Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 × 10⁹ m³ of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 × 10⁵ m³ s⁻¹ for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 × 10⁴ m³ s⁻¹) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 × 10⁴ m³ s⁻¹). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>10⁵ m³ s⁻¹) known worldwide and in the top ten largest floods in North America.     

Genesis and evolution of low-Al orthopyroxene in spinel peridotite xenoliths, Grand Canyon field, Arizona, USA

Orthopyroxene porphyroblasts zoned to interiors abnormally low in Al and Cr and containing numerous inclusions of olivine occur in some spinel peridotite xenoliths from the Colorado Plateau. Rims of these orthopyroxene grains contain 2.5–3.0 wt% Al₂O₃, consistent with equilibration in spinel peridotite at temperatures near 850 °C, but interiors contain as little as 0.20 wt% Al₂O₃ and 0.04 wt% Cr₂O₃. The Al-poor compositions are inferred to have equilibrated in chlorite peridotite, before porphyroblast growth during heating and consequent reactions that eliminated talc, tremolite, and chlorite. The distinctive orthopyroxene textures are inferred to have formed during reaction of talc and olivine. Rare intergrowths of orthopyroxene plus diopside are attributed to olivine-tremolite reaction. Al and Cr have gradients at grain rims that appear little modified by diffusion, but divalent elements are almost homogeneous throughout the porphyroblasts. Judging from the relative gradients, diffusion of Ca was at least 100 times faster than that of Al and Cr at the temperatures near and below 850 °C. Diffusion of Al and Cr was most effective along subgrain boundaries, and along these boundaries it appears to have been at least ten times faster than within the lattice: diffusion along such boundaries may be a dominant mechanism for re-equilibration of orthopyroxene at low mantle temperatures. Orthopyroxene with similar low Al and Cr occurs in chlorite peridotite xenoliths from the Navajo field, 300 km east of the Grand Canyon localities, and in spinel peridotite xenoliths from the Sierra Nevada, 500 km west across the extended Basin and Range province. Chlorite peridotite may therefore have been a significant minor component in much of the mantle lithosphere of western North America, although evidence for it would be erased at the higher temperatures recorded by xenoliths from the Basin and Range. Chemical changes during hydration may have been important in the evolution of these mantle volumes, and the case for addition of Sr is particularly strong. Dehydration reactions during heating could have influenced patterns of extension and crustal magmatism. Received: 1 July 1996 / Accepted: 2 December 1996     

Volatiles in amphiboles from xenoliths,Vulcan's Throne,Grand Canyon,Arizona, USA

Analyses of major element and volatile components of amphiboles from Vulcan's Throne, a Recent volcano on the north rim of the Grand Canyon, Arizona, USA, have been performed by using the electron microprobe and high temperature mass spectrometry. The amphiboles occur as megacrysts, as oikocrysts in peridotite and pyroxenite xenoliths, in amphibole-rich selvages on lherzolite xenoliths, and as grains in hornblendite xenoliths. Total volatiles range from 1.27 to 1.75 wt.%. In all samples, H₂O is the principal volatile species. Lesser amounts of structurally bound fluorine, chlorine, and oxygen were also released. The amphiboles studied are hydroxyl-deficient. The O(3) site is probably partially occupied by O^2?, which was detected as O₂ during degassing of the amphibole. Ti shows a strong positive correlation with the amount of hydroxyl deficiency in the amphiboles except for one oxidized sample. Thus, Ti probably is significant in charge balancing the substitution of O^2? for OH^? and the substitution probably occurred during crystallization rather than by dehydrogenation. Small amounts of both oxidized and reduced carbon and sulfur-bearing volatile species (e.g., CO₂, CO, CH₄, SO₂, H₂S) were detected in all samples. The observation of reduced carbon species supports the hypothesis that the oxygen fugacity of at least portions of the upper mantle is probably less than the quartz-fayalite-magnetite buffer.     

