DEPARTMENT OF GEOLOGICAL SCIENCES
(Dissertations Advisor(s); 1988-present
Abstracts; 1995-present)
2001
Eshete, Tefera Gashu, 2001, The deep structure of northeastern New Mexico from deep seismic reflection
profiles: Implications for Proterozoic tectonic evolution of southwestern North America; 143 p. (K.C. Miller)
Snelson, Catherine M., 2001, Investigating seismic hazards in the western Washington and crustal growth in the Rocky Mountains; 248 p. (K.C. Miller and G.R. Keller).(Outstanding Dissertation Award; Dept. Geol. Sci)
2000
Kilbride, Fiona, 2000, Receiver Function Studies in the Southwestern United States, and Correlation between
Stratigraphy and Poisson's Ratio in Southwest Washington State; 207 p. (Outstanding Dissertation Award; Dept. Geol.
Sci) (D.I. Doser and K.C. Miller)
Nandigam, Ravindra, 2000, Geology and Geochemistry of Newly Discovered Tertiary Carbonatite Occurrences near Villa Ahumada Area, Basin and Range Province, Chihuahua, Northern Mexico (K.F. Clark and E.Y. Anthony).
Rodríguez-Pineda, Jose Alfredo, 2000, A Geophysical, Geochemical, and Remote Sensing Investigation of the Water Resources at the City of Chihuahua, Mexico (N.E. Pingitorer)
1999
Seeley, John M., 1999 Studies of the Proterozoic Tectonic Evolution of the Southwestern United States (R.P.
Langford and G.R. Keller)
Steinhoff, Doerte, 1999, Selected Studies of Cretaceous mixed siliciclastic and carbonate sedimentation patterns in northern Mexico, Texas, and Montana (D.V. LeMone).
Williams-W., Wendi J., 1999, Evolution of Quaternary Intraplate Mafic Lavas Detailed Using 3He Surface Exposure and 40Ar/39Ar Dating, and Elemental and He, Sr, Nd, and Pb Isotopic Signatures: Potrillo Volcanic Field, New Mexico, U.S.A., and San Quintín Volcanic Field, Baja California Norte, México (E.Y. Anthony)
1998
Bartolini, Claudio, 1998, Stratigraphy, geochronology, geochemistry, and tectonic setting of the Nazas
Formation, North-Central México; 597 p. (W.C. Cornell).
Dial, Paul, 1998, An integrated geophysical study of North African and Mediterranean lithospheric structure; 226 p. (D.I. Doser and G.R. Keller)
Olmstead, Gary, 1998, A palynological analysis of Upper Jurassic dinoflagellate cysts from the Chihuahua Trough and Bisbee Basin, northern Mexico and southeastern Arizona; 221 p. (W.C. Cornell)
Rathjen, James D., 1998, Leonardian (Permian) Tubiphytes -- sponge bioherms of the Finlay Mountains, Marfa Basin, Trans-Pecos, Texas; 132 p. (D.V. LeMone)
Steinhoff, Ingo, 1998, Evaluation of H2S formation in the Basal Zechstein; Recipient: Department's Outstanding PhD Dissertation; (N.E. Pingitore)
Wong, Virgina, 1998, Gamma-ray characterization of uranium-series nuclides, and its application to the study of the Pena Blanca Natural Analogue Site, Chihuahua, México; 156 p. (E.Y Anthony and P.C. Goodell)
1997
Bader, Janet, 1997, Potential for bioremediation of a soil heavily contaminated with hexavalent chromium; 105 p.
(P.C. Goodell and S.D. Pillai)
Omenda, Peter, 1997, A petrologic study of the Kenya Rift; 106 p. (E.Y. Anthony and G.R. Keller)
1996
Machorro, Rudy, 1996, Water quality at Lake Izabal, Guatemala: Geochemical characterization and assessment
of trophic status; 261 p. (P.C. Goodell and J. Walton/Civil Engr.)
Simiyu, Silas, 1996, Integrated geophysical study of the crustal structure of the southern Kenya rift; 256 p. ( G.R. Keller)
Whitelaw, Julia, 1996, A seismic tomographic study of the Mojave region and geophysical constraints on thrust belt structure in eastern Tennessee; 308 p. (K.C. Miller and D.I. Doser)
1995
Adams, Donald C., 1995, Integrated studies of continental rifts in west Texas and New Mexico; 233 p. (G.R.
Keller and K.C. Miller)
John, Charles, 1995, The Blue Mountain transverse structure, Latimer County, Oklahoma; 192 p. (G.R. Keller and F.E. Julian)
Ouimette, Mark A., 1995, Petrology and geochemistry of Laramide and Tertiary igneous rocks in the Elk Mountains, Gunnison County, Colorado; 214 p.(E.Y. Anthony)
Ponce, Benjamin, 1995, Experimental and empirical investigation of the Ag2S--Ag2Se system and discrimination of topaz rhyolites by major-element composition; 103 p. (N.E. Pingitore)
Suleiman, Abdunnur, 1995, Geophysics of the rifts associated with the Anadarko Basin (North America), and the Sirte Basin, North Africa; 165 p. (G.R. Keller and D.I. Doser)
1994
Durrani, Bashir, 1994, An integrated crustal study of the Basin and Range Province-Colorado Plateau region,
western Arizona; 130 p.
Tekbali, Ali, 1994, Palynostratigraphy of the Mesozoic continental clastics in western and southeastern Libya; 380 p.
