Jemez Volcanic Field Research Articles

Petrogenesis of three East Fork Member rhyolites of the Jemez volcanic field, Valles caldera, New Mexico, USA

The most recent volcanism in the Valles caldera is represented, in stratigraphic order, by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), the VC-1 rhyolite (not studied herein), and Banco Bonito Flow (BBF), which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted approximately 74 to 68 ka after an approximate 460 ka lull in ring-fracture volcanism.Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides, in order of abundance. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (≥1 mm), typically resorbed or subhedral crystals represent one population, and small (≤0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have oligoclase to andesine rim overgrowths. 40Ar/39Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 3 ka and 575 ± 2 ka and total gas ages of 125 ± 1 ka and 129 ± 1 ka, respectively. High molar Mg numbers of large euhedral biotite and 40Ar/39Ar ages older than the accepted age range indicate these crystals are xenocrystic. Radiogenic isotopes are consistent with mixing between the mantle and lower crustal reservoirs. General trends are evident between whole-rock major and trace elements with increasing SiO2 and Nb, respectively. In general, incompatible trace elements increase and compatible trace elements decrease with increasing Nb. For the major elements, MgO, P2O5, Al2O3, FeO* show decreasing trends while K2O and Na2O show increasing trends with increasing SiO2. Incompatible trace element ratios (Ta/Yb, Y/Yb, Th/Yb, Th/Nb) indicate the presence of a single magma batch.The heterogeneity in crystal morphology and chemistry can be explained by a model in which partial melting of mid- to deep continental crust occurred due to an intrusion of an intermediate composition magma. Magma mixing and an exchange of crystals took place between the partial melt and the intruding magma. The hybrid magma rose to the upper crust, where it underwent fractional crystallization prior to eruption. The geochemical and isotopic data from this study are best explained by a modified version of the rapid production and eruption model put forth by Huppert and Sparks (1988).

Inferring Shallow Subsurface Density Structure from Surface and Underground Gravity Measurements: Calibrating Models for Relatively Undeformed Volcanic Strata at the Jemez Volcanic Field, New Mexico, USA

Imaging shallow subsurface density structure is an important goal in a variety of applications, from hydrogeology to seismic and volcanic hazard assessment. We assess the effectiveness of surface and subsurface gravity measurements in estimating the density structure of a well-characterized rock volume: the mesa (a small, flat-topped plateau) upon which the town of Los Alamos, New Mexico, USA is located. Our gravity measurements were made on the mesa surface above a horizontal tunnel and underground, within the tunnel. We demonstrate that, in the absence of other geophysical data such as seismic data or muon attenuation, subsurface (tunnel) gravity measurements are critical to accurately recovering geologic structure. Without the tunnel data, our resolution is limited to roughly the surface gravity station spacing, but by including the tunnel data we can resolve structure to a depth of ~ 10 times the surface gravity station spacing. Densities were obtained using both forward modeling and a Bayesian inverse modeling approach, incorporating relevant constraints from geologic observations. We find that Bayesian inversion, with geologically relevant prior, is a superior approach to the forward models in terms of both robustness and efficiency and correctly predicts the orientation and elevation of important geologic features.

The duration of a Yellowstone super-eruption cycle and implications for the age of the Olduvai subchron

Small-volume rhyolitic eruptions preceding and following a caldera-forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge. In this contribution, high-precision 40Ar/39Ar eruption ages were obtained on the three effusive eruptions bracketing the Huckleberry Ridge Tuff, which comprise Yellowstone's first volcanic cycle. These dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy. The Huckleberry Ridge Tuff (2.08 Ma) was preceded by an eruption at 2.14 Ma, and followed by eruptions at 1.98 and 1.95 Ma, all of which occurred during four distinct periods of geomagnetic instability within the Matuyama chron. The first volcanic cycle of Yellowstone has now been constrained to within a 200 kyr timespan, or half of the previously proposed duration, and similar to the duration of volcanic activity for caldera-forming systems in the Jemez Volcanic Field. The maximum duration for magmatic recharge for the first Yellowstone volcanic cycle is no greater than 100 kyr, and likely closer to 40 kyr. Furthermore, the combined 40Ar/39Ar eruption ages and paleomagnetic results provide polarity anchors for the Pre-Olduvai excursion and Olduvai subchron, which are often used as tie-points in studies of early Pleistocene hominin evolution.