Review: The distribution,flow, and quality of Grand Canyon Springs,Arizona (USA)

An understanding of the hydrogeology of Grand Canyon National Park (GRCA) in northern Arizona, USA, is critical for future resource protection. The ~750 springs in GRCA provide both perennial and seasonal flow to numerous desert streams, drinking water to wildlife and visitors in an otherwise arid environment, and habitat for rare, endemic and threatened species. Spring behavior and flow patterns represent local and regional patterns in aquifer recharge, reflect the geologic structure and stratigraphy, and are indicators of the overall biotic health of the canyon. These springs, however, are subject to pressures from water supply development, changes in recharge from forest fires and other land management activities, and potential contamination. Roaring Springs is the sole water supply for residents and visitors (>6 million/year), and all springs support valuable riparian habitats with very high species diversity. Most springs flow from the karstic Redwall-Muav aquifer and show seasonal patterns in flow and water chemistry indicative of variable aquifer porosities, including conduit flow. They have Ca/Mg-HCO₃ dominated chemistry and trace elements consistent with nearby deep wells drilled into the Redwall-Muav aquifer. Tracer techniques and water-age dating indicate a wide range of residence times for many springs, supporting the concept of multiple porosities. A perched aquifer produces small springs which issue from the contacts between sandstone and shale units, with variable groundwater residence times. Stable isotope data suggest both an elevational and seasonal difference in recharge between North and South Rim springs. This review highlights the complex nature of the groundwater system.     

Kaersutite-peridotite inclusions and kindred megacrysts in basanitic lavas,Grand Canyon,Arizona

Essentially two types of ultramafic inclusions occur in the basanitic lavas and ejecta deposits of the northwestern Grand Canyon, Arizona. Abundant, olivine-rich nodules contain an emerald green, chrome-rich diopside and chrome-rich spinels. A much less common group of inclusions generally containing poikilitic kaersutite have more variable modal compositions, more variable but iron-rich and chrome-poor mineral compositions, and are characterized by the presence of a titaniferous clinopyroxene which appears black in hand specimen. The nature and petrologic significance of these black clinopyroxene-bearing inclusions, together with megacrysts of kaersutite and black clinopyroxene, are discussed in this paper.Petrographic aspects indicate an origin as cumulates of fractionating basaltic magma. Compositions of pyroxenes suggest high pressures of crystallization. The co-precipitation of orthopyroxene, clinopyroxene, olivine and Mg-spinel from what in all probability was under-saturated magma, together with the total absence of feldspar as a cumulate or intercumulate phase, is compatible with crystallization near 10 kb, on the basis of quite limited experimental data on anhydrous basaltic compositions. Pressures of this sort are attained at depths close to the mantle-crust boundary in the western Grand Canyon. By way of comparison, cumulate-textured inclusions from central Nevada containing rare orthopyroxene, widespread plagioclase, and more Fe-enriched clinopyroxenes, kaersutites, olivines and spinels are postulated to have crystallized at lower temperatures (or at a more advanced stage of fractionation) and possibly at lower pressures.Numerous occurrences, worldwide, of kaersutite-bearing inclusions, always in undersaturated host rocks, have recently been reported. Compositionally, the kaersutites are quite uniform, whether coexistent with pyropic garnet-clinopyroxene (Kakanui, New Zealand), with ortho-pyroxene-clinopyroxene-olivine-Mg spinel (Grand Canyon), or with plagioclase-clinopyroxene-olivine-magnetite. The last assemblage is found in shallow-seated igneous bodies of alkalic, mafic composition, as well as in inclusions within basaltic rocks. These occurrences imply the precipitation of kaersutite amphibole over a broad range of pressures, and as high as those prevailing in the upper mantle.     

