 280 degree-Celsius liquid-dominated hydrothermal system.

Workshop participants walk toward some of the lacustrine deposits exposed in Valle San Antonio, Valles caldera. The north caldera wall appears in the background. These deposits overlie a post-caldera rhyolite dome sequence dated at 0.8 Ma. This section contains a rich assemblage of diatom fossils and has normal magnetic polarity. Image courtesy of the authors.

Over the past 30 years, various drilling projects have revealed a fascinating wealth of information on the geothermal potential of the caldera. Most recently, 65 U.S. scientists met in Los Alamos, N.M., to weigh the pros and cons of pursuing more drilling in order to understand climate change history through intracaldera sediments at this famous location. To learn more, visit www.geotimes.org.

Past efforts

Geothermal prospecting from 1960 to 1984 resulted in drilling of 24 deep exploration and production wells but a commercial reservoir was never developed for many reasons. From 1984 to 1988 the Continental Scientific Drilling Program drilled three core holes in Valles that produced a wealth of published information on the configuration of the hydrothermal system, the stratigraphy and structure of the resurgent dome, and analogues with fossil ore deposits. The last hole drilled was briefly the deepest, hottest core hole drilled in the conterminous United States — at 1,762 meters and 295 degrees Celsius.

Valles caldera also contains little known lacustrine sediments and hydromagmatic deposits dating from the inception of caldera formation to roughly 50, 000 years ago. More recent bog deposits also exist. Presumably, many of these deposits lie buried within the caldera moat, and are overlain and interbedded with post-caldera moat rhyolite eruptions. New geologic mapping at 1:24,000 scale shows that the best exposures of lacustrine rocks occur on the uplifted flanks of the central resurgent dome and as eroded remnants within the encircling valleys (Valle Grande, Valle Toledo, Valle San Antonio).

Very few absolute ages exist for these lacustrine deposits but geologic constraints indicate that a large lake formed in the caldera immediately after its creation and that several lakes existed in the valleys after 800,000 years ago. A 1952 pollen study by Paul Sears and Kathryn Clisby on shallow core and cuttings from a water well drilled in Valle Grande shows pronounced cycles of "dry" to "wet" pollen ratios that apparently mimic glacial-interglacial climate cycles. Presumably, the sediments of this study represent the youngest lacustrine deposits. Although nearly forgotten, this investigation suggests that the caldera sediments may contain a significant climate record and indicates that the Valles record correlates with a climate record as far away as Mexico City.

Next step

Most participants at the October workshop felt that the lacustrine sequences in Valles warranted further investigation before their climate change potential could be fully appreciated. Although no consensus was reached, many people suggested drilling a preliminary exploratory scoping corehole in Valle Grande to penetrate the youngest lacustrine sequences and to investigate their climate record using pollen, paleomagnetic, isotope, dating and other investigative techniques.

Many of the scientists also said that shallow seismic surveys were necessary in Valle Grande to determine the thickness and structure of the moat sediments, the position of possible buried volcanic domes, the absolute position of the buried ring-fracture zone, and the most suitable drilling site(s) for shallow and deep coring. Successful results from the scoping hole, they said, would justify additional and deeper holes for further climate research at Valles.

Drilling and funding requirements for the scoping hole are being evaluated. For additional information on the workshop, participants, and/or Valles lacustrine deposits, contact the authors.


Goff and Heikoop are both geologists at the Los Alamos National Laboratory in Los Alamos, N.M. Email: fraser@lanl.gov.

Link:

LANL Web site on Valles Caldera drilling

References:

Goff, F., 2002, Geothermal potential of Valles Caldera, New Mexico. Geo-Heat Center, Quarterly Bulletin, v. 23(4), p. 7-12.
Goff, F., and Gardner, J.N., 1994, Evolution of a mineralized geothermal system, Valles caldera, New Mexico. Economic Geology, v. 89, p. 1803-1832.
Sears, P.B., and Clisby, K.H., 1952, Two long climate records. Science, v. 116, p. 176-178.
Smith, R.L., and Bailey, R.A., 1968, Resurgent cauldrons: Geological Society of America, Memoir 116, p. 613-662.


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