The Goblin Colony: Spectacular Monoliths and Walls of Altered Bandelier Tuff South of the Valles Caldera, New Mexico

Valles Caldera: Preserving the “Yellowstone of the Southwest” Jeffrey M. Widener (bio) Located northwest of Santa Fe, New Mexico, are the majestic Jemez Mountains—a volcanic mountain range that began forming around 20 million years ago (figure 1) (Wolff et al. 2005). This range sits atop the western edge of the Rio Grande rift, an area of extreme volcanic activity, and is part of the Jemez lineament stretching from southeast Arizona to southern Colorado (Heiken et al. 1990). Recurrent tectonic activity formed and re-formed the Jemez Mountains into a range of lava domes, eroded cones, and incised canyons. Of particular interest to researchers are the eruptions of two small vents around 1.5 million years ago. These explosions left the surface unsupported and, as a result, the Toledo Caldera formed. Around 300,000 years later, another eruption, which was about 100 times stronger than the 1980 Mount St. Helens explosion in Washington, demolished the majority of the Toledo Caldera but formed a new one, the Valles Caldera, which measures almost 15 miles in diameter (Wolff and Gardner 1995; Valles Caldera Trust [VCT] 2003). Click for larger view View full resolution Figure 1. The Valles Caldera National Preserve is located in the volcanic Jemez Mountain range (in the distance)—taken from Santa Fe, NM (photo by author). [End Page 583] After a collapse that forms a caldera, resurgent domes can begin to form if uplift occurs owing to pressure pushing molten rock in the magma chamber below the caldera upward. In the Valles Caldera, this started over 70,000 years ago (Martin 2003, 6). Redondo Peak, at 11,254 feet, is the Valles Caldera National Preserve’s only resurgent dome. This resurgence drained the lakes, created rivers, and formed the Cañon de San Diego, where the Jemez settled in the 1700s. Geographers, geologists, and other researchers recognize the Valles Caldera as an exceptional example of an exposed caldera formation, and, indeed, some call it the “Yellowstone of the Southwest” (VCT 2003, 18; Solomon 2004). Like Yellowstone, it is a place worthy of preservation for its beautiful landscapes. It is also of great importance culturally and ecologically to several indigenous groups, particularly the Towa-speaking Jemez Pueblo. The government continues to seek ways to best preserve such areas for posterity. Over the past few decades, United States (US) government officials have tried using new methods of acquiring and paying for publicly owned land. In 1996, Congress placed the Presidio, a historic military base in San Francisco, under the direction of the Presidio Trust—the first charter land management program. Congress implemented this trust not only to “preserve the Presidio’s natural, scenic, cultural, and recreational resources” but also to help it pay for its own upkeep (Presidio Trust [PT] 2010; Fairfax, Gwin, and Huntsinger 2004). Thus, they established a difference between setting aside public land to be government supported and placing public land under a trust that would become self-supporting. In this case, the Presidio Trust manages 80 percent and the National Park Service (NPS) oversees 20 percent (PT 2010). The Valles Caldera National Preserve, established in 2000, is the second site placed in a trust program—the Valles Caldera Trust—but this region has a historical cultural geography much richer than and different from that of the Presidio (figure 2). While several authors have described the history and the problems associated with the land management practices of the NPS and the United States Department of Agriculture Forest Service (USFS) (Foresta 1984; Dilsaver 1994; Spence 1999; Burnham 2000; Williams 2006; Sellers 2009; Runte 2010), only a handful of scholars have researched the land management system of public land trusts (Benton 1998; Fairfax, Gwin, and Huntsinger 2004; Little, Berrens, and Champ 2005; DeBuys and Usner 2006; Sullivan 2008). On the surface, public land trusts are similar to the NPS and USFS models, but the unique trust model focuses more on actual land use and management for sustainability and resilience than the more familiar NPS [End Page 584] Click for larger view View full resolution Figure 2. The Valles Caldera National Preserve (cartography by author). and USFS models do. Both land management models, however, have a number of important roles in...

This report presents field, chemical, gas, and isotopic data for thermal and nonthermal waters of the southern Jemez Mountains, New Mexico. This region includes all thermal and mineral waters associated with Valles Caldera and many of those located near the Nacimiento Uplift, north of San Ysidro. Waters of the region can be categorized into five general types: (1) surface and near-surface meteoric waters; (2) acid-sulfate waters at Sulphur Springs (Valles Caldera); (3) thermal meteoric waters in the ring fracture zone (Valles Caldera); (4) deep geothermal waters of the Baca geothermal field and derivative waters in the Soda Dam and Jemez Springs area (Valles Caldera); and (5) mineralized waters near San Ysidro. Some waters display chemical and isotopic characteristics intermediate between the types listed. Data in this report will help in interpreting the geothermal potential of the Jemez Mountains region and will provide background for investigating problems in hydrology, structural geology, hydrothermal alterations, and hydrothermal solution chemistry.