The last 30,000 years of faunal history within the Grand Canyon, Arizona

The vertebrate fauna of the last 30,000 radiocarbon years in the Grand Canyon is reviewed. Faunas accompanied with 92 ¹⁴C dates have been analyzed from nine cave sites (four systematically excavated) and 50 packrat middens. Reasonably precise chronological and environmental data of late Pleistocene and Holocene age were obtained through dung studies in Rampart, Muav, and Stanton's Caves; from the numerous packrat middens; and from a ringtail refuse deposit in Vulture Cave. The desert tortoise, 8 species of lizards, 12 species of snakes, 68 species of birds, and 33 species of mammals are identified. Extinct animals include the avian carrion feeder, Teratornis merriami, and the mammalian herbivores, Oreamnos harringtoni, Camelops cf. hesternus, Equus sp., and Nothrotheriops shastense. There is no apparent abrupt end to the late Pleistocene as observed in the Grand Canyon fossil faunal or floral record. Animal and plant taxa of the Grand Canyon responded individually to the changes in climate of the last 30,000 yr. Both animal and plant fossil assemblages indicate that a pre-full glacial, a full glacial, and a late glacial woodland community with many less dominant desert taxa were slowly replaced by a Holocene desert community. All woodland taxa were absent from the lower elevations of the Grand Canyon by 8500 yr B.P.     

Oxygen isotope composition of annually banded modern and mid-Holocene travertine and evidence of paleomonsoon floods, Grand Canyon, Arizona, USA

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ¹⁸O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is 0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ¹⁸O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.     

Amphibole-bearing Cumulate Inclusions, Grand Canyon, Arizona and their bearing on Silica-Undersaturated Hydrous Magmas in the Upper Mantle

Rare inclusions in Holocene basanite within the western GrandCanyon are comprised of poikilitic titaniferous amphibole togetherwith variable proportions of relatively Fe-rich clinopyroxene,orthopyroxene, olivine, Cr-poor spinel, and pyropic garnet,magnesian ilmenite, and titaniferous phlogopite. No feldsparhas been found in the 219 inclusions investigated. Availableexperimental data suggest crystallization at approximately 20kb (65 km depth) in a region where the crust is 30–40km thick. On the basis of their fabric, the inclusions appear to representcumulates, but other modes of origin cannot be completely ruledout. Anhydrous grains, including some considered to be postcumulusprecipitates, experienced extensive resorption into the interstitialhydrous melt before it ultimately crystallized, perhaps 100?C below liquidus temperatures, as the poikilitic amphibole.In spite of these crystal-melt reactions, and some probablesubsolidus recrystallization as well, systematic variationsin cumulus phase compositions exist and indicate one main precipitationsequence was ol+sp, ol+sp+cpx, cpx+cpx+sp, cpx+sp, cpx+sp+ilm.The local pyropic garnet appears postcumulus in the last threecumulus assemblages. The igneous bodies represented by the inclusions comprised arelatively small portion of the upper mantle sampled by theascending basanitic magma. But in contrast to the thin amphibole-bearingveins in the mantle-derived massif at Etang de Lherz, the igneousbodies beneath the Grand Canyon are considered to be substantiallylarger in dimension, on the order of at least meters ratherthan a few centimeters. Primary nephelinite-basanite melts produced by variable butsmall degrees of partial melting of hydrous upper mantle arenot represented by the poikilitic amphiboles because of complexprocesses at the site of emplacement, including reactions betweenmelt and chromian-spinel peridotite wall rocks.     

