• Spatial Ability Development in the Geosciences

      Hall-Wallace, Michelle; Baldwin, Tammy Katherine; Hall-Wallace, Michelle; Wallace, Terry C.; Butler, Robert; Baldwin, Tammy Katherine (The University of Arizona., 2003)
      We designed an experiment to evaluate change in students' spatial skills as a result of specific interventions. Our test subjects included high school students in earth science classes, college level non-science majors enrolled in large enrollment introductory geoscience courses and introductory level geoscience students. All students completed spatial tests to measure their ability to mentally rotate three-dimensional objects and to construct a three-dimensional object from a two-dimensional representation. Results show a steady improvement in spatial skills for all groups. They also indicate that students choosing science majors typically have much higher spatial skills as they enter college. Specific interventions to improve spatial skills included having a subgroup of the non-science majors and high school students complete a suite of Geographic Information System (GIS) activities. The intervention at the high school level was more extensive and resulted in significant improvements in both categories of spatial ability. At the college level, the non-science majors that received the intervention showed no significant difference from those that did not, probably because the time spent on the intervention was too short. The geoscience majors had nearly three times the improvement of non-science majors in both categories of spatial ability attributed to hands-on weekly laboratory experiences. These results reveal a wide range of abilities among all groups of students, and suggest that we evaluate teaching strategies in all courses to ensure that students can interpret and understand the visual imagery used in lectures.
    • Hillside Gullies and Possible Glacial Landforms Associated with the Degradation of Highland Craters on Mars

      Berman, Daniel Craig; Baker, Victor R.; Berman, Daniel Craig (The University of Arizona., 2003)
      The discovery of recent water and ice related landforms on Mars has led to a new understanding of the planet. Hundreds of examples of gullies and tongue-shaped ridges have been found on hillsides in images from the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). I conducted a thorough survey of these images and compiled a database of those containing gullies and tongue-shaped ridges, which may form as the result of flow of ice-rich materials. These features are found in similar mid-latitude bands. Crater counts on the ice flow features show estimated ages of the order 10⁶ y to 10⁷ y, similar to the timescale calculated for the last episode of high obliquity, when Models suggest ice deposition at these latitudes. Specific craters and various ice flow features were studied and compared in three regions: Newton Basin, Eastern Hellas, and Northern Elysium, leading to a model for crater degradation. Gullies, tongue ridges, debris aprons, and mantle deposits all may stem from recent cycles of ice deposition.
    • Quaternary Ostracode Paleoecology and Its Link to Climate Change in the Bonneville Basin: A Detailed Study of the Glad800 Core GSL00-4, Great Salt Lake, Utah

      Cohen, Andrew; Balch, Deborah P.; Cohen, Andrew; Flessa, Karl W.; Davis, Owen; Balch, Deborah P. (The University of Arizona., 2003)
      We report the results of a detailed paleoecological study of the Bonneville Basin covering the last ~240,000 years. Our study used fossil ostracodes and a sedimentological record obtained from the August 2000 GLAD800 drilling operation at the Great Salt Lake. We analyzed 125 samples, taken at ~1 meter intervals from core GSL00-4, for ostracodes and other paleoecologic and sedimentological indicators of environmental change. Multivariate analyses applied to the ostracode data indicate an alternation between three major environments at the core site over the cored interval. The environments fluctuated most often between shallow saline, open -water lake conditions (when the lake was high enough to inundate the core site) and salt or freshwater, spring -fed marsh (when the water level was at or lower than the core site). But occasionally, the core site was submerged by deep fresh water. Immediately following deep lake phases, crashes in lake level from rapid desiccation resulted in the deposition of thick evaporite units. These environmental changes are consistent with shoreline studies of regional lake level fluctuations, but provide considerable new detail on both the timing and environmental conditions associated with the various lake phases. Our age model (using dates obtained from ¹⁴C, U- series, tephra and biostratigraphic chronologies) allowed us to associate the core's record of regional paleohydrology to the marine oxygen isotope stages record of global climate change. The core contains high resolution, continuous records for the last three glacial/interglacial sequences. In each case we found that fresh open-water conditions (i.e. lake highstands) correspond with maximum glacial advances, except for the smaller, less intense OIS 4 glaciation, when the lake remained saline. Salt and freshwater marshes were dominant environments for most of the interglacials. However, throughout most of the Quaternary, this basin has contained a shallow, saline open-water lake.
    • Variable Denudation in the Evolution of the Bolivian Andes: Controls and Uplift-Climate-Erosion Feedbacks