1993
Gridley, James M., 1993, Crustal structure of western Washington state; 244 p.
Isik, Iskender, 1993, White brick manufacture utilizing Kline Mountain, New Mexico clay; 260
p.
McLemore, Virginia, 1993, Mineralization and alteration in the Steeple Rock mining district, Grant County, New Mexico and Greenlee County, Arizona; 522 p.
1992
Abushagur, Sulaiman, 1992, The Lower San Andres Formation (Permian) Levelland and Slaughter Fields,
Northern Shelf of the Midland Basin, West Texas; 265 p.
Gutierrez, Melida, 1992, Competitive adsorption interactions of strontium, cobalt and cesium in suspensions of calcium montmorillonite: Electrochemical behavior and modeling; 278 p.
Luo, Ming, 1992, Generation and development of the overpressure system in the eastern Delaware Basin, west Texas and southern New Mexico; 206 p
Yuan, DeRen, 1992, Some aspects of the inversion problem of surface wave dispersion data; 163 p
1991
Deng, Qingping, 1991, Geology and trace element geochemistry of the Hollister gold deposit, Ivanhoe District,
Elko County, Nevada; 312 p.
Farraro, John T., 1991, Geochemical evaluation of the petroleum source potential of selected Paleozoic rocks of the Franklin Mountains of El Paso County, Texas and Dona Ana County, New Mexico; 186 p. (D.V. LeMone)
1990
Saenz, Guadalupe, 1990, Application of surface organic geochemistry to petroleum exploration and
environmental pollution; 154 p.
Schneider, Robert V., 1990, An Integrated geophysical and geological investigation of the transition zone between the Colorado Plateau, Rio Grande Rift and Basin and Range Provinces: Arizona and New Mexico; 220 p.
1989
Mickus, Kevin L., 1989, Backus and Gilbert inversion of two and one-half dimensional gravity and magnetic
anomalies and crustal structure studies in western Arizona and the eastern Mojave Desert, California; 225 p.
1988
Baker, Mark R., 1988, Quantitative interpretation of geological and geophysical well data; 145 p. (Best Dissertation
Award; Department of Geological Sciences)
Kwarteng, Andrews Y., 1988, Remote sensing applied to the exploration for uranium-mineralized breccia pipes in northwestern Arizona; 220 p.
Knoll, Martin A., 1988, Tertiary basin evolution, eastern Mojave Desert; 201 p.
Lueth, Virgil W., 1988, Studies of the geochemistry of the semimetal elements: Arsenic, antimony, and bismuth; 187 p
1987
Figuers, Sands H., 1987, Structural geology and geophysics of the pipeline complex, northern Franklin
Mountains, El Paso, Texas; 299 p.
Harder, Vicki M., 1987, Fission tracks in fluorite and apatite with geological applications; 93 p.
Yousef, Ali A., 1987, A surface wave dispersion study of the lithospheric structure of Africa; 128 p.
1986
Harder, Steven H., 1986, Inversion of seismic velocities for the anisotropic elastic tensor; 84 p.
1984
Aluka, M. Innocent, 1984, Stratigraphy of early Ordovician El Paso Group of the southern Hueco Mountains,
Hudspeth County, Texas; 260 p.
Edwards, Gerald, 1984, Petrography and geochemistry of the Allamoore Formation, Culberson and Hudspeth Counties, Texas; 313 p.
Orr, Cynthia D., 1984, A seismotectonic study and stress analysis of the Kermit Seismic zone, Texas; 286 p.
Simpson, Ronald D., 1984, Permian brachiopoda from far west Texas and southwestern New Mexico; 415 p.
Sinno, Yehia A., 1984, Crustal structure of the southern Rio Grande Rift determined from seismic refraction, surface wave dispersion and gravity data; 193 p.
Suleiman, Ibrahim S., 1984, Gravity and heat flow studies in the Sirte Basin, Libya; 198 p.
1983
Zamzow, Craig E., 1983, Tertiary volcanics of the eastern Eagle Mountains, Eagle Mountains, Hudspeth
County, Texas; 186 p.
Wilkerson, Gregg, 1983, Geology of the Batopilas mining district, Chihuahua, Mexico; 320 p.
1982
Hoffer, Roberta L., 1982, Uranium geochemistry of selected rock units from the Marysvale Volcanic Field, Piute
County, Utah; 281 p.
Lance, James O., 1982, Frequency domain analysis of least squares polynomial surfaces with application to gravity data in the Pedregosa Basin area; 202 p.
Weise, James R., 1982, Stratigraphy, lithofacies, and biofacies of the U-Bar Formation (Aptian-Albian) of the Big Hatchet Mountains, Hidalgo County, New Mexico; 354 p.
1981
Evans, Kathryn C., 1981, Geochemical reconnaissance for uranium in middle Tertiary ashflow tuffs of the Morey
Peak Quadrangle, northern Nye County, Nevada; 333 p.
Orajaka, I. Paul, 1981, Mineralogy and uranium geochemistry of selected volcaniclastic sediments in the western United States: An exploration model; 392 p.
Robinson, Bob R., 1981, Geology of the D-Cross Mountain quadrangle, Socorro and Catron Counties, New Mexico; 224 p.
Taylor, Bruce, 1981, Heat flow studies and geothermal exploration in western Trans-Pecos, Texas; 339 p.
Trentham, Robert C., 1981, Leaching of uranium from felsic volcanics and volcanoclastics model, experimental studies and analysis of sites; 225 p.