U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona

Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain. Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite data from the third sample exhibit dispersion in both the ²⁰⁷Pb/²⁰⁶Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by incomplete dissolution or excess (thorogenic) ²⁰⁶Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the ²⁰⁷Pb/²⁰⁶Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes. Received: 9 June 1996 / Accepted: 18 October 1996     

Thermal history of the Permian formations from the Breccia Pipes area (Grand Canyon region,Arizona)

The thermal history of the Permian formations from the Breccia Pipes area in the Colorado Plateau has been assessed using different methods. Burial reconstruction indicates maximum temperatures between 100 and 140 °C for a 30 °C/km geothermal gradient. A temperature of 140±5 °C integrated over 10 to 50 Ma. has been deduced from fission track analysis. This range of temperatures is consistent with trapping temperatures obtained by fluid inclusions study (75–140 °C). Fission track and burial hypotheses have been introduced in a kinetic model for organic matter maturation. The computed Tmax average 445 and 421 °C respectively. A comparison with organic geochemistry data from the Permian Toroweap formation is carried out and demonstrates that the two above hypotheses constitute a thermal framework for the organic matter evolution. Main conclusions referring to beginning of oil window and regional uplift ages (Laramide orogeny) as well as maximum temperature ranges are deduced from this study.

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Zusammenfassung Die thermal bedingte Entwicklung permischer Schichten in der »Breccia Pipes« Region der Colorado-Hochebene wurde mittels dreier Geothermometer rekonstruiert. Die Rekonstruktion der Ablagerungsgeschichte deutet auf maximale Temperaturen zwischen 100 ° und 140 °C hin. Eine Spaltspuren-Analyse erbrachte als Ergebnis eine maximale Temperatur von 137 °C, die über 50 Ma. integriert wurde. Diese Ergebnisse stimmen mit der Minimum-Temperatur überein, die mit Hilfe von Flüssigkeitseinschlüssen (75°–140 °C) errechnet wurden. Die Hypothesen der Spaltspuren-Analyse und der Ablagerungsgeschichte wurden in ein kinetisches Modell der Reifung von organischen Substanzen miteinbezogen. Die errechneten Durchschnitts-temperaturen liegen bei 445° und 421 °C. Ein Vergleich mit Daten der organischen Geochemie aus der permischen »Toroweap« Formation zeigt, daß die zwei oben genannten Hypothesen es erlauben, thermale Rahmenbedingungen für die Entwicklung organischer Substanzen festzusetzen. Die wichtigsten Schlüsse, die aus dieser Studie gezogen werden können sind das Alter des Beginns der Ölbildungsphase und der regionalen Hebungsphase (Laramid-Orogenese) sowie die Spannweite der maximalen Temperaturen.

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Résumé L'histoire thermique des formations permiennes de la région des Breccia Pipes dans le plateau du Colorado a été reconstituée grâce à l'utilisation de méthodes différentes. Les modélisations géologiques indiquent des températures maximales comprises entre 100 et 140 °C pour un gradient géothermique de 30 °C/km. Une température de 140±5 °C intégrée sur une période comprise entre 10 et 50 Ma est déduite de l'analyse des traces de fission. Ces résultats sont compatibles avec les températures de piégeage obtenues lors de l'étude de inclusions fluides (75–140 °C). Les histoires thermiques découlant de l'analyse des traces de fission et des reconstitutions géologiques ont été introduites dans un modèle cinétique d'évolution de la matière organique. Les Tmax calculées sont, respectivement, de 445 et 421 °C. Une comparaison avec les caractéristiques des kérogènes de la formation du Toroweap (Permien) démontre que ces hypothèses fixent les limites inférieure et supérieure de la maturation de la matière organique. Les conclusions principales de cette étude permettent de déterminer les âges correspondant au début de la fenêtre à huile et à la principale phase orogénique régionale (Laramienne) ainsi que les différentes températures associées.

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«Breccia Pipes» . 100140°. 137°, 50 . , , 75–140°. . , , 445 421 °?. «Toroweap» , , , . , — Laramid' — .