Hydrothermal alteration in the Valles caldera ring fracture zone and core hole VC-1: evidence for multiple hydrothermal systems

Seismic perspectives from the western U.S. on magma reservoirs underlying large silicic calderas

This report is a review and summary of the core drilling operations of the first Valles Caldera research borehole (VC-1) under the Thermal Regimes element of the Continental Scientific Drilling Program (CSDP). The project is a portion of a broader program that seeks to answer fundamental scientific questions about magma, rock/water interactions, and volcanology through shallow (<1-km) core holes at Long Valley, California; Salton Sea, California; and the Valles Caldera, New Mexico. The report emphasizes coring operations with reference to the stratigraphy of the core hole, core quality description, core rig specifications, and performance. It is intended to guide future research on the core and in the borehole, as well as have applications to other areas and scientific problems in the Valles Caldera. The primary objectives of this Valles Caldera coring effort were (1) to study the hydrogeochemistry of a subsurface geothermal outflow zone of the caldera near the source of convective upflow, (2) to obtain structural and stratigraphic information from intracaldera rock formations in the southern ring-fracture zone, and (3) to obtain continuous core samples through the youngest volcanic unit in Valles Caldera, the Banco Bonito rhyolite (approximately 0.1 Ma). All objectives were met. The high percentage of core recovery and the excellent quality of the samples are especially notable. New field sample (core) handling and documentation procedures were successfully utilized. The procedures were designed to provide consistent field handling of the samples and logs obtained through the national CSDP.

Eruption of reverse-zoned upper Tshirege Member, Bandelier Tuff from centralized vents within Valles caldera, New Mexico

Field, chemical, and isotopic data for 95 thermal and nonthermal waters of the southern Jemez Mountains, New Mexico are presented. This region includes all thermal and mineral waters associated with Valles Caldera and many of those located near the Nacimiento Uplift, near San Ysidro. Waters of the region can be categorized into five general types: (1) surface and near surface meteoric waters; (2) acid-sulfate waters (Valles Caldera); (3) thermal meteoric waters (Valles Caldera); (4) deep geothermal and derivative waters (Valles Caldera); and (5) mineralized waters near San Ysidro. Some waters display chemical and isotopic characteristics intermediate between the types listed. The object of the data is to help interpret geothermal potential of the Jemez Mountains region and to provide background data for investigating problems in hydrology, structural geology, hydrothermal alterations, and hydrothermal solution chemistry.

Over 5% of heat in the western United States is lost through Quaternary silicic volcanic centers, including the Valles caldera in north central New Mexico. These centers are the sites of major hydrothermal activity and upper crustal metamorphism, metasomatism, and mineralization, producing associated geothermal resources. We present new heat flow data from Valles caldera core hole 1 (VC‐1), drilled in the southwestern margin of the Valles caldera. Thermal conductivities were measured on 55 segments of core from VC‐1, waxed and wrapped to preserve fluids. These values were combined with temperature gradient data to calculate heat flow. Above 335 m, which is probably unsaturated, heat flow is 247±16 mW m−2. The only deep temperature information available is from an uncalibrated commercial log made 19 months after drilling. Gradients, derived from uncalibrated temperature logs, and conductivities are inversely correlated between 335 and 737 m, indicating a conductive thermal regime, and component heat fluxes over three depth intervals (335–539 m, 549–628 m, and 628–737 m) are in excellent agreement with each other with an average of 504±15 mW m−2. Temperature logs to 518 m depth with well‐calibrated temperature sensors result in a revised heat flow of 463±15 mW m. We use shallow thermal gradient data from 75 other sites in and around the caldera to interpret the thermal regime at the VC‐1 site. A critical review of published thermal conductivity data from the Valles caldera yields an average thermal conductivity of ≥1 W m−1 K−1 for the near‐surface tuffaceous material, and we assume that shallow gradient values (°C km−1) are approximately numerically equal to heat flow (mW m−2). Heat loss from the caldera is asymmetrically distributed, with higher values (400 mW m−2 or higher) concentrated in the west‐southwestern quadrant of the caldera. This quadrant also contains the main drainage from the caldera and the youngest volcanism associated with the caldera. We interpret the shallow thermal gradient data and the thermal regime at VC‐1 to indicate a long‐lived hydrothermal (and magmatic) system in the southwestern Valles caldera that has been maintained through the generation of shallow magma bodies during the long postcollapse history of the caldera. High heat flow at the VC‐1 site is interpreted to result from hot water circulating below the base of the core hole, and we attribute the lower heat flow in the unsaturated zone to hydrologic recharge.