1980
Thomann, William F., 1980, Petrology and geochemistry of the Precambrian Thunderbird Formation, Franklin
Mountains, El Paso County, Texas; 176 p.
1979
Massingill, Gary, 1979, Geology of Riley-Puertecito area, southeastern margin of the Colorado Plateau, Socorro
County, New Mexico; 316 p.
Return to About the Department Page Geophysics of the Rifts Associated with the Anadarko Basin (North America), and the Sirt Basin, North Africa
Suleiman, Abdunnur
Department of Geological Sciences
In this study, a significant amount of geophysical and geological data has been gathered for both the Sirt and Anadarko basins. The primary intent of most of the geophysical data gathered in the Sirt basin (North Africa) and the Anadarko basin (North America) areas was petroleum oriented, and published studies (especially in the case of Sirt basin) are very limited. The purpose of this study was to analyze these rifted areas of North Africa and North America.
For the Sirt basin area, an integrated analysis of subsurface and geophysical data was conducted using gravity, seismicity, and heat flow data. These data suggest that the Sirt basin has a number of features in common with modern and older continental rifts throughout the world. A total of 5331 gravity values were used in the gravity investigations of the Sirt basin rift system. The Bouguer gravity map indicates that the Sirt basin is marked by elongate regional gravity maxima and minima trending NW-SE showing a strong correlation with the structural highs (associated with platforms and carbonate buildups) and lows (associated with sedimentary filled troughs) known from drilling. This map also shows that the Han graben is marked by a long wavelength negative gravity anomaly and very steep gravity gradients which delineate faults along the sides . Large deep-seated structures are also predominant on the Bouguer gravity map. Thus in some cases, it is very difficult to recognize smaller or shallower features. The third order polynomial surface map was chosen as the best approximation of the regional gravity field over the Sirt basin area, The gravity values shown in this map generally increase in a regular manner from the SE to the NW as a result of a major density/structural change in the lithosphere marking the transition from the continent to the ocean. The major upper-crustal features (which were of interest) were enhanced by removing the very short wavelength (15 km) and very long wavelength (75 km) anomalies from the third order residual data. A long gravity profile was constructed from the western shelf in the western flank of the Hun graben to the Cyrenica shelf in the northeast. Computer modeling of this profile suggests that the crust is thicker under the Western shelf (40 km) and thinner to the northeast underneath the Cyrenaica shelf (35 km).
The seismicity indicates that the rift margins, especially the western margin, are still tectonically active. The first motion results suggest strike-slip and normal faulting is occurring throughout the region. The waveform, modeling results for the 1935 main shock and the two largest aftershocks suggest a strike-slip mechanism with one fault plane similar to the strike of the eastern edge of the Hun graben. The modeled focal depth is 21±3 km for the main shock, and 14-16 km for the two aftershocks. Inversions of focal mechanisms within the Min graben for stress direction suggest maximum compressive stress oriented 97 degrees. East of the relatively aseismic portion of the central basin, dip-slip faulting is common and P axes trend N-S to NE-SW. These seismic observations suggest that both the NW-SE distinctive faults seen in the gravity and associated NE-SW trending cross faults seen on geologic maps (and to a lesser extent in the gravity data) continue to play an important role in the western Sirt basin. Although these faults were normal faults from Cretaceous through Eocene time, they appear to behave as strike-slip faults in the present stress-regime.
The heat flow data show above average heat flow values in portions of the rift. The thick (>5 km) sedimentary section indicated from gravity studies for the eastern Sirt basin may mask any higher heat flow in this region. The heat flow-heat production relationship also indicates a tectonic setting between that of rifting and stable continental interiors.
The Southern Oklahoma Aulacogen (SOA) area was analyzed through subsurface and geophysical data particularly those from a wide-aperture seismic field experiment conducted in 1985. The data were first reprocessed using an interactive two-dimensional seismic processing and data analysis software system. The data were then interpreted using a two-dimensional, ray-tracing program. The resulting velocity model for the SOA region was in turn constrained using gravity and drilling data along the seismic profile to produce a final interpreted crustal model. This earth model indicates that the SOA has experienced an impressive amount of modification of the crust. When reactivated by late Paleozoic events at the continental margin, vertical displacements alone exceeded 12 km. Quaternary stratigraphic relationships and ten 14C age dates demonstrate that the latest movement on the Meers fault (part of a major zone of NW-trending fault that forms the boundary between the Wichita Mountains and the Anadarko basin) is late Holocene in age.
Seismic and gravity data suggests that the Anadarko basin shows a striking lack of symmetry with a great truncation at the south side of the basin along the frontal fault zone. Seismic data also indicates that the base of the crust is layered and transitional in character rather than a simple sharp boundary. The Moho is about 45 km deep beneath the frontal thrust zone of the Wichita uplift shallowing to about 41 km in both ends of the modeled line. An average upper crustal compressional seismic velocity of 6.25 km/sec, and an average lower crustal compressional seismic velocity of 6.85 km/sec were also suggested.
The rifted regions that were investigated in this study represent two of the major continental rift structures. Both the Sirt basin of North Africa
and the Anadarko basin of North America have experienced multiple reactivations and form some of the most important petroleum provinces
in the world. The detailed individual investigations of both regions suggest that the two rifted regions are not similar in many geophysical
aspects primarily due to their age difference.