     

Patterns in the Landscape and Erosion of Cultural Sites Along the Colorado River Corridor in Grand Canyon,USA

The geologic and geomorphic template of Grand Canyon influences patterns in the archaeological record, including sites where apparent increases in erosion may be related to Glen Canyon Dam. To provide geoarchaeological context for the Colorado River corridor and such issues, we explore first‐order trends in a database of field observations and topographic metrics from 227 cultural sites. The patterns revealed may be expected in other river‐canyon settings of management concern. The spatial clustering of sites along the river follows variations in width of the valley bottom and the occurrence of alluvial terraces and debris fans, linking to bedrock controls. In contrast, the pattern of more Formative (Ancestral Puebloan, 800–1250 A.D.) sites in eastern Grand Canyon and Protohistoric (1250–1776 A.D.) sites in western Grand Canyon does not follow any evident geomorphic trends. In terms of site stability, wider reaches with more terrace and debris fan landforms host a disproportionate number of sites with acute erosion. This links most directly to weak alluvial substrates, and the primary erosion process is gullying with diffusive‐creep processes also pervasive. Although Glen Canyon Dam does not directly influence these erosion processes, overall sediment depletion and the loss of major flooding leaves erosion unhampered along the river corridor.     

The Grand Canyon of the Colorado

The Grand Canyon is not only one of the greatest erosional features on Earth, it also reveals one of the finest stratigraphic records anywhere.     

雅鲁藏布江大峡谷的形成

通过对雅鲁藏布大峡谷流域地貌形成响应时间域的定量估算,大峡谷与上游河道特征的对比,以及大峡谷入口处河湖阶地的沉积分析和定年研究,结合构造研究的新进展和数值地貌分析成果,系统论证了雅鲁藏布大峡谷的形成。研究结果表明,现今的雅鲁藏布大峡谷与大峡谷上游的河道在大峡谷形成之前分属不同的河流体系,大约在距今30ka BP前后,原属于帕隆藏布江水系支流的扎曲—直白河段因溯源侵蚀,袭夺了位于现今直白河段上游的古雅鲁藏布江水系,使得此前向南经南伊沟（纳伊普曲）流出高原的古雅鲁藏布江与帕隆藏布江合二为一,雅鲁藏布大峡谷得以贯通和强烈的侵蚀下切,形成现今著名的大峡谷和大拐弯式样的流域结构。     

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.     

U-Pb monazite, xenotime and titanite geochronological constraints on the prograde to post-peak metamorphic thermal history of Paleoproterozoic migmatites from the Grand Canyon, Arizona

The petrology and U-Pb geochronology of pelitic migmatite and calc-silicate gneiss reveal a detailed prograde to post-peak metamorphic thermal history for a single outcrop of Paleoproterozoic supracrustal rocks in the eastern part of the Grand Canyon. Metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 Ma and continued to grow until about 1697 Ma. The U-Pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting, which involved the breakdown of muscovite to sillimanite, commenced at about 1702 Ma, prior to the metamorphic peak. Partial melting continued until about 1690 Ma, the youngest U-Pb date from magmatic monazite in the leucosomes. Field and petrographic evidence, as well as inheritance patterns in monazites from the leucosomes, suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region. Between 1702 and 1690 Ma, the migmatite package heated to peak metamorphic conditions of about 720 °C and 6 kbar, cooled to about 675 °C at a cooling rate >30 °C/million years, and decompressed to about 4 kbar. The U-Pb geochronological data for metamorphic titanite from a calc-silicate gneiss exhibit a clear relationship between grain size and the ²⁰⁷Pb/²⁰⁶Pb date indicating that the titanite crystals record cooling ages. These data, combined with the titanite Pb diffusion data of Cherniak (1993), yield a cooling rate of 5.4_−0.9 ^+1.7 °C/million years, integrated over the interval 1690 to 1676 Ma and suggest that by 1675 Ma, the cooling rate slowed to less than 2 °C/million years. The rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large-scale tectonic processes associated with the accretion of juvenile crust to the margin of Laurentia. Juvenile arc crust appears to have been assembled, accreted and stabilized into Laurentian lithosphere in less than 30 million years. Received: 21 January 1998 / Accepted: 27 August 1998     

Strontium isotopic evidence for prehistoric transport of gray‐ware ceramic materials in the eastern Grand Canyon region,USA