      Pelletier, Jon D.; Barnes, Jason B.; Chase, Clement G.; DeCelles, Peter G.; Barnes, Jason B. (The University of Arizona., 2002)
      Controls on denudation in the eastern Bolivian Andes are evaluated by synthesis of new and existing denudation estimates from basin-morphometry, stream - powered fluvial incision, landslide mapping, sediment flux, erosion surfaces, thermochronology, foreland basin sediment volumes, and structural restorations. Centered at 17.5 °S, the northeastern Bolivian Andes exhibit high relief, a wet climate, and a narrow fold- thrust belt. In contrast, the southeastern Bolivian Andes have low relief, a semi-arid climate, and a wide fold-thrust belt. Basin -morphometry indicates a northward increase in relief and relative denudation. Stream-power along river profiles shows greater average incision rates in the north by a factor of 2 to 4. In the south, profile knickpoints with high incision rates are controlled by fold-thrust belt structures such as the surface expressions of basement megathrusts, faults, folds, and lithologic boundaries. Landslide and sediment-flux data are controlled by climate, elevation, basin morphology, and size and show a similar trend; short -term denudation-rate averages are greater in the north (1- 9 mm/yr) than the south (0.3-0.4 mm/yr). Long-term denudation-rate estimates including fission track, basin fill, erosion surfaces, and structural restorations also exhibit greater values in the north (0.2-0.8 mm/yr) compared to the south (0.04-0.3 mm/yr). Controls on long-term denudation rates include relief, orographic and global atmospheric circulation patterns of precipitation, climate change, glaciation, and fold-thrust belt geometry and kinematics. The denudation synthesis supports two conclusions: 1) denudation rates have increased towards the present 2) an along-strike disparity in denudation (greater in the north) has existed since at least the Miocene and has increased towards the present. Denudation rates and controls suggest that Bolivian mountain morphology is controlled by both its orientation at mid-latitude, and the feedbacks between uplift, kinematics, orographic effects on precipitation, glaciation, and the increased erosion that accompanies orogenesis.
    • A Re-Os Study of Sulfides from the Bagdad Porphyry Cu-Mo Deposit, Northern Arizona, USA