Valles Caldera: a geologic history

The consists of a twelve-mile-wide collapsed volcanic crater and more than ten post collapse volcanic domes in New Mexico's Jemez Mountains. For over a century, it was safeguarded within the 89,000-acre Baca Ranch. In the year 2000, Congress passed the Preservation Act, creating the Trust to purchase the ranch and create a nine-member board of trustees responsible for the protection and development of the National Preserve. With special permission, qualified geologists interested in volcanic processes and hydrothermal systems have been allowed to conduct research on the preserve. One of those volcanologists, Fraser Goff, collaborated with the Trust to provide an accessible scientific overview of the caldera's geologic wonders. Presented in two parts, Valles Caldera first offers a summary of significant geologic events that have taken place in the area. Then Goff presents the geology, volcanology, and geothermal characteristics of the and the Jemez volcanic field. Geologic terms and names unfamiliar to all but professional geologists are defined in a summarizing glossary.

(U–Th)/He dating of volcanic phenocrysts with high-U–Th inclusions, Jemez Volcanic Field, New Mexico

We measured (U–Th)/He ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the ∼1.6 Ma Otowi (Lower) Member of the Bandelier Tuff, Jemez Volcanic Field. U and Th, the major parent nuclides of radiogenic 4He, are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains. We analyzed 36 fayalite (21 non-abraded + 15 abraded) and 13 hedenbergite (7 non-abraded + 6 abraded) phenocrysts, and applied alpha-ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts. A subset of phenocrysts was subjected to abrasion to remove the alpha-ejection affected regions. The alpha-ejection correction factors ( F T ) of the non-abraded samples range between 0.90 and 0.96, whereas those of the abraded samples are virtually unity. The fayalite and hedenbergite crystals yield mean ages of 1.60 ± 0.10 Ma (S.D.) and 1.69 ± 0.13 Ma, respectively. The overall mean age of 1.62 ± 0.11 Ma (S.D.) is indistinguishable from previously published 40Ar/ 39Ar ages of 1.61 ± 0.01 Ma (1 σ). Our results suggest that other inclusion-bearing volcanic phenocrysts and their host units may be dateable by the (U–Th)/He method.

Geologic Framework of a Groundwater System on the Margin of a Rift Basin, Pajarito Plateau, North‐Central New Mexico

The Pajarito Plateau is an important source of abundant potable groundwater for Los Alamos National Laboratory (LANL) and the communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau‐wide hydrogeological investigation to develop conceptual models of the groundwater system as a framework for numerical simulations of groundwater flow. The Pajarito Plateau is located in the western part of the Española basin where rocks of the Jemez and Cerros del Rio volcanic fields overlie and interfinger with Neogene basin‐fill sedimentary rocks. The vadose zone is about 200 m (600 ft) thick beneath mesas on the east side of the plateau and more than 375 m (1245 ft) thick on the west side. Groundwater occurs as shallow groundwater in canyon‐floor alluvium, moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone. The most productive rocks of the regional aquifer occur in a westward‐thickening wedge of coarse‐grained, Miocene and Pliocene volcaniclastic rocks derived from the Jemez volcanic field. Eastern aquifer rocks consist of fine‐grained, somewhat less productive Miocene sedimentary deposits derived from highland sources to the east and north. Intermediate and mafic lavas interbedded with the Miocene and Pliocene sedimentary deposits are components of the regional aquifer locally. The hydrogeology of the Pajarito Plateau is probably typical of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment‐filled basins receive most of their recharge from adjacent mountainous areas.