Long-range core-drilling operations and initial scientific investigations are described for four sites in the Valles caldera, New Mexico. The plan concentrates on the period 1986 to 1993 and has six primary objectives: (1) study the origin, evolution, physical/chemical dynamics of the vapor-dominated portion of the Valles geothermal system; (2) investigate the characteristics of caldera fill and mechanisms of caldera collapse and resurgence; (3) determine the physical/chemical conditions in the heat transfer zone between crystallizing plutons and the hydrothermal system; (4) study the mechanism of ore deposition in the caldera environment; (5) develop and test high-temperature drilling techniques and logging tools; and (6) evaluate the geothermal resource within a large silicic caldera. Core holes VC-2a (500 m) and VC-2b (2000 m) are planned in the Sulphur Springs area; these core holes will probe the vapor-dominated zone, the underlying hot-water-dominated zone, the boiling interface and probable ore deposition between the two zones, and the deep structure and stratigraphy along the western part of the Valles caldera fracture zone and resurgent dome. Core hole VC-3 will involve reopening existing well Baca number12 and deepening it from 3.2 km (present total depth) to 5.5 km, this core hole will penetrate the deep-crystallized silicic pluton, investigate conductive heat transfer in that zone, and study the evolution of the central resurgent dome. Core hole VC-4 is designed to penetrate deep into the presumably thick caldera fill in eastern Valles caldera and examine the relationship between caldera formation, sedimentation, tectonics, and volcanism. Core hole VC-5 is to test structure, stratigraphy, and magmatic evolution of pre-Valles caldera rocks, their relations to Valles caldera, and the influences of regional structure on volcanism and caldera formation.

New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative P wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km. The three principle features of our model for Valles caldera are: (1) near‐surface low velocities of −17% beneath the Toledo embayment and the Valle Grande, (2) midcrustal low velocities of −23% in an ellipsoidal volume underneath the northwest quadrant of the caldera, and (3) a broad zone of low velocities (−15%) in the lower crust or upper mantle. Crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast. Near‐surface low velocities are interpreted as thick deposits of Bandelier tuff and postcaldera volcaniclastic rocks. Lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast, beneath the Valle Grande. We interpret the midcrustal low‐velocity zone to contain a minimum melt fraction of 10%. While we cannot rule out the possibility that this zone is the remnant 1.2 Ma Bandelier magma chamber, the eruption history and geochemistry of the volcanic rocks erupted in Valles caldera following the Bandelier tuff make it more likely that magma results from a new pulse of intrusion, indicating that melt flux into the upper crust beneath Valles caldera continues. The low‐velocity zone near the crust‐mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle. In either case, this low‐velocity anomaly indicates that underplating by mantle‐derived melts has occurred.

K/Ar dates of hydrothermal clays from core hole VC-2B, Valles caldera, New Mexico and their relation to alteration in a large hydrothermal system

Valles Caldera 1 (VC‐1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC‐1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856‐m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC‐1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480‐, 540‐, and 845‐m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.

Mobility and depositional controls of radioelements in hydrothermal systems at the Long Valley and Valles calderas

About 1.25 million years ago a volcanic eruption in New Mexico’s Jemez Mountains created the Valles Caldera. The Valles Caldera is particularly susceptible to erosion induced by wildfires because of its steep slopes. Gullies that feed the bottom grassland locations are reactivated after a fire season when protective vegetation is burned off and the slopes of the caldera are subjected to increased erosion. In July 2011, the Las Conchas Fire, started by an electrical power line on nearby private land, burned 120 km2 of the Valles Caldera National Preserve causing enormous erosion in a short amount of time. A 3 m exposure was found in a gully, after the 2011 fire, which revealed alternating light and dark bands of material (15 – 25 cm in width). The abrupt and unusual nature of the contact between different colored material strongly suggests deposition events related to either fire cycles or heavy precipitation events on a fire-ravaged landscape. Two competing theories for the formation of the layers are that they reflect the fire history of the caldera by directly transporting and depositing charcoal or burned sediment or that these layers are part of an organic soil formation in a local wet spot (or marsh) that is periodically buried by alluvium or debris flows after fires. Fifteen OSL samples of the dark bands and three OSL samples of the light bands were measured for quartz OSL. When dated with OSL, material near the bottom is about 7,000 years old and material near the top is about 2,500 years old but there is no apparent pattern to the formation of dark bands. The quartz was unexpected since the majority of the local sediments are sourced from tuff, although there are a few local exposures of sandstone near the top of the Preserve rim. Luminescence characteristics show that it is blown in and incorporated into the sediment. Particle size analyses reveals that both the light and dark sediment is 80% sand, 5% clay and 15% silt, with some variations, but no large swings, which is puzzling if the darker bands are organic soil, since one would expect a finer grain size. Elemental concentrations reveal no substantial difference in major or minor elements between light or dark layers. A young mollisol was sampled further upstream, along with a modern marsh deposit, and reveals that the darker layers are almost certainly mollisols. This presentation will focus on why these mollisols are unusual and what they indicate about past climatic events in Valles Caldera.

This field trip guide has been compiled from extensive field trips led at Los Alamos National Laboratory during the past six years. The original version of this guide was designed to augment a workshop on the Valles Caldera for the Continental Scientific Drilling Program (CSDP). This workshop was held at Los Alamos, New Mexico, 5-7 October 1982. More stops were added to this guide to display the volcanic and geothermal features at the Valles Caldera. The trip covers about 90 miles (one way) and takes two days to complete; however, those who wish to compress the trip into one day are advised to use the designated stops listed in the Introduction. Valles Caldera and vicinity comprise both one of the most exciting geothermal areas in the United States and one of the best preserved Quaternary caldera complexes in the world.