Experimental and Empirical Investigation of the Ag2S--Ag2Se System and Discrimination of Topaz Rhyolitesby Major-element Composition
Ponce-S., Benjamin
Department of Geological Sciences
This dissertation comprises an investigation of the Ag2S--Ag2Se system and the discrimination of the topaz rhyolites by major-element composition, and consists of a brief introduction and four chapters. Each chapter has a separate objective and was or will be published as an independent paper. Chapters I and II were published in the Journal of Materials Research, Chapter III is in manuscript form and will be submitted to Canadian Mineralogist after approval from the International Mineralogical Association (IMA), Chapter IV was published in the Journal of Geochemical Exploration. The study encompasses the compositional and structural variations of silver sulfide-selenide minerals and synthetic compounds, and the multivariate statistical discrimination of three groups of rhyolites.
Solid Solutions in the System Ag2S--Ag2Se
Optical, electron microprobe, and x-ray diffraction analysis of 88 samples of various compositions between Ag2S and Ag2Se were synthesized
at high temperature in sealed quartz tubing. Chemical and x-ray diffraction analyses indicate the presence of two solid-solution series in this
system at ambient (room) conditions. One series extends from Ag2S to approximately Ag2SO0.4Se0.6 and has the Ag2S-III-type structure
(monoclinic). The second series ranges from Ag2S0.3Se0.7 to the Ag2Se and is characterized by the Ag2Se-II-type structure (orthorhombic).
Members of both series, in appropriate proportions, characterize the compositional gap between the two solid solutions. Gradual shifts in the
locations of the x-ray diffraction peaks along the compositional gradient of each solid solution reveal an expansion of the d-spacing as the
larger Se ion is substituted for S in the Ag2S-III-type structure and a contraction as S is substituted for Se in the Ag2Se-II-type structure
(orthorhombic). The reported discrete phase, Ag4SSe (aguilarite, orthorhombic) appears to be simply a member of the monoclinic
Ag2S-III-type solid solution.
Calorimetric Analysis of the System Ag2S-Ag2Se
Two solid-solution series have been established in the Ag2S--Ag2Se system. The first solid-solution series presents a monoclinic structure
where 0.0 Se0.6, an orthorhombic structure where 0.7Se1.0, and a two-phase zone or miscibility gap is formed when 0.6Se0.7.
It was demonstrated that within the compositional range of the two series, each compound is a single phase, rather than an intergrowth of end members, by observing a single transition from low-temperature to high-temperature allotropes in a differential scanning calorimeter. Calorimetry confirmed the continuous nature of each solid solution showing that the transition temperatures vary smoothly with composition.
Calorimetric and chemical analyses indicated that within the miscibility gap, the high-temperature form dissociates into two immiscible phases with the corresponding end member compositions. The dissociation is rapid and reversible and occurs below 100oC, depending on composition. The coexistence of the two phases was suspected when double peaks were found, instead of single peaks, in some positions of the X-ray powder patterns. The bulk composition of the mineral grain or compound was close to that of the solid-solution. Determination of the entropy of the transitions was consistent with the model of two solid solutions.
Silver Sulfide-selenide Minerals from Mexico: Evidence for Two Solid Solutions from Acanthite to Naumannite.
The Ag2S--Ag2Se system was traditionally (Petruk et al., 1974, Canadian Mineralogist, v. 12, p. 365) considered composed of the
end-member elemental compositions corresponding to the well known minerals acanthite (Ag2S, monoclinic) and naumannite (Ag2Se,
orthorhombic); only a single intermediate species, aguilarite (Ag4SSe, orthorhombic), with a defined stoichiometry has been described
formally. There have been several reports of compositions between these three minerals, most notably that of Morales and Borodayev (1982,
Dok. Akad. Nauk SSSR, v. 264, p. 685).
Electron probe microanalysis of more than 100 mineral grains from ore concentrates and vein samples collected in the Mexican silver districts of Guanajuato and Zacatecas provided compositions ranging from pure silver sulfide, Ag2S, to pure silver selenide, Ag2Se. The analyses probably correspond to the two solid-solution series which laboratory syntheses recently have demonstrated to characterize the system Ag2S-Ag2Se (Pingitore et al., 1992, Jour. MaterialsResearch, v. 7, p. 2219). In the light of such experimental data, the full range of intermediate compositions in nature should not be unexpected.
Discrimination of Topaz Rhyolites by Major Element Composition: A Statistical Routine for Geochemical Exploration.
The recognition of topaz-bearing, calc-alkaline, and peralkaline rhyolites at an early stage of an exploration program may be of both geologic
interest and of economic significance due to the different mineral deposits characteristically associated with each of these rock suites. Such
discrimination could result in better definition of target areas and commodities to explore within a selected region.
A geochemical database of major element analyses of calc-alkaline and topaz rhyolites, and comendites from western North America was assembled. Multiple discriminant function analysis classified each sample statistically as a member of the calc-alkaline, topaz or peralkaline groups, using only major element composition. The discrimination routine correctly identified 90% of the rhyolites as members of the groups to which they actually belong. In effect major element composition can serve as a proxy for fluorine content either when no fluorine analysis is available or when it is suspected that fluorine was not preserved in the rock -due to loss of fluorine during emplacement or subsequent processes.
A simplified procedure was developed to use this discrimination model to classify silicic rocks of unknown affiliation. This model may help to
evaluate the economic potential of both new or previously explored areas by analyzing data files containing major-element chemical analyses.
