Photonic dating of prehistoric irrigation canals at Phoenix,Arizona, U.S.A.*

A number of archeological features, including in‐filled irrigation canals of uncertain prehistoric age, occur within the Holocene floodplain of the Salt River at Phoenix, Arizona. In the first attempt to date irrigation‐canal sediments using luminescence methods, we obtained age estimates of 1640 ± 190 yr B.P. (1σ) (multi‐aliquot or MA) and 1621 ± 95 yr B.P. (post‐IR single‐aliquot‐regenerative‐dose or SAR) for a single sample from the base of the oldest canal‐infilling deposits (all IR‐PSL ages reported in this article are in calendar years before A.D. 2001). For the remaining canal samples, weighted mean luminescence ages of 819 ± 45 yr (MA) and 826 ± 32 yr (post‐IR SAR) were obtained. Thus from photonic dating we can resolve the first and last phases of canal use at this Phoenix site: initiation at ca. 1600 years ago and final use at ca. 800 years ago. These results demonstrate the power of SAR luminescence sediment dating to enhance our understanding of prehistoric irrigation‐canal development and usage here and elsewhere in the world. © 2004 Wiley Periodicals, Inc.     

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     

Geochemical exploration for mineralized breccia pipes in northern Arizona,U.S.A.

Thousands of solution-collapse breccia pipe crop out in the canyons and on the plateaus of northern Arizona. Over 80 of these are known to contain U or Cu mineralized rock. The high-grade U ore associated with potentially economic concentrations of Ag, Pb, Zn, Cu, Co and Ni in some of these pipes has continued to stimulate mining and exploration activity in northern Arizona, despite periods of depressed U prices. Large expanses of northern Arizona are comprised of undissected high plateaus; recognition of pipes in these areas is particularly important because mining access to the plateaus is far better than to the canyons. The small size of the pipes, generally less than 600 ft (200 m) in diameter, and limited rock outcrop on the plateaus, compounds the recognition problem. Although the breccia pipes, which bottom in the Mississippian Redwall Limestone, are occasionally exposed on the plateaus as circular features, so are unmineralized near-surface collapse features that bottom in the Permian Kaibab and Toroweap Formations. The distinction between these two classes of circular features is critical during exploration for this unique type of U deposit.Various geochemical and geophysical exploration methods have been tested over these classes of collapse features. Because of the small size of the deposits, and the low-level geochemical signatures in the overlying rock that are rarely dispersed for distances in excess of several hundred feet, most reconnaissance geochemical surveys, such as hydrogeochemistry or stream sediment, will not delineete mineralized pipes.Several types of detailed geochemical surveys made over collapse features, located through examination of aerial photographs and later field mapping, have been successful at delineating collapse features from the surrounding host rock: (1) Rock geochemistry commonly shows low level Ag, As, Ba, Co, Cu, Ni, Pb, Se and Zn anomalies over mineralized breccia pipes; (2) Soil surveys appear to have the greatest potential for distinguishing mineralized breccia pipes from the surrounding terrane. Although the soil anomalies are only twice the background concentrations for most anomalous elements, traverses made over collapse features show consistent enrichment inside of the feature as compared to outside; (3) B. Cereus surveys over a known mineralized pipe show significantly more anomalous samples collected from within the ring fracture than from outside of the breccia pipe; (4) Helium soil-gas surveys were made over 7 collapse features with discouraging results from 5 of the 7 features.Geophysical surveys indicate that scaler audio-magnetotelluric (AMT) and E-field telluric profile data show diagnostic conductivity differences over mineralized pipes as compared to the surrounding terrane. These surveys, coupled with the geochemical surveys conducted as detailed studies over features mapped by field and aerial photograph examination, can be a significant asset in the selection of potential breccia pipes for drilling.     