Three‐dimensional Kirchhoff migration: Imaging of the Jemez volcanic field using teleseismic data

We computed a reflectivity image for the Earth beneath the Jemez volcanic field, New Mexico, using a novel adaptation of petroleum exploration seismic imaging. This image was obtained by applying the Kirchhoff wave field imaging method to digitally recorded teleseismic data. The volume imaged has a lateral extent of 30 km, extends to 45 km depth, and lies beneath the Valles caldera, to the west of Los Alamos, New Mexico. The derived picture of the Earth is a three‐dimensional (3‐D) map of the locations of impedance changes within the crust and upper mantle below the Jemez volcanic field, with a spatial resolution on the order of a kilometer. Significant features seen in the image include the base of the caldera fill; several reflectors in the crust we interpret to be associated with intrusions coming from the mantle and/or other crystallized chambers such as a low‐velocity zone seen in the tomographic image; two strong reflectors coincident with the crust‐mantle interface, and a zone of layered reflections at the base of the crust which may be basaltic underplating. This method proved to be a powerful tool for imaging the Earth's crust and mantle when combined with other methods, such as tomography.

Structural and Stratigraphic Evolution of the Rio Grande Rift, Northern New Mexico and Southern Colorado

The well-known Pliocene to Quaternary Rio Grande rift of northern New Mexico and southern Colorado is distinctly different from the Miocene rift, especially in structural style. Prior to approximately 21 Ma, there was little extension or rift-basin development. Uppermost Oligocene and Lower Miocene strata were deposited as broad volcaniclastic aprons, with no significant evidence of syn-depositional faulting, in contrast to younger deposits. The only documented areas of extensional faulting and stratal rotation older than 21 Ma occur within or close to magmatic centers. Early rift basins (21-10 Ma) developed as half grabens progressively tilted in hanging walls of normal faults that primarily reactivated Laramide (Eocene) reverse faults: (1) the San Luis basin tilted eastward as the Sangre de Cristo normal fault reactivated westward-dipping Laramide reverse faults; (2) the Tesuque basin tilted westward as normal faults reactivated eastward-dipping Laramide reverse faults of Sierra Nacimiento and related features; and (3) the Belen basin experienced complex tilting as diverse normal faults reactivated variably dipping Laramide reverse faults. Some of these early-rift faults remain active, whereas others became inactive starting near 10 Ma, as new faults broke across Laramide and early-rift features. The Embudo transfer zone linked normal faults along the east side of the San Luis basin to the Pajarito, La Bajada, San Francisco, and Rincon fault zones at this time. Normal faults along the northwest side of the Miocene Tesuque basin became inactive at the same time that rapid uplift of the Sandia Mountains as a footwall block began at about 10 Ma. This shifting of normal-fault activity resulted in reversal of tilt direction from westward for the Miocene Tesuque basin to eastward for the modern Albuquerque basin. Uplift and erosion of early-rift deposits along the northwest side of the Albuquerque basin have resulted. This two-stage model for evolution of the Rio Grande rift in north-central New Mexico and southern Colorado is fundamentally different from previous two-stage models, which described Oligo-Miocene volcaniclastic aprons as “early rift deposits,” and related them to extensional structures. Rather, development of half grabens began around 21 Ma, with dominance of negative inversion of Laramide reverse and thrust faults. Regional change in extension direction led to the abandonment of some faults and the initiation of new faults at 10-8 Ma in the Rio Grande rift. The biggest change occurred in the Tesuque basin, as the western boundary fault became inactive during growth of the Jemez volcanic field, and the Sandia Mountains began their rapid rise as the northern Albuquerque basin tilted to the east. Continued regional uplift, and integration and incision of the Rio Grande and tributaries, have occurred during the last 5 million years, with the course of the river tending to follow the downdropped side of each modern half graben.