Dissertation Abstract; 1995
Petrology and Geochemistry of Laramide and Tertiary Igneous Rocks in the Elk Mountains, Gunnison County, Colorado
Ouimette, Mark A.
Department of Geological Sciences
The Elk and Sawatch Mountains of eastern Gunnison County, Colorado, contain felsic igneous rocks that were emplaced episodically during three intervals of time. The intervals were between 75 to 55 Ma, 42 to 26 Ma, and 25 to 0 Ma. Thus, they span a period from Late Cretaceous Laramide compression through the Late Tertiary Rio Grande rift extension. This study reports the changes in magmatic style and source region that have occurred through this period of time. Patterns of trace elements (from rare earth and multi-element variation diagrams) and major elements for whole rock chemistry were studied for 13 igneous rock units.
Four groups with distinctive geochemical signatures were identified. Each group has different source region characteristics and mineralization aspects. Group I includes the Laramide Dustin Gulch and Fossil Ridge stock, and the middle Tertiary Gold Hill intrusive. These stocks are metaluminous and have Ta decoupled from Nb. The group also possesses a normative mineralogy that is high in anorthite and pyroxene, and low in quartz and orthoclase. They formed from an oxidized, mafic source. The group is similar to rocks that contain Laramide Cu-porphyry mineralization, but the stocks in this study are barren of mineralization.
Group II includes the Hillerton, Pitkin, Seeton Mine, Buck Gulch, and South Matchless Mountain stocks. They are non-mineralized and middle Tertiary in age. They are mildly-to strongly- peraluminous, have normative mineralogy high in quartz and corundum, and are low in pyroxene. Tantalum is decoupled from Nb, the concentrations Y and the heavy rare earth elements are variable and depleted. This suggests that garnet was an important mineral phase in the magmatic source. The Group II source was oxidized and meta-sedimentary.
Group III is associated with precious-and base-metal mineralization and includes the middle Tertiary Crested Butte laccolith and the quartz monzonite phases of the Italian Mountain and Mount Princeton intrusive centers and the Late Tertiary Round Mountain. This group is metaluminous, has normative mineralogy high in quartz and orthoclase, but is low in anorthite and corundum. They are different from the stocks of Group I and II in that both Ta and Nb have low concentrations. These rocks formed from a less oxidized felsic meta-igneous (or gneissic) source.
Group IV is limited to the Late Tertiary Boston Peak topaz rhyolite.. It is highly fractionated, possesses a strong negative Eu anomaly, is light rare earth element depleted and heavy rare earth element enriched compared to Groups I, II, or III. It has characteristics typical of the igneous rocks associated with stockwork Mo deposits (Climax-type) and formed from a F-enriched crustal source.
The groups (and stocks) reflect the geochemical characteristics associated with the change from subduction to extensional tectonics. Laramide subduction-related magmas occurred between 76 to 71 Ma. The source for these magmas was mafic and produced, the stocks of Group 1. During the second magmatic pulse, between 41 to 37 Ma, magma was produced from mafic, meta-sedimentary, and meta-igneous sources (Groups I, II, and III, respectively). At about 30 Ma North America overrode the Pacific spreading center and subduction ceased. Between 33 to 29 Ma the only sources to produce magmas were from Groups II and M. Afterward, the magmatic style changed to the bimodal magmatism typical for the extensional tectonics of the Rio Grande rift. Between 14 to 10 Ma magmas were produced in this area only from meta-igneous (Group M) and F-enriched crustal (Group IV) sources.
This study shows that felsic igneous rocks that range in age between Late Cretaceous to Late Tertiary were emplaced in a limited area that is
transitional between the central-and southern Colorado Mineral Belt. The study shows that at least three different source regions produced
magmas. These changes in the source region reflect the changes in regional plate tectonics that affected the Colorado Mineral Belt during this
interval of time. The regional division of igneous rocks from this period of time by Stein and Crock (1990) is temporally and spatially
expanded by this study. The boundaries previously recognized are modified by the expanded geochemical consideration offered by this study.
Dissertation Abstract; 1995
The Blue Mountain Transverse Structure, Latimer County, Oklahoma
John, Charles
Department of Geological Sciences
This research investigates a previously suggested transverse structure in the Blue Mountain area of west-central Latimer County, Oklahoma. The study is located in a 100 sq mi area between the Choctaw and Ti Valley thrust faults of the Ouachita Mountains frontal belt of southeastern Oklahoma.
The investigative approach includes the construction of a series of dip and strike cross sections in order to determine the structural geometry, chronology, and other features of the study area. The cross sections are based on surface geology, available oil and gas wireline geophysical surveys (commonly known as electric logs), which measure a variety of physical properties, and published seismic data. The cross sections are used to determine (1) the fault geometry and chronology, (2) thrust distances, and (3) shortenings at a specific horizon.
The following features associated with the Choctaw thrust system of the Ouachita frontal belt are specifically addressed:
The Ouachita orogen is the belt of deformed Paleozoic rocks flanking the southern margin of the North American craton. It is the result of a Late Paleozoic tectonic event that involved the closing of an ocean by the collision of Gondwanaland with the North American craton (Laurasia). This action caused subduction of the North American plate beneath a growing accretionary prism and created companion provinces, the Ouachita Mountains and the Arkoma basin of western Arkansas and southeastern Oklahoma.
The Ouachita Mountains proper lie south of the Arkoma basin and are characterized by imbricate strike ridges and valleys. In Oklahoma, the Ouachita Mountains are separated into three belts based on stratigraphy and structural style which are, from north to south, the frontal belt , the central belt, and Broken Bow uplift.