      Ruiz, Joaquin; Barra-Pantoja, Luis Fernando; Ruiz, Joaquin; Patchett, P. Jonathan; Titley, Spencer R.; Barra-Pantoja, Luis Fernando (The University of Arizona., 2001)
      Use of Re-Os systematics in sulfides from the Bagdad porphyry Cu-Mo deposit provide information on the timing of mineralization and the source of the ore -forming elements. Analyzed samples of pyrite, chalcopyrite and molybdenite mainly from the quartz monzonite and porphyritic quartz monzonite units are characterized by a moderate to strong potassic alteration (secondary biotite and K- feldspar). Rhenium concentrations in molybdenite are between 330 and 730 ppm. Two molybdenite samples from the quartz monzonite and porphyritic quartz monzonite provide a Re-Os isotope age of 71.7 ± 0.3 Ma. A third sample from a molybdenite vein in Precambrian rocks yields an age of 75.8 ± 0.4 Ma. These molybdenite ages support previous suggestions of two mineralization episodes in the Bagdad deposit. An early event at 76 Ma and a later episode at 72 Ma. Pyrite Os and Re concentrations range between 0.008-0.016 and 3.9-6.8 ppb, respectively. Chalcopyrite contains a wide range of Os (6 to 91 ppt) and Re (1.7 to 69 ppb) concentrations and variable ¹⁸⁷Os/¹⁸⁸Os ratios that range between 0.13 to 22.27. This variability in the chalcopyrite data may be attributed to different copper sources, one of them the Proterozoic volcanic massive sulfides in the district, or to alteration and remobilization of Re and Os. Analyses from two pyrite samples yield an eight point isochron with an age of 77 ± 15 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 2.12. This pyrite Re-Os isochron age is in good agreement with the molybdenite ages. We interpret the highly radiogenic initial 1870s/188Os as an indication that the source of Os and, by inference, the ore-forming elements for the Bagdad deposit, was mainly the crust. This conclusion agrees with previous Pb and Nd isotope studies and supports the notion that a significant part of the metals and magmas have a crustal source.
    • A Flexural Model for the Paradox Basin: Implications for the Tectonics of the Ancestral Rocky Mountains

      Barbeau, David Longfellow Jr.; Dickinson, William R.; DeCelles, Peter G.; Chase, Clement G.; Demko, T.; Geslin, Jeff K.; Garzione, C.; Sussman, A.; Bump, A.; Barbeau, David Longfellow Jr. (The University of Arizona., 2000)
      The Paradox Basin is a large (200 km x 265 km) asymmetric basin that developed along the southwestern flank of the basement-involved Uncompahgre uplift during the Pennsylvanian-Permian (Desmoinesian-Wolfcampian) Ancestral Rocky Mountain (ARM) orogenic event. Traditionally interpreted as a pull-apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement-cored uplifts of the Late Cretaceous-Eocene Laramide orogeny. The width, shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre-Paradox system are exemplary of typical 'immobile' foreland basin systems. Along the southwest-vergent Uncompahgre thrust, ~5 km of coarse-grained syntectonic Desmoinesian-Wolfcampian sediments were shed from the Uncompahgre uplift by alluvial fans and were reworked by fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift-parallel barrier ~250 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite-shale and biohermal carbonate facies in the medial and distal, submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian. Reconstruction of an end-Permian two-dimensional uplift-basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space (i.e., foreland basin depozones) generated by Pennsylvanian-Permian subsidence. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust (Te = 25 km; D = 10²³ N m) by a model Uncompahgre uplift (A(c-s) = 214 km², ρ = 2670 kg /m³) and accompanying synorogenic sediments (ρ = 2325 kg /m³). The Paradox Basin's lack of flexural interference by competing loads, its location on the relatively undeformed Colorado Plateau, and its well-exposed and well-studied basin-fill provide a rare glimpse into a Paleozoic intraforeland flexural basin. Other thrust-bounded basins of the Ancestral Rocky Mountains are not so optimal for study. However, similarities in basin profiles, structural relationships and facies architectures suggest that many ARM basins share a geodynamic and tectonic history similar to the Paradox Basin's. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to also consider the province's intraforeland flexural basins.
    • Time-Space Variations in Mesozoic and Cenozoic Meteoric Waters, Southwestern North America

      Becker, Jennifer L.; Titley, Spencer R.; Quade, Jay; Barton, Mark D.; Becker, Jennifer L. (The University of Arizona., 1999)
      Mesozoic and Cenozoic hydrothermal systems of the southwestern North American Cordillera contain a complex record from which meteoric water stable isotope compositions (δ¹⁸O and δ D) can be inferred. This record is therefore of interest as a proxy for climate. New analytical results combined with systematic review of isotopic values from more than 200 locations in the southwestern North American Cordillera show regular isotopic patterns in time and space. Jurassic isotopic ratios are high, and Late Cretaceous values are more negative. During the Oligocene, there is a transition to more negative values. The ancient dD values are higher from most locations when compared to younger and present day values. This enrichment is compatible with warmer climates in the past and with changes in tectonic environments and paleoelevation and paleolatitude estimates over the same time interval. Complications in the application of the data include uncertainties in the estimated temperatures, alteration ages, isotopic disequilibrium, and incorporation of multiple fluids.
    • A Palynological Analysis of Part of Death Valley Core DV93-1: 166-114 KA