Effusive eruptions from a large silicic magma chamber: the Bearhead Rhyolite, Jemez volcanic field, NM

Large continental silicic magma systems commonly produce voluminous ignimbrites and associated caldera collapse events. Less conspicuous and relatively poorly documented are cases in which silicic magma chambers of similar size to those associated with caldera-forming events produce dominantly effusive eruptions of small-volume rhyolite domes and flows. The Bearhead Rhyolite and associated Peralta Tuff Member in the Jemez volcanic field, New Mexico, represent small-volume eruptions from a large silicic magma system in which no caldera-forming event occurred, and thus may have implications for the genesis and eruption of large volumes of silicic magma and the long-term evolution of continental silicic magma systems.40Ar/39Ar dating reveals that most units mapped as Bearhead Rhyolite and Peralta Tuff (the Main Group) were erupted during an ∼540ka interval between 7.06 and 6.52Ma. These rocks define a chemically coherent group of high-silica rhyolites that can be related by simple fractional crystallization models. Preceding the Main Group, minor amounts of unrelated trachydacite and low silica rhyolite were erupted at ∼11–9 and ∼8Ma, respectively, whereas subsequent to the Main Group minor amounts of unrelated rhyolites were erupted at ∼6.1 and ∼1.5Ma.The chemical coherency, apparent fractional crystallization-derived geochemical trends, large areal distribution of rhyolite domes (∼200km2), and presence of a major hydrothermal system support the hypothesis that Main Group magmas were derived from a single, large, shallow magma chamber. The ∼540ka eruptive interval demands input of heat into the system by replenishment with silicic melts, or basaltic underplating to maintain the Bearhead Rhyolite magma chamber.Although the volatile content of Main Group magmas was within the range of rhyolites from major caldera-forming eruptions such as the Bandelier and Bishop Tuffs, eruptions were smaller volume and dominantly effusive. Bearhead Rhyolite domes occur at the intersection of faults, and are cut by faults, suggesting that the magma chamber was structurally vented preventing volatiles from accumulating to levels high enough to trigger a caldera-forming eruption.

ESR dating of quartz phenocrysts in some rhyolitic extrusive rocks using Al and Ti impurity centres

ESR has been employed to determine the eruption date of rhyolitic rocks from three different tectonic settings. Samples from the Olkaria complex, Kenya rift valley, yielded ages of 31±3 and 38±5 ka (148a) and 23±4 and 34±8 ka (570) from the Al and Ti centres, respectively. These compare with an inferred formation age of 65±12 ka (148a), obtained from U-series isochrons, and an eruption age of 5.7–9.7 ka (570), based on 14 C and lake stands. Glass (melt) inclusions in sample 570 may have distorted the ESR age. The Youngest Toba Tuff (YTT), Sumatra, gave an average ESR eruption age of 81±17 ka which compares well with K/Ar and 40 Ar / 39 Ar ages in the range 73–75 ka. However, there is evidence of thermal annealing in the Oldest (OTT) and Middle (MTT) Toba Tuffs by the more recent YTT eruption, with mean ages of ca. 349 ka and 326 ka, respectively, significantly below the K/Ar age of 840±30 ka (OTT) and the 40 Ar / 39 Ar age of 501±5 ka (MTT). A sample from the Battleship Rock Tuff, Jemez Volcanic Field, New Mexico, gave a mean ESR age of ca. 247 ka which contrasts markedly with a previously determined ESR age of 59±6 ka, but compares more favourably with previous K/Ar ages of 278±52, 130±70 and 180±70 ka. Subsequent geothermal activity and high ambient temperatures in this area may be responsible for these apparent discrepancies.