There are three definite zones of faulting in the Ouachitas. They are: zone 1, from the Choctaw fault to the Ti Valley fault; zone 2, between the Ti Valley fault and the Winding stair fault; and zone 3, south of the Winding stair fault. The area of this study is within zone 1. Rocks in zone 1 are primarily Carboniferous and consist of deep water turbidites.
Analyses of the cross sections lead to the following conclusions:
Integrated Studies of Continental Rifts in West Texas and New Mexico
Adams, Donald C.
Department of Geological Sciences
West Texas and New Mexico have experienced a complex tectonic history including the formation of continental rifts on at least two occasions since the Middle Proterozoic. An integration of multiple geophysical methods combined with geologic information has been used to study the structure, geology and tectonic setting of the present day Rio Grande rift and the late Middle Proterozoic Delaware aulacogen.
The Rio Grande rift study focused on the structure of the rift in south-central New Mexico. Seismic data and well drilling information were used to constrain gravity models of the crust. These models suggest that the Engle and Palomas basins are half graben structures bounded by normal faults. The Tularosa basin with a full graben structure is the best developed basin in this part of the rift. An analysis of the deeper structure based on these models suggests that the crust is 28-33 km thick, which is thin compared to the unextended crust surrounding the rift.
A series of four studies of the Precambrian basement geology of the Permian basin region make up remainder of the dissertation. These studies are focused on understanding the geology, structure and tectonic setting of the Abilene gravity minimum and the Pecos mafic intrusive suite. The Abilene gravity minimum is a 600 km long gravity low extending from the Texas -Oklahoma border to the Diablo platform in west Texas. This anomaly has been previously interpreted as marking the northern extent of Grenville deformation in Texas. Based on geologically constrained gravity modeling and basement geology from drill holes I prefer an alternative model. The Abilene gravity minimum may represent a continental margin are batholith equivalent in scale to the Sierra-Nevada batholith in California. Based on its apparent relationship to regional geology it may be a result of subduction along the southern margin of the continent during the formation of the southern Granite-Rhyolite province. Because of these results, I propose that the location of the Grenville deformation front is farther south and alined along a series of east-west oriented gravity highs extending between the Llano and Van Horn uplifts.
The Pecos mafic intrusive suite is represented by a series of layered mafic intrusions in the basement under the Central basin platform and Roosevelt uplift in the Permian basin. The Pecos mafic intrusive suite contains the igneous rocks associated with formation of the Delaware aulacogen. These intrusions were identified by the North American Royalties, #1 Nellie well which penetrated 4.5 km of layered mafic basement rocks at the southern end of the Central basin platform. Paired gravity and magnetic maxima indicate that the intrusion may be composed of four large intrusive centers which extend north under the rest of the Central basin platform and Roosevelt uplift covering a minimum area of 9200 km2. Seismic reflection data over this intrusive suite are characterized by sub-horizontal layered reflectivity which is tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram for the Nellie well. Integration of the seismic interpretation with gravity models shows that the Pecos mafic intrusive suite is 3-10 km thick and 50-100 km wide. The overall structure of the suite is that of a sill with a dike-like keel. In places the intrusion forms a sill, and the base of the sill is imaged in the reflection data. I interpret this massive intrusion to be associated with rifting in the foreland of the Grenville orogeny, in a tectonic setting much like that of the Oslo rift. Structural inversion of the rift during the Ouachita orogeny led to the uplift of the Central basin platform of the Permian basin, and the completion of an aulacogen cycle.
This rift event is contemporaneous with formation of the Midcontinent rift system, Pikes Peak Batholith and the intrusion of diabase sills in
California, Nevada and Arizona. Taken together these events indicate that the late Middle Proterozoic in North America was a time of
extensive igneous activity and rifting on a scale and complexity comparable to the combined Mesozoic and Cenozoic African rift systems.
Dissertation Abstract; 1996
Integrated Geophysical Study of the Crustal Structure of the Southern Kenya Rift
Simiyu, Silas
Department of Geological Sciences
A significant amount of geophysical and geological data have been recently gathered around the Kenya rift, primarily associated as a result of the Kenya Rift International Seismic Project (KRISP). These data indicate the presence of a major mantle seismic velocity anomaly and crustal thickening beneath the Kenya dome, an uplift centered on the southern part of the Kenya rift. Detailed crustal studies north of the dome show thinning of the crust northward with an increase in lithospheric extension.
This study employed refraction and teleseismic data from KRISP, surface geology, gravity data from the UTEP data base and 5,600 new gravity measurements, drill hole data from geothermal exploration wells, and mantle xenolith and petrochemical results to constrain the construction of integrated cross-sectional models of lithosphere by gravity modeling. The approach taken in this study was to first interpret the deep regional tectonics affecting the whole of the East African Plateau on which the Kenya rift is superimposed. It was important to understand rifting in the broader context of the East African Plateau and the interplay between mantle and crustal structures and their contribution to the full spectrum of gravity anomalies. The long wavelength anomalies associated with mantle structures were modeled in a regional sense and the remaining short wavelength anomalies of crustal origin were then modeled in detail.