      Davis, Owen K.; Bader, Nicholas E.; Davis, Owen K.; Quade, Jay; Dettman, David; Bader, Nicholas E. (The University of Arizona., 1999)
      Salt Core DV93 -1, from Badwater Basin in California's Death Valley, spans the past 192 ka. An analysis of fossil palynomorphs from 151.8 m (ca. 166 ka) to 103.5 m (ca. 114 ka) delimits four pollen zones. Zone 1, the "cheno -am" zone (151.8 to 143.5 m depth, 166 -154 ka), contains high percentages of Chenopodiaceae /Amaranthus pollen, and correlates with marine Oxygen Isotope Stage (OIS) 7. Zone 2, the juniper zone (143.5 to 117.3 m, 154 -124 ka), correlates with OIS 6 and contains high percentages of Cupressaceae pollen and low percentages of Ambrosia pollen. A simultaneous drop in juniper and increase in oak (Quercus) pollen, followed by replacement of Artemisia with Ambrosia, occurs at the Zone 2 /Zone 3 (oak zone) boundary (124 ka), corresponding to OIS Termination II warming. Zone 4, the Asteraceae zone (108.8 to 103.5 m, 119 -115 ka), contains higher percentages of Asteraceae and cheno -am pollen, indicating further warming.
    • The Nucleation and Evolution of Riedel Shear Zones as Deformation Bands in Porous Sandstone

      Davis, George H.; Ahlgren, Stephen G.; Davis, George H.; Chase, Clement G.; DeCelles, Peter G.; Ahlgren, Stephen G. (The University of Arizona., 1999)
      Riedel shear zones are geometric fault patterns commonly associated with strike-slip fault systems. The progressive evolution of natural Riedel shear zones within the Navajo Sandstone of southern Utah is interpreted from the spatial evolution of small-scale, incipient Proto-Riedel Zones (PRZs) to better-developed Riedel shear zones using field mapping and three-dimensional digital modeling. PRZs nucleate as a tabular zone of localized shearing marked by en èchelon deformation bands, each of which is no more than a few mm wide and tens of cm long, and oriented at 55° - 85° to the trend of the zone. With increasing strain, deformation bands and sedimentary markers are sheared ductily through granular flow and assume a sigmoidal form. The temporal and spatial evolution of bands comprising a Riedel shear zone suggests that PRZs nucleate as transitional-compactional deformation bands under localized, supra-lithostatic fluid pressure. Subsequent bands develop under modified regional stresses as conjugate shear fractures within the strain- hardened axis of the PRZ. These antithetic driven systems are not compatible with traditional synthetic driven models of Riedel shear zones. Unlike most synthetic driven examples, these antithetic driven systems are not controlled by preexisting "basement" structures, thus their geometries reflect a primary propagation or secondary passive deformation mechanism.
    • Migration of Recharge Water Downgradient from the Santa Catalina Mountains into the Tucson Basin Aquifer

