Scientific Drilling Project by DOSECC Core Drilling Services
The Long Valley Exploratory Well (LVEW) is situated on the resurgent dome of Long Valley Caldera, at the boundary of the Sierra Nevada and the Basin and Range Province in eastern California (Figure 1). Long Valley Caldera was created by a catastrophic, voluminous eruption of high-silica rhyolite, approximately 760,000 years ago. Since that time, minor volcanic activity in the area has recurred every few hundred years and major volcanic episodes associated with the caldera magma system occur at a frequency of about every 200,000 years (Bailey, 1989). LVEW was drilled to assess the potential of a deep geothermal resource beneath the resurgent dome, help improve assessments of volcanic risk from major or minor volcanic eruptions emanating from the central caldera, and provide a “hole of opportunity” for testing drilling instruments and technology (Figure 2).
LVEW began in 1989 as an engineering and scientific research well for the U. S. Department of Energy’s Magma Energy Program. Original plans called for drilling to a total depth of 18,000 feet to be accomplished in four phases. Phase I was completed in 1989 to a depth of 2650 feet. In 1991, Phase II deepened the hole to 7200 feet, with emphasis on exploring the geothermal energy potential of a deep magmatic source. Data from Phase II suggested that downhole temperatures, which reached 100¡C at the bottom of the well, were too cool for geothermal development (Sass and others, 1991), although, mineralogical evidence suggested that, in the past, the temperatures had been as high as 350¡C for the same depth (McConnell and others, 1997).
Figure 1 – Long Valley location map.
The Long Valley Coring Project (LVCP), operated during July through September 1998, was the third phase of drilling at LVEW. This phase deepened the existing hole from 7180 feet to a final depth of 9831 feet (2188 to 2996 meters) during the summer of 1998. Unlike the previous two phases of drilling in which core was recovered only at specific intervals, LVCP recovered continuous core throughout the entire section. LVCP was initiated to obtain additional information on magmatic processes both past and present in the caldera. It was envisioned that by deepening the preexisting borehole it would be possible to gain information about the presence of small magmatic intrusions, the brittle-ductile transition above the inferred magma body, the nature of the temperature gradient, and perhaps the presence of a magma chamber or neogranite. In addition, drilling was expected to provide more details about geothermal energy potential, volcanic hazards and the stratigraphy of Long Valley Caldera, as well as to field-test a newly developed hybrid drilling unit (Sass and others, 1998). To achieve these goals, LVCP was slated to deepen the existing hole to a depth between 11,500 and 13,000 feet. However, unexpected delays and short bit runs, attributed to thick layers of metaquartzite, decreased daily recovered footage and cost precious drilling time and money. The final depth achieved was 9831 feet. Nevertheless, the retrieval of over 2500 feet of continuous core provided geologists with a wealth of new data and material for further research efforts.
During Phases I and II, LVEW passed through the volcanic caldera fill and into the metamorphosed intrusions and metasedimentary basement rock. LVCP continued to drill through a thick sequence of metasedimentary rocks (Figure 3). The metamorphic material is correlative with the Mt Morrison roof pendant which is intruded by Sierra Nevada granites to the west and south of the drillsite. Specifically, graphitic, banded metapelites, metaquartzites, calcareous sandstones, and calc-silicate rocks most resembles the sequence of metamorphic rocks that make up the nearby Ordovician Mt. Aggie Formation (McConnell and others, 1998). Metamorphic patterns in the recovered core are indicative of several episodes of deformation suggesting that multiple processes of metamorphism were active, either simultaneously or sequentially, in the area. Indeed, an increase in contact metamorphic grade in the deepest portion of the core suggests that drilling approached the contact aureole of an intrusive body. Possible sources include Cretaceous age Sierra Nevada granite or neogranite representing the chilled margin from the Long Valley Caldera magma chamber.
LVCP drilling intersected several igneous intrusions in the metamorphic rocks. Their chemistry and alteration vary greatly, suggesting intrusion at different times or multiple episodes of emplacement and magmatic activity in the caldera. Several intrusions are petrographically similar to hypabyssal sills in the overlying volcanic fill found during the earlier phases of drilling. Their cumulative thickness and presence is suggested to account for much of the elevation of the resurgent dome (McConnell and others, 1995). These intrusions may ultimately provide information about the underlying magma chamber.
The presence or absence of geothermal potential should be reflected in changes of temperature with depth. Temperature’s in LVEW are unexpectedly low (Figure 4). The last kilometer in the well is isothermal and the bottomhole temperature remains just above 100 ¡C. Despite these anomalously low temperatures, the core provides abundant evidence of deep hot water circulation. Thick zones of hydrothermal alteration and open veins of pristine crystals of hydrothermal minerals, such as quartz, epidote, chlorite, sulfide minerals, and blade calcite at depths of 8500-8550 and 9100 feet, suggest that vigorous hot water circulation was present in the recent past
Figure 2 – Long Valley cross section
In addition to locally stretched and ductilely deformed metasedimentary rocks, the core contains lithologic and structural evidence of recent faulting. Faulting is indicated by fresh fault gouge, open vein brecciation, and open fractures. Two sections show the most significant evidence for recent faulting. Thick fault gauges occur between 8630 and 8910, presumable indicating the largest and most active faults. A deeper sequence stretches from approximately 9420 to 9800 feet, repeating the package of faulted rocks many times; suggesting an active extensional fault zone. This depth coincides with the calculated hypocenters of the Long Valley Caldera seismic swarm from September 1997 to January 1998 (oral communication, Steve Hickman). This intriguing evidence for young and contemporaneous faulting may allow for interpretations of the state of stress in the area of the resurgent dome.
Future plans for the wellsite include continued temperature logging and the emplacement of a package of instruments including seismometers and flow meters to increase the monitoring capabilities of the Long Valley Observatory. A series of add-on science research projects are also underway including detailing geochemistry of fluid inclusions within the hydrothermal minerals, Alpha recoil track age determinations of volcanic rocks, and geophysical modeling of the thermal regime of the caldera.
[The above project summary was modified from Sacket et al., 1999.]
Bender-Lamb, Sylvia, Magma energy exploratory well, Long Valley Caldera, California Geology, 44 (4), p. 85-92, 1991.
Hill, David P., Sorey, Michael L., Ellsworth, William L., Sass, John H., Scientific drilling continues in Long Valley Caldera, California, Eos, Transactions, American Geophysical Union, 79 (36), p. 429, 432, 1998.
Martini, B. A., Cochran, S. A., Potts, D. C., Silver, E. A., Pickles, W. L., Carter, M. R., Priest, R. E., Wayne, B. M., White, W. T., III, Geobotanical characterization of a geothermal system using hyperspectral imagery; Long Valley Caldera, CA, USA, Anonymous, Proceedings of the thirteenth international conference; applied geologic remote sensing, Proceedings of the Thematic Conference on Geologic Remote Sensing, 13 (1), p. 337-341, 1999. Meeting: Thirteenth international conference on Applied geologic remote sensing, Vancouver, BC, Canada, March 1-3, 1999.
Sackett, Penelope C., McConnell, Vicki S., Roach, Angela L., Priest, Susan S., Sass, John H., Long Valley Coring Project, Inyo County, California, 1998; preliminary stratigraphy and images of recovered core, Open-File Report – U. S. Geological Survey , OF 99-0158, p. (1 disc), 1999.
Sass, John H., Finger, John T., McConnel, Vicki, The Long Valley coring project, Bulletin – Geothermal Resources Council, 27 (2), p. 44-46, 1997.
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