Heterogeneous structure around the Jemez volcanic field, New Mexico, USA, as inferred from the envelope inversion of active-experiment seismic data

SUMMARY We analyse active-experiment seismic data obtained by the 1993 Jemez Tomography Experiment (JTEX) programme to elucidate the heterogeneous structure of the Jemez volcanic field, which is located at the boundary between the Colorado Plateau and the Rio Grande Rift. Using a single isotropic scattering assumption, we first calculate the envelope Green's functions for the upper and lower crust and the uppermost mantle. By fitting the theoretical envelopes with the observed three-component data, we estimate depth-dependent features of the scattering coefficients around Valles Caldera. We estimate the ratios of scattering coefficients, rather than scattering coefficients themselves, because of the uncertainty of the seismic efficiency of the explosive sources and knowledge of absolute site-amplification factors. The strongest scattering coefficients are observed at a shallow depth beneath the Valles Caldera. This is considered to be related to the complex structure caused by two episodes of caldera formation and the ensuing resurgent uplift in the caldera, etc. The depth-dependent characters of the scattering coefficients for the Colorado Plateau and the Rio Grande Rift are similar to each other: a transparent upper crust and a heterogeneous lower crust (small and large scattering coefficients, respectively). However, the scattering coefficients beneath the Rio Grande Rift are several times larger than those beneath the Colorado Plateau. Depths of the lower crust and the Moho boundary beneath the Rio Grande Rift are shallower than those of the Colorado Plateau. From their geological settings and other geophysical results around the region, we infer that the larger scattering coefficients of the rift are associated with rift formation and volcanic activity, such as magma ascent from the upper mantle to the crust.

Cerro Toledo Rhyolite, Jemez Volcanic Field, New Mexico: 40Ar/39Ar geochronology of eruptions between two caldera-forming events

The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.

Plio-pleistocene pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA

At least four times during the late Pliocene and early Pleistocene pyroclastic eruptions in the Jemez volcanic field, northern Rio Grande rift, flooded the ancestral Rio Grande with gravel-sized pumice. Following as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Rio Grande rift. A combination of reversal magnetostratigraphy and single-crystal sanidine 40Ar/39Ar dating constrains the ages of pumice-clast conglomerates at 3.1, ∼2.0, 1.6, and 1.3 Ma. The coarsest pumice beds (cobbles, boulders) were deposited as antidune-like bedforms in a fluvial channel and as a crevasse-splay sheet. Granule and pebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clasts in the gravel fraction (60–100%) suggests very rapid transport downriver, probably in a few days or weeks. The two older pumice-clast conglomerates correlate with the Puye Formation in the Jemez volcanic field, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.

Overview of electromagnetic methods applied in active volcanic areas of western United States

Abstract A better understanding of active volcanic areas in the United States through electromagnetic geophysical studies received foundation from the many surveys done for geothermal exploration in the 1970's. Investigations by governmental, industrial, and academic agencies include (but are not limited to) mapping of the Cascades. Long Valley/Mono area, the Jemez volcanic field, Yellowstone Park, and an area in Colorado. For one example — Mt. Konocti in the Mayacamas Mountains, California — gravity, magnetic, and seismic, as well as electromagnetic methods have all been used in an attempt to gain a better understanding of the subsurface structure. In each of these volcanic regions, anomalous zones were mapped. When conductive, these anomalies were interpreted to be correlated with hydrothermal activity and not to represent a magma chamber. Electrical and electromagnetic geophysical methods can offer valuable information in the understanding of volcanoes by being the method which is most sensitive to change in temperature and, therefore, can best map heat budget and hydrological character to aid in prediction of eruptions.

Replenishment and crystallization in epicontinental silicic magma chambers: evidence from the Bandelier magmatic system

The Cerro Toledo Rhyolite (Jemez volcanic field, New Mexico) records the volcanism that occurred after the eruption of the Lower Bandelier Tuff ignimbrite, which formed Toledo caldera at 1.51 ± 0.03 Ma. Pumice samples from the top of this ignimbrite and from the base of the Cerro Toledo Rhyolite tephra sequence are stratigraphically and geochemically distinct from younger Cerro Toledo Rhyolite tephras that overlie this basal suite. All samples exhibit decreasing La/Lu with increasing Lu concentrations. For a given Lu content, however, the Lower Bandelier Tuff and basal member Cerro Toledo Rhyolites have higher La/Lu compared to overlying Cerro Toledo Rhyolites. This difference can be explained either by (1) differentiation of a single batch of magma in which trace mineral solubilities increased with time due to higher volatile contents, or (2) replenishment of the magma chamber by an input of silicic magma after caldera collapse at 1.51 Ma. Large-scale replenishment of epicontinental silicic magma chambers may be restricted to timescales of 10 5 years.