      Long, Austin; Barger, Erin E.; Long, Austin; Eastoe, Christopher J.; Bassett, R. L.; Barger, Erin E. (The University of Arizona., 1996)
      Aquifers in the arid alluvial basins of the southwestern U.S. are recharged predominantly by infiltration from streams within the basins and by water entering along the margins of the basins from surrounding mountains (mountain -front recharge). The Tucson Basin of Southeastern Arizona is such a basin. The Santa Catalina Mountains form the northern boundary of this basin and receive more than twice as much precipitation (about 70 cm/yr) as the basin does (about 30 cm/yr). In this study environmental isotopes were employed to investigate the migration of precipitation basinward through joints and fractures. Water samples were obtained from springs in the Santa Catalina Mountains. Stable isotopes and thermonuclear bomb-produced tritium enabled qualitative characterizations of flow paths and flow velocities. Stable isotopic measurements fail to display a direct altitude effect. Tritium values indicate that although a few springs discharge pre-bomb water, most springs discharge waters from the 1960's or later.
    • Linkages of Laramide Thrusts, Northern Sangre de Cristo Range, Colorado

      Chase, Clement G.; Bedford, Janice M.; Chase, Clement G.; Davis, George H.; Coney, Peter J.; Bedford, Janice M. (The University of Arizona., 1994)
      Laramide thrust belts in the Colorado Rocky Mountains have been mapped as discrete units with little investigation into the linkage between displacements. The Elk Range-Sawatch and Elkhorn thrust systems displaced Precambrian, Paleozoic, and Mesozoic rocks toward the southwest. The Sangre de Cristo Range and Wet Mountains thrust systems displaced rocks toward the northeast. The opposite vergence and oppositely directed displacements between these systems must be accommodated, both at present levels of exposure and at depth. Mapping of the Kerber Creek area west of the northern Sangre de Cristo Range by J.M. Bedford helped answer the question of the linkage between the opposing Elk Range-Sawatch and Sangre de Cristo Range thrusts. In the Elk Range-Sawatch system the westward displacement on the thrusts is interpreted as a minimum of 11 km in the southern Elk Range with displacement decreasing toward the north. Bryant (1966) interpreted the timing of faulting as Paleocene in age. In the Sangre de Cristo Range northeast-verging thrust system the minimum estimated eastward displacement is 8 km. Burbank and Goddard (1937) interpreted the displacement as Eocene in age. The timing of the faulting in the two areas is not necessarily different. The Kerber Creek area lies between the two regions of oppositely facing displacement. Its internal structure most closely resembles that of the more proximal Sangre de Cristo deformation. Thrusts in the Kerber Creek area place Precambrian rocks over Laramide(?) and Paleozoic sedimentary rocks. The structures verge toward the north-northeast and represent a minimum of several kilometers of displacement. These Laramide structures are exposed where overlying Tertiary volcanics are eroded. In the Northern Sangre de Cristo Range a set of E-W trending faults intersects the generally N-S trending Laramide thrusts, possibly representing a partitioning of northeast transport into N-S and E-W components. The E-W trending deformation can be correlated across the San Luis Valley with the thrust faults in the Kerber Creek area. Correlation of the Kerber Creek thrusts with Sangre de Cristo faults extends the northeast directed Laramide deformation 22 km northwest of the edge of the Sangre de Cristo Range. Thus the link between the oppositely verging structures must continue toward the northwest, possibly beneath the Bonanza volcanic field. Further mapping toward the southern extent of the Elk Range-Sawatch thrust system may reveal how the opposite vergence and minimum 8-10 km of displacement are accommodated.
    • Primary Sediment Production from Granitic Rocks in Southeastern Arizona

      Acaba, Joseph Michael; Schreiber, Joseph F., Jr.; Acaba, Joseph Michael (The University of Arizona., 1992)
      Isolated granitic rock bodies (granites, granodiorites, quartz monzonites) in the vicinity of Benson in southeastern Arizona were studied to trace the behavior of rock weathering. Thin sections of fresh granites were examined to characterize the original mineralogy which consisted mainly of quartz, feldspars, and micas. The weathering products show up on the granites as grus and soil profiles as well as down slope in the basin deposits. X -ray diffraction studies of the < 2 micrometers fraction of the weathering products proved illite, smectite, illite-smectite mixed layer, and kaolinite to be the dominant clays; quartz and feldspar also persisted into this size fraction. Silt sized material produced similar results. The quartz monzonite of Texas Canyon afforded a special study of the initial weathering stages of feldspars and micas. In the < 2 micrometers fraction obtained from granitic material placed in an ultra sonic bath, the feldspars weathered to a Na-montmorillinite while biotite weathered to vermiculite.
    • Resistivity and Induced-Polarization Responses Over Two Different Earth Geometries

