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     

Provenance of sand temper in Hohokam Ceramics,Arizona

Sand temper compositions of regionally distinct Hohokam pottery sherds were determined by detailed point counts using sedimentary petrographic methods. Different temper compositions from different sites were compared with maps of sand composition zones within the geographic range of the pottery in order to establish the probable provenance of each sherd. A number of probable instances of intraregional pottery exchange were identified. The larger number of sand sources in undecorated vs. decorated pottery at each site suggests that undecorated pottery was made in more places than decorated vessels, and may have been traded differently as well. Petrographic analysis of temper is a useful method for studying exchange of homogeneous pottery in geologically diverse areas, and for investigating prehistoric Hohokam interaction in the Tucson and Red Rock Basins on a scale not possible with traditional archaeological techniques.     

Groundwater modeling of the Silver Bell Mountains, Arizona, USA

This study examines the groundwater characteristics in the Silver Bell Mountains, Arizona, USA, using a numerical model. Groundwater modeling is developed to describe the flow pattern in the study area and subsequently explores the possible interaction with regional porphyry copper deposits. A conceptual model is developed for the study area and regional hydrogeological conditions are simulated using the finite-difference groundwater flow model, MODFLOW-2005. The model results show that groundwater flow in the Silver Bell Mountains is strongly influenced by topography and its velocity varies with depth. In addition, the numerical model supports the idea of a continuous sustained interaction between groundwater flow and porphyry copper deposits in the Silver Bell Mountains. This interaction may result in continuing leaching of trace elements from the ore deposit, an important implication for continuing supergene alteration and enrichment of the porphyry copper deposit.     

Organic geochemistry of silicified wood,Petrified Forest National Park,Arizona

Lignin derivatives have been isolated for the first time from carbonaceous sections of the silicified (~90% SiO₂) conifer Araucarioxylon arizonicum. The products released by sequential high vacuum pyrolysis and identified by combined gas chromatography-mass spectrometry (GC-MS) include a wide variety of alkyl-substituted, phenolic and condensed aromatic compounds.Brauns spruce lignin was pyrolized and analyzed by GC-MS as a comparison for the fossil wood data. The primary pyrolyzates at the 300°C step were CO₂, H₂O, ethanol and propanol. The main product at 450°C was 4-methyl-2-methoxyphenol (methyl guaiacol), but at 600°C the pyrolyzates were similar both in product composition and in relative abundance to those from silicified wood. The results suggest that the fossil wood experienced a mild thermal event during which the ether bonds were ruptured and loss of oxygen occurred along with the rearrangement of the original wood into a highly stable polymer.     