An updated data base of more 156,000 gravity reading was used to construct a Bouguer anomaly map, a suite of filtered gravity maps and regional profiles across the East African Plateau which were modeled. These models together with gravity maps demonstrate lithospheric thinning beneath the rift valleys and suggest the existence of a deep mantle anomaly centered beneath the East African Plateau associated with the Tanzania craton. The mantle anomaly is widely believed to be a plume and our models indicate that the diameter of its head at 130 km depth is about 600 km. In our models, two arms from the plume head with a diameter less than 250 km penetrate the lithosphere to shallow levels under the rifts. In the western rift and the southern part of the Kenya rift (Tanzania), the mantle anomaly is deeper than under Kenya. This study provides additional evidence that the long wavelength Bouguer gravity anomaly over the East African Plateau can be explained by lithospheric heating and thinning by thermal erosion of the cratonic root. A regional model along the axis of the Kenya rift suggests that the rift is propagating southwards into central Tanzania.
The result of detailed integrated interpretation of the crustal structure of the southern Kenya rift show the following: 1) there is no north-south
crustal thickness symmetry with respect to the apex of the Kenya dome along the rift axis; 2) the rift graben master fault is on the western
flank and there is no evidence for half-graben polarity (master fault side) reversals along the entire southern part of the Kenya rift; 3) the
pre-existing lithospheric contrast between the Archean and Precambrian tectonic grains plays a significant role in the rift's location and
structural geometry; 4) along axis crustal thickness is less related to crustal thickening by underplating than pre-rift crustal type and thickness.
Dissertation Abstract; 1996
Water Quality at Lake Izabal, Guatemala: Geochemical Characterization and Assessment of Trophic Status
Machorro, Rudy
Department of Geological Sciences
Lago de Izabal, the largest lake of Guatemala (717 km) with a mean depth of 11.6 m, is a unique freshwater body in which geology, land use, hydrology, and climate have combined to produce anomalous concentration of heavy metals in the sediments and an ongoing eutrophication process. The lake has been monitored over a period of one year generating a comprehensive set of chemical analyses which include nearly 9000 water chemical analyses and 900 sediment analyses. Lake-surface waters were sampled for a full year at 17 sites. A physical and chemical characterization of Lago de Izabal is presented. Spatial and temporal variations were examined. A hydrologic analysis, including rainfall runoff relationships for the watershed as well as determination of the water budget of the lake, has been conducted to calculate rates of inflow and outflow of chemicals in the lake. The pH is uniform during the entire year with an average of 7.8. Temperature vertical profiles show that the lake becomes isothermal during the low rainfall season and weakly stratified during the high rainfall season. The lake is well oxygenated throughout the year but anoxic conditions dominate the bottom waters with a simultaneous drop in pH from 8 to 7 when the lake develops thermal stratification.
The controversial issue of saline incursions from the Atlantic Ocean into Lago de Izabal has been analyzed. The average K, Cl, and TDS content of Lago de Izabal in comparison to that of average seawater indicates no penetration of saline waters into the lake. TDS values calculated from resistivity profiles conducted between San Felipe, the outlet of Lago de Izabal, and the ocean show that saline water penetrates areas located half way between the lake and the ocean, but they do not reach the lake.
Chemical weathering and subsequent soil erosion are major processes controlling water quality in the lake. Heavy metals in sediment, including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb, have been analyzed with the Factor of Enrichment (FE) and the Index of Geoaccumulation (I-geo). The FE shows that the sediments are enriched with respect to average crust of the earth in Cr, Co, Ni, and Pb. The I-geo indicates that the sediments of the lake present moderate contamination by Ni and Cr, and moderate to strong contamination by Co and Pb. Outcrops of ophiolites located in the north shore of the lake have been identified as the major source of Cr, Co, and Ni. Pb-tailings, located at the edge of the Cahabon watershed, are a potential source of Pb, but an anthropogenic input is also suggested.
Extensive deforestation and subsequent agricultural runoff has accelerated the nutrient input into the lake resulting in an incipient
eutrophication process. Total P, total N, chlorophyll, transparency, and primary productivity indicate eutrophic conditions. Trophic state
indices suggest hypereutrophic conditions based on chlorophyll and eutrophic conditions based on Total P and transparency. The
Vollenweider model shows that the annual P loading to the lake has been strong enough to initiate the process of eutrophication. Diatoms
constitute the major phytoplanktonic community (76 %) with green algae (14 %) and blue-green algae (10 %) as secondary communities. A
mass balance for silica reveals that the lake behaves as a sink for H4SiO4. This causes the high Si:P ratio responsible for the overwhelming
abundance of diatoms. Rios draining the ophiolites have the highest silica content of the area suggesting that chemical weathering on these
rocks is an important source of silica to the lake.
A Petrologic Study of the Kenya Rift
Omenda, Peter
Department of Geological Sciences
The southern portion of the Kenya rift is characterized by voluminous eruption of mafic to felsic magmas during the Quaternary. These include transitional basalts, basaltic trachyandesites, trachyandesites, phonolites, trachytes, pantellerites, and comendites within the rift axis and silica undersaturated basanites on the eastern flanks. To understand the petrogenesis of these lavas, the basanites from the eastern flank volcanic field at Chyulu are used to represent the most primitive magma associated with rifting. Results of modeling imply that the Chyulu basanites are products of 1-2% partial melting of a chondritic to primitive mantle. The transitional basalts of the rift axis represent 5-14% partial melting of a similar mantle. The lower value is preferred since it is consistent with the generation of the magmas that evolve into the transitional basalts of the rift axis.