      Akman, Hulya Hayriye; Wait, James R.; Sumner, J. S.; Nabighian, M. N.; Sternberg, B.; Akman, Hulya Hayriye (The University of Arizona., 1988)
      The object of the thesis is to obtain the apparent- resistivity curves and induced-polarization (IP) effects that are utilized in geophysical exploration. Two different earth geometries, the thin horizontal conductive layer and vertical dike, were studied. The solution for both cases is identical. First, quasi- static electrical conditions were assumed, so that the problem could be solved using potential fields. The exact solution to the problem was obtained by using the Bessel integral formulation. Also, the image method was employed to find the potential fields. We noticed that the image -type series converges best when the dike or layer was thick (ratio of thickness to electrode spacing, b/a, is large) and the reflection coefficient was not near ±1. Otherwise, it is preferable to employ the thin conductive sheet model. The next step was to determine the dilution and distortion factors which are relevant to the induced polarization response. Finally, numerical results were obtained using a Fortran computer program. These calculations were compared with some results taken from the literature and good agreement is seen.
    • Regional Structure and Stratigraphy of Sierra El Aliso, Central Sonora, Mexico

      Stewart, John; Bartolini, Claudio; Coney, Peter J.; Ruiz, Joaquin; Bartolini, Claudio (The University of Arizona., 1988)
      Assemblages of Paleozoic age and less significant Triassic and possibly Cretaceous-Tertiary volcanic rocks constitute the Sierra El Aliso, 186 km east-southeast of Hermosillo, Sonora. The Paleozoic section consists of approximately 2000 m of allochthonous Ordovician to Permian pelagic and hemipelagic deposits that accumulated in continental slope, continental rise and ocean floor (?) environments. The lower Paleozoic is characterized by graptolitic black shale and radiolarian chert, quartzite, argillite and local limestone. The upper Paleozoic is predominantly turbidite carbonates rich in benthonic foraminifera, and conodont faunas, subordinate bedded chert, siltstone, sandstone and chert-clast conglomerate. After Early Permian time, but prior to the deposition of the Late Triassic Barranca Group the Paleozoic section was imbricated along south-southeast vergent thrust faults. The Triassic rocks unconformably overlie the Paleo-zoic strata and all thrust faults. The Triassic and older rocks are overlain by the Cretaceous-Tertiary volcanics.
    • Provenance and Petrofacies, Upper Devonian Sandstones, Philip Smith Mountains and Arctic Quadrangles Brooks Range, Alaska

      Coney, Peter J.; Anderson, Arlene Verona; Coney, Peter J.; Anderson, Arlene Verona (The University of Arizona., 1987)
      A petrographic study of upper Devonian sandstones (Endicott and Hammond Terranes), Philip Smith Mountains and Arctic quadrangles, Brooks Range, Alaska, shows that the sand-sized detritus was derived from two petrographic provenances. Detrital modes, calculated from point counts of thin sections, show that the provenance for the Devonian clastic wedge (Endicott Terrane) was a recycled orogenic belt with major components of quartz, chert, and lithic fragments. Three petrofacies are distinguished. Their distribution indicates compositional changes vertically and laterally which reflect changing compositions in the source area. A petrographically different provenance supplied the sandstones that overlie the Skajit Limestone (Hammond Terrane). Characterized by high feldspar and abundant volcanic rock fragments, this petrofacies indicates first-cycle deposition close to the source area. A magmatica arc provenance is suggested.
    • Correlation of Some Mid-Mesozoic Redbeds and Quartz Sandstones in the Santa Rita Mountains, Mustang Mountains, and Canelo Hills, Southeastern Arizona