Origin of birefringence in andradite from Arizona, Madagascar, and Iran

The crystal structure of four birefringent andradite samples (two from Arizona, one from Madagascar, and one from Iran) was refined with the Rietveld method, space group $Ia\overline{3} d$ , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Each sample contains an assemblage of three different cubic phases. From the electron-microprobe (EMPA) results, fine-scale intergrowths in the Arizona-2 and Madagascar samples appear homogeneous with nearly identical compositions of {Ca_2.99Mg_0.01}_Σ3[ ${\text{Fe}}_{1.99}^{3 + }$ ${\text{Mn}}_{0.01}^{3 + }$ ]_Σ2(Si_2.95Al_0.03 ${\text{Fe}}_{0.02}^{3 + }$ )_Σ3O₁₂, Adr₉₈ (Arizona-2), and Adr₉₇ (Madagascar). Both samples are near-end-member andradite, ideally {Ca₃}[ ${\text{Fe}}_{2}^{3 + }$ ](Si₃)O₁₂, so cation ordering in the X, Y, or Z sites is not possible. Because of the large-scale intergrowths, the Arizona-1 and Iran samples contain three different compositions. Arizona-1 has compositions Adr₉₇ (phase-1), Adr₉₃Grs₄ (phase-2), and Adr₈₇Grs₁₁ (phase-3). Iran sample has compositions Adr₈₆Uv₁₂ (phase-1), Adr₆₉Uv₃₀ (phase-2), and Adr₇₆Uv₂₂ (phase-3). The crystal structure of the three phases within each sample was modeled quite well as indicated by the Rietveld refinement statistics of reduced χ² and overall R (F ²) values of, respectively, 1.980 and 0.0291 (Arizona-1); 1.091 and 0.0305 (Arizona-2); 1.362 and 0.0231 (Madagascar); and 1.681 and 0.0304 (Iran). The dominant phase for each sample has the following unit-cell parameters (Å) and weight fractions (%): a = 12.06314(1), 51.93(9) (Arizona-1); 12.04889(1), 52.47(1) (Arizona-2); 12.06276(1), 52.21(8) (Madagascar); and 12.05962(2), 63.3(1) (Iran). For these dominant phases, the distances and site occupancy factors (sofs) in terms of neutral atoms at the Ca(X), Fe(Y), and Si(Z) sites are as follows: <Ca–O> = 2.4348, Fe–O = 2.0121(6), Si–O = 1.6508(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.930(2), and Si(sof) = 0.917(2) (Arizona-1); <Ca–O> = 2.4288, Fe–O = 2.0148(7), Si–O = 1.6476(7) Å; Ca(sof) = 0.953(2), Fe(sof) = 0.891(2), and Si(sof) = 0.927(2) (Arizona-2); <Ca–O> = 2.4319, Fe–O = 2.0220(6), Si–O = 1.6460(6) Å; Ca(sof) = 0.955(2), Fe(sof) = 0.941(2), and Si(sof) = 0.939(2) (Madagascar); and <Ca–O> = 2.4344, Fe–O = 2.0156(8), Si–O = 1.6468(8) Å; Ca(sof) = 0.928(2), Fe(sof) = 0.908(2), and Si(sof) = 0.932(3) (Iran). The sofs based on the EMPA results are similar to those obtained from the Rietveld refinement. Each phase in the HRPXRD results can be correlated with a specific chemical composition. For example, the Iran sample composition Adr₆₃Uv₃₀ corresponds to phase-3 that has the smallest unit-cell parameter; Adr₇₆Uv₂₂ corresponds to phase-1 that has the intermediate cell value; and Adr₈₆Uv₁₃ corresponds to phase-2 that has the largest unit-cell parameter. The bond distances compare well with those obtained from radii sum. The three different cubic phases in each sample cause strain that arises from the mismatch of the cubic unit-cell parameters and give rise to birefringence.     

Early agriculture in the Eastern Grand Canyon of Arizona,USA

Abandoned fields in Colorado River alluvium in the eastern Grand Canyon show signs of primitive agriculture. Presence of maize pollen in association with buried soils near Comanche Creek suggests that farming began prior to 3130 yr B.P. Cotton pollen, identified in buried soils near Nankoweap Creek, dates to 1310 yr B.P., approximately 500 years earlier than previously reported anywhere on the Colorado Plateau. Farming spanned three millennia in this reach of the canyon. Entrenchment, starting approximately 700 yr B.P., making water diversion to fields infeasible, was likely responsible for field abandonment. © 2000 John Wiley & Sons, Inc.     

Facies architecture of a Lower Cretaceous coral-rudist patch reef, Arizona

The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontia initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework.Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous faunal transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks.     

Noble gases in ultramafic xenoliths from San Carlos,Arizona

This work reports the results of noble gas (Ne, Ar, Kr, Xe) analyses of accidental mantle xenoliths from San Carlos, Arizona. Except for the addition of radiogenic ⁴⁰Ar and mass fractionation effects, the isotopic structures of these gases are indistinguishable from atmospheric composition. The absence of ¹²⁹Xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths. The dominant influence on the noble gas abundances in the San Carlos xenoliths appears to have been diffusive gas loss, which may have occurred in a mantle metamorphic event or during contact with the host basanite magma. Evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals; the proportion of each gas in the inclusions increases with increasing atomic weight of the gas, possibly reflecting solubility effects. The noble gases are present in greater concentration in pyroxenes than in olivine, similar to the behavior of other incompatible elements.     

Data fusion and porphyry copper prospectivity models,southeastern Arizona

Mineral exploration programs commonly use a combination of geological, geophysical and remotely sensed data to detect sets of optimal conditions for potential ore deposits. Prospectivity mapping techniques can integrate and analyse these digital geological data sets to produce maps that identify where optimal conditions converge. Three prospectivity mapping techniques – weights of evidence, fuzzy logic and a combination of these two methods – were applied to a 32,000 km² study area within the southeastern Arizona porphyry Cu district and then assessed based on their ability to identify new and existing areas of high mineral prospectivity. Validity testing revealed that the fuzzy logic method using membership values based on an exploration model identified known Cu deposits considerably better than those that relied solely on weights of evidence, and slightly better than those that used a combination of weights of evidence and fuzzy logic. This led to the selection of the prospectivity map created using the fuzzy logic method with membership values based on an exploration model. Three case study areas were identified that comprise many critical geological and geophysical characteristics favourable to hosting porphyry Cu mineralisation, but not associated with known mining or exploration activity. Detailed analysis of each case study has been performed to promote these areas as potential targets and to demonstrate the ability of prospectivity modelling techniques as useful tools in mineral exploration programs.     