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

Abstract Seventeen K/Ar dates were obtained on illitic clays within Valles caldera (1.13 Ma) to investigate the impact of hydrothermal alteration on Quaternary to Precambrian intracaldera and pre-caldera rocks in a large, long-lived hydrothermal system (∼ 1.0 Ma to present). Clay samples came from scientific core hole VC-2B (295°C at 1762 m) which was spudded in the Sulphur Springs thermal area and drilled into the boundary between the central resurgent dome and the western ring-fracture zone. Six illitic clays within Quaternary caldera-fill debris flow, tuffaceous sediment, and ash-flow tuff (48 to 587 m depth) yield ages from 0.35 to 1.09 Ma. Illite from Miocene pre-caldera sandstone (765 m) gives an age of 6.74 Ma. Two dates on illite from sandstones in Permian red beds (1008 and 1187 m) are 4.33 and 4.07 Ma, respectively. Surprisingly, three dates on illites from altered andesite pebbles within the red beds (1010–1014 m) are 0.95 to 1.06 Ma. Four illite dates on variably altered Precambrian quartz monzonite (1615–1762 m) range from 2.90 to 276 Ma. Post-Valles age illite is not correlated with alteration style (argillic to propylitic). Rather, post-Valles ages are uniformly obtained from illites in highly fractured, intensely altered, caldera-fill rocks and the Permian volcanic clasts. Generally, finer clay fractions from identical samples yield younger ages. Plots of 40 Ar/ 36 Ar versus 40 K/ 36 Ar and 40 Ar∗ versus 40 K for the illites in caldera-fill rocks lie close to a 1-Ma isochron. Most illite dates older than Valles caldera are difficult to interpret because they correspond to the ages of pre-Valles volcanic and hydrothermal episodes in the Jemez volcanic field (⩽ 13 Ma). In addition, older dates may be caused by co-mingling of different illites during sample preparation, or by inherited argon or lost argon in illites from rocks with potentially complex hydrothermal histories. However, the range of ages obtained from illites in Permian sands and pebbles and from Precambrian crystalline rocks indicates that Valles hydrothermal activity is overwhelming illite produced by earlier geologic events.

Temporal patterns of tephra alterations in various geologic settings: The Jemez Volcanic Field and the adjacent Morrison Formation in New Mexico and Colorado

Authigenic mineral assemblages and temporal constraints of tephra layers altered by hydrothermal activities, subaerial weathering, and burial diagenesis are described. Clay-size assemblages (< 2 mm) from hydrothermally altered, Miocene to Quaternary volcanic rocks of the Jemez volcanic field are characterized by argillic minerals (kaolinite, quartz, smectite, alunite, pyrite, etc.) from acid-sulfate reactions and propylitic minerals (illite, chlorite, quartz, calcite, pyrite, etc.) that formed from neutral-chloride fluids. Unlike the hydrothermally altered rocks, the diagenetic minerals from the altered Jurassic tephra layers (bentonites) of the Morrison Formation are zonally distributed from a marginal smectite zone that grades laterally to zeolites, feldspars, and illite/smectite clays toward the center of the basin. In the Jenez volcanic field, the hydrothermal illite/smectites in core hole VC-2A range between 0.66 and 0.83 Ma for subsurface Plio-Pleistocene tephra deposits (1.14–1.78 Ma) and interbedded volcaniclastic units, whereas similar clays from altered Miocene rocks yield ages between 5.6 and 8.1 Ma. These clays formed during hydrothermal alterations related to the waning phases of the various volcanic episodes of the area. Clinoptilotites from the zeolitized tephra units from the Late Jurassic Morrison Formation gave discordant dates ranging from 154 to 83 Ma, whereas the illite/smectites from the bentonite beds of the same formation yielded two age groups (29.69–38.2 and 53.7–56 Ma). All the K Ar results were interpreted using the K Ar isochron method and in most cases the isochron ages are similar to the conventional K Ar results. The altered tephra layers provide diagnostic regional marker horizons in addition to recording concisely the time of diagenetic processes in various geologic settings.