      Chase, Clement G.; Beatty, Barbara; Chase, Clement G.; Dickinson, William R.; Reynolds, Stephen J.; Shafiqullah, Muhammad; Beatty, Barbara (The University of Arizona., 1987)
      Mid-Mesozoic redbeds in the Santa Rita Mountains, Mustang Mountains, and Canelo Hills are lithologically and petrographically correlative. Quartz sandstone overlying the redbeds in the Mustang Mountains and Canelo Hills is also lithologically and petrographically correlative. The sediments were deposited during one or more breaks in volcanism associated with a continental magmatic arc that was active in southern Arizona during Triassic (?) and Jurassic time. The exact timing of sedimentation cannot be determined without more accurate dating of volcanics associated with the redbed strata. Redbeds of the Gardner Canyon Formation in the Santa Rita Mountains were probably deposited during and after eruption of the Mount Wrightson volcanics, the lower member of which is presently dated at 210 ±3 Ma. The Monkey Canyon redbeds and the Dark Canyon sandstone in the Canelo Hills and identical rocks in the Mustang Mountains are probably equivalent and were deposited between or during one or more hiatuses in two volcanic episodes presently dated at 165-185 Ma and 150-155 Ma, or before the 165-185 Ma volcanism and possibly as early as the period when the Gardner Canyon Formation was deposited. The Gardner Canyon Formation and Monkey Canyon redbeds are sequences of interbedded mudstone, siltstone, fine- to coarse-grained volcaniclastic sandstone, and volcanic conglomerate. They are interpreted as the product of meandering fluvial systems developed in close proximity to local volcanic sources, probably in distal alluvial fan or floodplain environments. The Dark Canyon and upper member Mount Wrightson quartz sandstones are bimodal, fine- and medium-grained, and average 93 percent monocrystalline quartz. They were probably deposited by both fluvial processes in stream channels and as windblown sand.
    • Paleomagnetism of Miocene Volcanic Rocks in the Mojave Region of Southeastern California

      Acton, Gary Dean; Butler, Robert F.; Acton, Gary Dean (The University of Arizona., 1986)
      Paleomagnetic data were collected from Miocene volcanic rocks in the Turtle Mountains, Clipper Mountain, Colton Hills, and Piute Range of the southern Basin and Range (SBR) province in southeastern California as well as in the Soledad Mountains of the Mojave block in southern California. The data from these two tectonic provinces yield significantly different paleomagnetic directions, which probably indicates the existence of a major crustal and /or lithospheric discontinuity in the area between the Barstow Basin and the Clipper Mountain. Comparing the mean direction from the SBR data to the Miocene expected direction indicates no statistically significant rotation (R = -0.2° ± 18.2°) or flattening (F = -6.5° ± 9.2°). A similar comparison for the Soledad Mountain data, which were combined with data of Burke et al. (1982) from the Barstow Basin, yields a significant rotation of -43.5° ± 12.9° and flattening of 19.3° ± 10.6° for the Mojave block. These Mojave block values may be exaggerated a few degrees due to inadequate averaging of secular variation and possible improper structural corrections.
    • Stratigraphy, Taphonomy, and Fauna-Substrate Associations in a Gulf of California Pleistocene Marine Terrace Near Punta Chueca, Sonora, Mexico