Geochemical and isotopic characteristics of lower crustal xenoliths, San Francisco Volcanic Field, Arizona, U.S.A

A wide variety of xenoliths has been entrained in Miocene-to-Recent alkali olivine and hypersthene-normative basalts in the San Francisco Volcanic Field (SFVF), northern Arizona, U.S.A. Based on petrography, mineralogy, bulk rock chemistry and Sr-Nd isotopic characteristics, SFVF xenoliths can be divided into two major groups: cumulates and granulites. The cumulates are genetically related to the Cenozoic volcanic rocks and represent under- and/or intraplated additions to the crust of the Colorado Plateau. Assemblages are mafic to ultramafic and are dominated by clinopyroxene-orthopyroxene-plagioclase-spinel-amphibole-olivine. The granulites are probably Proterozoic in age, mafic-to-intermediate/felsic in bulk composition, either two pyroxene-plagioclase-spinel or plagioclase-alkali feldspar-quartz-magnetite-amphibole-biotite assemblages. Many of the granulites show evidence of partial melting. Some high SiO₂, very high Rb/Sr glasses are close in composition to erupted rhyolites, and probably represent end-member melts that have interacted with basalt to produce a variety of hybrid intermediate lavas. The major element, trace element and Sr-Nd isotope geochemistry is highly variable in the SFVF xenoliths. Extremely high Ba contents and Ba/Nb of a number of the granulites are equivalent to values characteristics of modern supra-subduction zone magmas. The considerable variation of chemical and isotopic composition depends upon mineral proportions, assemblages and chemistry. Isotopically, three end-members can be identified within the granulites: (i) lowest ⁸⁷Sr/⁸⁶Sr (0.702870) with low ¹⁴³Nd/¹⁴⁴Nd (0.511541, εNd-21.4); (ii) high ⁸⁷Sr/⁸⁶Sr (0.711069) with the lowest ¹⁴³Nd/¹⁴⁴Nd (0.511434, εNd-23.5); (iii) highest ⁸⁷Sr/{86}Sr (0.715306) with low ¹⁴³Nd/¹⁴⁴Nd (0.511793, εNd-16.5). Two important age ranges deduced from the isotopic data probably relate to episodes of crustal-growth beneath the SFVF (1.88 ± 0.33 Ga and Cenozoic). Thermobarometric calculations assuming equilibrium show that the xenoliths are derived from the lower crust (0.6–1.3 GPa, 850–1050 °C). The average SFVF lower crust is mafic in composition. In the absence of partial lithospheric delamination, the lower crust may become mafic with time due to under- and intraplating of continental crust by mafic magmas derived from the mantle.     

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.     

Iron deposition as acidic groundwater encounters carbonates in the alluvium of Pinal Creek,Arizona, U.S.A.

In a column experiment, acidic groundwater from Pinal Creek Arizona, a Cu mining area, was eluted through a composited alluvial sample obtained from a core that had been removed from a well downgradient of the acidic groundwater. The minerals present in typical grains and flakes in the alluvium before and after the elution were determined by X-ray diffraction (XRD), scanning electron microscopy, and energy dispersive multichannel analyses (EDX). The concentrations of Fe, Ti, Mn, Si, Al, Na, Ca, K, Mg and S in these grains and flakes and in their microcrystalline surface coatings were measured by EDX.In addition to magnetite, hematite, and Fe-Ti oxides, Fe was most concentrated in micas (especially biotite-like flakes) and in the microcrystalline coatings. The measured elements in these microcrystalline coatings were primarily K, Fe, Al, and Si. The microcrystalline coatings on the mica flakes also contained Mg. The approximate l:3 Mg:Si atomic ratios (ARs)_of the biotite-life flake both before and after the elution would suggest that the Fe deposited during the elution had not substituted for Mg in these flakes. As a result of the elution, assuming no loss of Si, the averaged recorded Fe:Si AR of the microcrystalline coatings increased from (0.46 to 0.58):3.00.Iron deposition on the typical grains and flakes may relate to the presence of Fe in the particle on which it is deposited or to the presence of Fe in the microcrystalline surface coatings before elution. The data here are not sufficient for a statistical evaluation, but elution caused the following trends: (l) The Fe:Si AR increased in the (K,Fe,Al,Si)-microcrystalline surface coatings; (2) For the mica flakes, there was more than a 2-fold increase in the Fe:Si AR for the microcrystalline surface coatings of the Fe-rich biotite-like flakes but no measurable increase of the Fe:Si AR for the microcrystalline surface coatings of the muscovite-like flakes that contained 3–5 times less Fe; (3) Also for the biotite-like flakes, the increase in Fe:Si AR was greater in the flakes that had a higher Fe:Si AR; (4) The Fe deposition on the Fe-rich microcrystalline surface coatings of the feldspar was much greater than on the Fe-poor, beige quartz and feldspar grains that, prior to elution, had only CaS0₄ microcrystalline coatings; and (5) No Fe was deposited on Fe-poor grains with no microcrystalline surface coating.     

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.