Provenance Analysis of Obsidian from Two Late Prehistoric Archaeological Sites in Kansas

Six obsidian artifacts from two protohistoric archaeological sites in Kansas were subjected to non-destructive energy dispersive x-ray fluorescence analysis to determine the parent obsidian source used in their manufacture. Results indicate all six specimens were derived from two obsidian sources located in the Jemez volcanic field of northern New Mexico. Silicic volcanic glass (obsidian) are well known from various parts of the Far West, and for the last two decades x-ray fluorescence and neutron activation analysis have been employed by geochemists and archaeologists to determine the parent source of origin for artifacts recovered from prehistoric archaeological sites. Such research has been conducted intensively in California (Jack, 1976; Ericson, 1981; Hughes, 1986) and the Great Basin (Hughes, 1983, 1985; Hughes and Bennyhoff, 1986). Despite the relative paucity of obsidian samples, some important research along these same lines has been undertaken in the Midwest and Plains (Anderson et al., 1986; Frison et al., 1968; Griffin et al., 1969; Baugh and Nelson, 1987, 1988). Baugh and Nelson (1987) for example, have identified changes in the amounts and sources of obsidian used in several southern Plains archaeological sites, and have advanced hypotheses to account for observed changes. One of the more conspicuous gaps in current understandings about prehistoric obsidian conveyance in the Plains concerns Kansas archaeological sites. Although archaeological obsidian was first noted in Kansas over a century ago (Putnam, 1880), it wasn't until the systematic fieldwork of Waldo Wedel between 1937-1940 the presence of obsidian in the state became known to a wider audience of professional archaeologists. Despite the distance from sources of volcanic glass, a surprising amount of obsidian has been documented from Kansas sites. Wedel (1959, pp. 265, 314, 335, 367This content downloaded from 207.46.13.128 on Tue, 06 Sep 2016 06:08:34 UTC All use subject to http://about.jstor.org/terms VOLUME 94, NUMBERS 1-2 39 368, 411, 545), for example, reported more than 80 obsidian artifacts (mostly flakes) from seven sites in Kansas. Since then, obsidian has been a common, if low frequency, find on Kansas sites, particularly on Late Ceramic period (post AD 1500) sites associated with the Great Bend aspect (protohistoric Wichita) in Rice County. At least thirty years ago Wedel speculated obsidian from the central and northern Plains was obtained from Obsidian Cliffs, Wyoming, but that in sites from the Republican Valley southward is more likely from New Mexico or Colorado deposits (Wedel, 1961, p. 274; cf. 1959, p. 264). Although not using Kansas data, Baugh and Nelson (1987) recently observed prior to AD 1450 obsidian found in Southern Plains sites originated from in New Mexico, Idaho, and west-central Utah but after AD 1450, obsidian was derived exclusively from New Mexico. Baugh and Nelson (1987) thus postulated a major change in exchange relationships from north-south to an east-west orientation to account for this change in source use. The present study places both models of Wedel, and Baugh and Nelson to empirical test through analyses of a sample of artifacts from two archaeological sites in the Southern Plains in Marion and Comanche counties, Kansas (see Fig. 1). The two samples analyzed from Marion County come from the Mem site (14MN328), a Great Bend aspect (protohistoric Wichita) site excavated in 1986 in conjunction with a highway salvage project. As of this writing, these specimens are the only obsidian artifacts recovered from this, or any other, Great Bend site in Marion County (Lees et al., 1989; Rohn and Emerson, 1984). Sample 20599 was recovered from the upper levels of a large cache (storage) pit and was associated with European-made artifacts, one of which was a glass bead dated to around AD 1700 (Lees, 1990a). The other sample (10128) was recovered unassociated in the 0-30 cm level of the site. The four samples from Comanche County were excavated from the Booth site (14CM406) during 1989 research by the Kansas State Historical Society and the Kansas Anthropological Association (Lees and Reynolds, 1989). Obsidian made up about 5 percent of the entire chipped stone inventory at the Booth site, an unusually high percentage for Kansas and the Plains in general. Recent chronological information from the Booth site indicates a Late Ceramic period affiliation (suggesting contemporaneity with the Mem site), although the possibility of an earlier occupation during the late prehistoric Middle Ceramic period (AD 1000-1500) also exists. The identity of the Native American occupants of this site is, however, not currently known (Lees, 1990b). METHODS AND PROCEDURES Laboratory analyses on these samples were performed on a Spectrace 5000 (Tracor X-ray) energy dispersive x-ray fluorescence spectrometer This content downloaded from 207.46.13.128 on Tue, 06 Sep 2016 06:08:34 UTC All use subject to http://about.jstor.org/terms 40 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE Cerro KANSAS del Medio