      Chase, Clement G.; Beckvar, Nancy; Chase, Clement G.; Kidwell, Susan M.; Prisrid, P. P.; Beckvar, Nancy (The University of Arizona., 1986)
      A richly fossiliferous Pleistocene terrace located near Punta Chueca, Sonora, Mexico, contains sediments that were deposited at the interface of an alluvial fan and shallow marine environment. Shell beds range from extremely dense fossil concentrations in sand, gravel, and cobble sized sediments to sparsely fossiliferous shell hashes. Three subenvironments were recognized: 1) shallow-subtidal to lower intertidal; 2) mid- to upper intertidal; and 3) supratidal. Shallow-subtidal to lower intertidal facies consist of shell beds with infaunal bivalves in life position, shell beds with fauna not in life position, and a Porites biostrome. Mid- to upper-intertidal facies include shell hash layers, and pebble and cobble lenses that are characterized by abundant autochthonous epi- faunal gastropods (i.e. limpets). Sparsely fossiliferous supratidal sands are overlain by Holocene alluvial fan deposits. Coarse conglomerates were not reworked by marine processes whereas finer conglomerates were, as evidenced by horizontal bedding and segregation of gravel and sand. The coarsest sediments - metamorphic cobbles - are relict and were probably derived from an earlier terrace. The following criteria were used to interpret the mode of shell bed formation: encrustation frequency, valve articulation, bivalve orientation, shell condition, and shell density (hardpart abundance). Storms played a major role in the formation of fossil concentrations. Four shell beds were interpreted as storm beds and one shell bed was interpreted as a condensed bed. Storm beds differ from condensed beds in having lower encrustation frequencies, higher percentages of articulated bivalves, and shells in very good condition. Association of hard-substrate faunas with gravel sediments and of infaunal molluscs with sand substrates suggests that little transport between habitats occurred. The high percentage of articulated valves, unworn appearance of most shells, predominance of concave-up oriented valves, and strong association of fauna with grain size all reflect a generally low energy environment, but one periodically disturbed by storm events.
    • The Tertiary Igneous Terrain in the Vicinity of the King Tonopah Mine, Tonopah, Nevada: An Exploration Case Study

      Barker, Walter Blaine; Eastoe, C. J.; Ruiz, Joaquin; Titley, Spencer R.; Barker, Walter Blaine (The University of Arizona., 1986)
      Uneconomic epithermal precious metal mineralization and associated alteration occur in the Tonopah Property, and are similar in style, although much less intense, to the deposits of the Tonopah camp two miles south. Mineralization is localized within a set of northwest-trending faults within the Tonopah, Mizpah, and King Tonopah Member of the Fraction-Tuff formations, and is associated with widespread propylitic and sparse fracture-localized potassic and argillic alteration. A younger set of Mn-calcite veins, anomalous in manganese, mercury, arsenic, and antimony, occurs in northeast-trending faults cutting older formations as well as the younger Tonopah Summit Member of the Fraction Tuff. This mineralization is possibly associated with silicification, zeolitization, and clay-alteration of the Fraction Tuff. The Tonopah Summit Member of the Fraction Tuff is reinterpreted as younger than the King Tonopah Member. Mega-breccia and basin morphology in the northeast may indicate an eruptive vent in this area.
    • Geologic Implications of a Geo-Chemical Study of Three Two-Mica Granites in Southern Arizona

      Damon, Paul E.; Guilbert, John; Arnold, Andrew Herbert; Damon, Paul E.; Ruiz, Joaquin; Arnold, Andrew Herbert (The University of Arizona., 1986)
      The biotite + muscovite ± garnet-bearing Texas Canyon (TC), Presumido Peak (PP), and Gunnery Range (GR) granites are members of an enigmatic suite of Eocene age granites in southern Arizona. The late orogenic granites intrude Precambrian through Jurassic metasediments and metavolcanics. The major and minor element geochemistry of the high silica, weakly peraluminous granites is rather uniform. However, trace element concentrations, REE patterns, and isotopic compositions imply gross similarities between the TC and GR granites when both are compared to the PP granite. The TC and GR granites were derived from a depleted Precambrian lower crustal source area with low Rb/Sr, while the PP granite was the result of anatectic melting of an enriched Precambrian mid-to-upper crustal source. The PP granite is an integral part of a metamorphic core complex, and this tectonic setting accounts for the geochemical differences between it and both the TC and GR granites.