Through a novel application of strontium (Sr) isotopic analysis, we evaluate geological sources for prehistoric ceramics in the eastern Grand Canyon region of northern Arizona, focusing on two gray‐ware traditions in the Upper Basin of the Coconino Plateau. Building on a conceptual framework for the general potential of Sr isotopes in the analysis of geological materials, we suggest that the eastern Grand Canyon is specifically well suited archaeologically and geologically for: (1) exploring the utility of Sr isotopes for ceramic provenance research and (2) testing long‐standing hypotheses that gray‐ware ceramics were invariably made with local materials. Sr isotopic compositions indicate that the ceramic samples represent at least three different geological sources, and that different raw materials were used in the manufacture of the two gray‐ware traditions found in the Upper Basin. One of the gray‐ware traditions is not compositionally consistent with local geology, indicating that either the ceramics or the raw materials were transported at least 20 km to the Upper Basin. © 2011 Wiley Periodicals, Inc.     

Grand Canyon caves, breccia pipes and mineral deposits

The Redwall Limestone of the Grand Canyon forms a major mineral district, its breccia pipes being host to rich uranium and base-metal deposits. Yet karst processes as a mechanism for preparing host rocks for ore deposition have been little studied.     

Causes of sinks near Tucson, Arizona, USA

 Land subsidence in the form of sinks has occurred on and near farmlands near Tucson, Pima County, Arizona, USA. The sinks occur in alluvial deposits along the flood plain of the Santa Cruz River, and have made farmlands dangerous and unsuitable for farming. More than 1700 sinks are confined to the flood plain of the Santa Cruz River and are grouped along two north-northwestward-trending bands that are approximately parallel to the river and other flood-plain drainages. An estimated 17,000 m³ of sediment have been removed in the formation of the sinks. Thirteen trenches were dug to depths of 4–6 m to characterize near-surface sediments in sink and nonsink areas. Sediments below about 2 m included a large percentage of dispersive clays in sink areas. Sediments in nonsink areas contain a large component of medium- to coarse-grained, moderately to well sorted sand that probably fills a paleochannel. Electromagnetic surveys support the association of silts and clays in sink areas that are highly electrically conductive relative to sand in nonsink areas. Sinks probably are caused by the near-surface process of subsurface erosion of dispersive sediments along pre-existing cracks in predominantly silt and clay sediments. The pre-existing cracks probably result from desiccation or tension that developed during periods of water-table decline and channel incision during the past 100 years or in earlier periods. Submitted, April 1997 · Revised, November 1997 · Accepted, April 1998     

Groundwater arsenic in the Verde Valley in central Arizona,USA

Forty-one water samples were collected and analyzed from throughout the Verde Valley watershed to identify the source of As in well water used for domestic and agricultural purposes. Each water sample was analyzed for anions, cations and trace chemical constituents by atomic absorption spectroscopy, anion chromatography and traditional wet chemical procedures. Arsenic concentrations ranged from 10 to 210 μg/l, with the highest values observed for water pooled on tailings from an abandoned Cu mine. Geostatistical analysis of the data revealed the primary source of As to be groundwater in contact with the Supai and Verde formations, as opposed to runoff from the abandoned mine tailings. Montezuma Well, a collapsed travertine spring, contained the highest levels of naturally occurring As (> 100 μg/l). Arsenic in Montezuma Well water was shown to be 100% arsenate. X-ray absorbance near edge spectra (XANES) of Potomogeton illinoiensis, an endemic plant species of Montezuma Well, demonstrate that As is absorbed as arsenate, reduced to arsenite in the plant and retained as an organic glutathione complex. XANES spectra of Montezuma Well sediments show 4 forms of As present: arsenate (∼54%), As(III)-glutathione complex (∼32%) and an As-organic complex (∼14%) containing dimethylarsinic acid and arsenobetaine. This is the first report of As(III)-glutathione in sediments.