THE VALLES/TOLEDO CALDERA COMPLEX, JEMEZ VOLCANIC FIELD, NEW MEXICO

Valles caldera is famous as an example of a resurgent caldera and for its high-temperature geothermal system of volcanic origin (Smith & Bailey 1968, Dondanville 1978). Although the caldera and its hydrothermal sys­ tem are the products of events that have occurred during the last 1.12 Myr, the Jemez volcanic field has experienced continuous volcanism for the last 13 Myr and is known to have had at least three major periods of hydrothermal activity (Gardner et al 1986, WoldeGabriel & Goff 1989). This paper does not review all aspects of research concerning the Jemez

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

Two types of hydrothermal mineral assemblages, kaolinite-alunite and sericite-chlorite were recognized in outcrop and core samples from the Jemez volcanic field, New Mexico. Active and fossil acid-sulfate hydrothermal activity, producing kaolinite-alunite alteration occurs along the caldera ring fracture and within the resurgent dome, whereas alterations of the chlorite-sericite type are mostly confined to subsurface rocks. K/Ar dates on illitic clays (<2 μm, 2-0.25 μm and <0.25 μm) separated from hydrothermally altered samples are used to document hydrothermal episodes in pre-caldera rocks of the Jemez volcanic field. The clay separates are illitic (75–95% illite layers) with K2O contents of 8.24–8.95% in the core samples and 4.22–6.67% in the outcrops. Three episodes of hydrothermal alteration are now recognized in the Jemez Mountains region based on K/Ar age dates of illites. The earliest event (≤17-11 Ma, n=4) occurred within altered and illite-rich (≥90% illite layers) lowermost Paleozoic rocks in core hole VC-1 and may correlate with the inception of volcanism in the Jemez region (≤16.5 Ma). Illitic clays from altered andesite and rhyolite from the topographic rim of the Valles caldera were dated at 8.2 and 6.96 Ma, suggesting that a hydrothermal event related to the waning stages of the Keres Group volcanism (13-5.8 Ma) was responsible for the alteration. These dates overlap minimum ages obtained on slightly altered basalt (8.05 Ma) and andesite lava (7.07 Ma) from the southern and northwestern topographic rim of the Valles caldera, respectively. Ages of 1.21 and 1.34 Ma from illite-rich (≥90% illite layers), hydrothermally altered Paleozoic sandstone at a depth of 479 m in VC-1 suggest hydrothermal activity contemporaneous with the formation of the Toledo (1.45 Ma) and Valles (1.12 Ma) calderas. Ore minerals of sphalerite, chalcopyrite, galena, barite, and molybdenite mineralization occur in the lower half of VC-1 and are related to the Toledo and Valles caldera-forming eruptions.