A middle Wisconsin pollen record from Hay Lake,Arizona

The lower part of a 5-m core from Hay Lake (34°N, 109° 25′W) at 2780 m in east-central Arizona provides a pollen record for the middle Wisconsin. Identification of fossil pines is based on a key modified from Hansen and Cushing (1973, Geological Society of America Bulletin84, 1181–1200). Pinus edulis and P. monophylla are similar in size and morphology but are significantly different from P. flexilis. Haploxylon pines dominate the pollen record. The abundance of pinyon pines during the middle Wisconsin is interpreted as indicating that this group was widespread at lower elevations. The local vegetation was mixed conifer forest consisting of Picea, P. aristata, P. flexilis and/or P. strobiformis, and with P. ponderosa and/or P. contorta after about 26,000 yr B.P. Tree line was above the elevation of Hay Lake. The middle Wisconsin climate is inferred to have been cooler than today and is marked by more available moisture that permitted pinyon pines to grow at low elevations.     

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.     

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.     

Geochemical soil sampling for deeply-buried mineralized breccia pipes,northwestern Arizona

Thousands of solution-collapse breccia pipes crop out in the canyons and on the plateaus of northwestern Arizona; some host high-grade uranium deposits. The mineralized pipes are enriched in Ag, As, Ba, Co, Cu, Mo, Ni, Pb, Sb, Se, V and Zn. These breccia pipes formed as sedimentary strata collapsed into solution caverns within the underlying Mississippian Redwall Limestone. A typical pipe is approximately 100 m (300 ft) in diameter and extends upward from the Redwall Limestone as much as 1000 m (3000 ft).Unmineralized gypsum and limestone collapses rooted in the Lower Permian Kaibab Limestone or Toroweap Formation also occur throughout this area. Hence, development of geochemical tools that can distinguish these unmineralized collapse structures, as well as unmineralized breccia pipes, from mineralized breccia pipes could significantly reduce drilling costs for these orebodies commonly buried 300–360 m (1000–1200 ft) below the plateau surface.Design and interpretation of soil sampling surveys over breccia pipes are plagued with several complications. (1) The plateau-capping Kaibab Limestone and Moenkopi Formation are made up of diverse lithologies. Thus, because different breccia pipes are capped by different lithologies, each pipe needs to be treated as a separate geochemical survey with its own background samples. (2) Ascertaining true background is difficult because of uncertainties in locations of poorly-exposed collapse cones and ring fracture zones that surround the pipes.Soil geochemical surveys were completed on 50 collapse structures, three of which are known mineralized breccia pipes. Each collapse structure was treated as an independent geochemical survey. Geochemical data from each collapse feature were plotted on single-element geochemical maps and processed by multivariate factor analysis. To contrast the results between geochemical surveys (collapse structures), a means of quantifying the anomalousness of elements at each site was developed. This degree of anomalousness, named the “correlation value”, was used to rank collapse features by their potential to overlie a deeply-buried mineralized breccia pipe.Soil geochemical results from the three mineralized breccia pipes (the only three of the 50 that had previously been drilled) show that: (1) Soils above the SBF pipe contain significant enrichment of Ag, Al, As, Ba, Ga, K, La, Mo, Nd, Ni, Pb, Sc, Th, U and Zn, and depletion in Ca, Mg and Sr, in contrast to soils outside the topographic and structural rim; (2) Soils over the inner treeless zone of the Canyon pipe show Mo and Pb enrichment anf As and Ga depletion, in contrast to soils from the surrounding forest; and (3) The soil survey of the Mohawk Canyon pipe was a failure because of the rocky terrane and lack of a B soil horizon, or because the pipe plunges. At least 11 of the 47 other collapse structures studied contain anomalous soil enrichments similar to the SBF uranium ore-bearing pipe, and thus have good potential as exploration targets for uranium. One of these 11, #1102, does contain surface mineralized rock. These surveys suggest that soil geochemical sampling is a useful tool for the recognition of many collapse structures with underlying ore-bearing breccia pipes.