The second phase of this study was to interpret the relationship between the mafic and felsic magmas. In order to accomplish this, the characteristics of pantellerites from Eburru, comendites from Olkaria, and phonolites from Suswa were studied. Major and trace element models show that the pantellerites, trachytes, and associated transitional basalts are cogenetic, and that evolution was dominated by polybaric fractional crystallization processes. Results obtained from stable oxygen isotopes (delta O18 = +5.7 to 6.6%) indicate that the rocks are mantle derived and that crustal assimilation was not important in their genesis. Trace elements indicate that initial crystallization of the rift axis magmas occurred at the Moho. A subsequent low pressure crystallization is indicated by the observed phenocrystic mineral assemblage that includes olivine, augite, and plagioclase. The models show that the transitional basalts are highly evolved and experienced 67 to 80% pre-eruption crystallization compared to 20 to 40% for the Chyulu basanites. The phonolites, trachytes and pantellerites represent more than 95% crystallization of basaltic parents. Least squares major element models reveal that the resultant cumulates have sufficiently high densities (2.90 g cm-3) to cause the gravity anomalies observed under the volcanoes in the rift axis.
Suswa is in a transition zone where the lithosphere begins to thicken southward. The volcano displays earliest shield building lavas of transitional basalt-trachyte association similar to the others in the rift. However, the evolution of the syn-caldera trachytes and post-caldera phonolites require more alkaline parental magma similar to the Chyulu-type sources. Comendites from Olkaria are not part of the fractional crystallization evolutionary paths. The highly evolved nature of these magmas and their major and trace element patterns for the compositional range trachyte to comendite are inconsistent with fractional crystallization. The data suggest, rather, that the comendites are generated by crustal anatexis. Models reveal that 20 to 30% partial melting of a syenitic rock can generate the observed major and trace element compositions of the comendites.
The distribution of the Quaternary lavas associated with rifting can be used to infer the tectonic evolution of the Kenya rift. The occurrence of
basanites on the eastern flank of the rift indicates thicker lithosphere than that under the rift axis where Quaternary transitional basalts and
their felsic derivatives dominate. Geophysical models indicate thin lithosphere under the rift. The eruption of phonolites at Suswa implies a
change to lower degrees of partial melting in the mantle under the volcano during the Holocene time. This is primarily caused by the
thickening lithosphere, south of Suswa which results in deeper partial melting in the mantle.
Dissertation Abstract; 1997
Potential for Bioremediation of a Soil Heavily Contaminated with Hexavalent Chromium
Bader, Janet
Department of Geological Sciences
This dissertation consists of an investigation of the potential for bioremediation of a soil heavily contaminated with hexavalent chromium [Cr(VI)]. The introductory chapter briefly reviews the chemistry and biogeochemistry of chromium. It is followed by three chapters in manuscript form which will be published as three independent scientific papers. Chapter 11 was submitted to Water, Air, and Soil Pollution; Chapter III will be submitted to Applied and Environmental Microbiology, and Chapter IV will be submitted to Bioresource Technology. The purpose of this study was to determine if the indigenous microbial populations in a soil heavily contaminated with Cr(VI) were capable of catalyzing Cr(VI) reduction, thereby decreasing Cr toxicity and bioavailability, and to determine how these populations compared to those in soils with no history of Cr(VI) contamination. Chapters II, III, IV describe the results and conclusions from different but closely related aspects of this topic. Chapter V comprises a brief summary and conclusions. The following paragraphs are the individual abstracts from each paper.
Chromium-resistant Bacterial Populations from a Site Heavily Contaminated with Hexavalent Chromium
Chromium-containing industrial effluents are primarily responsible for environmental contamination by toxic and highly mobile, hexavalent
chromium. The dilution plate-count method, using media amended with Cr(VI) at concentrations ranging from 0 to 1000 mg L-1, was used to
compare the sizes of Cr(VI)-resistant bacterial populations from a soil contaminated with 25,100 mg kg-1 total Cr [12,400 mg kg-1 Cr(VI)] to
those isolated from a slightly contaminated soil (99.6 mg kg-1 total Cr) and two other soils without any history of Cr contamination. Bacterial
populations resistant to 500 mg L-1Cr(VI) were isolated from all soils except the heavily contaminated soil. To determine whether Cr-resistant
bacterial populations were indigenous to both the contaminated and the uncontaminated soils, enrichment cultures containing Cr(VI)
concentrations ranging from 0 to 1000 mg L-1 were employed. Bacterial populations, as high as 105 CFU g-1 soil, tolerant of 500 mg L-1Cr(VI)
were isolated from all soils within 48 hr of enrichment suggesting that the presence of aerobic Cr(VI)-resistant bacterial populations is
unrelated to contamination levels or contamination history. However, identification of these resistant bacteria using fatty acid profiles was
unsuccessful suggesting that these populations may have unique characteristics. Fungal colonies resistant to 1,000 mg L-1Cr(VI) were
routinely isolated from both uncontaminated and contaminated soils. The results suggest that Cr-resistant microorganisms may be present in
most soils, even those with no history of Cr contamination.
Aerobic Reduction of Hexavalent Chromium by Indigenous Soil Microorganisms
Chromium in the environment exists in two stable oxidation states, +3 and +6. Hexavalent Cr is highly mobile and toxic whereas trivalent Cr
is insoluble above pH 5.5 and is an essential nutrient for mammals. Thus, reducing Cr(VI) to Cr(III) greatly reduces the toxicity of Cr. Soil
containing 25,100 mg kg-1 total Cr [12,400 mg kg-1 Cr(VI)] was collected from a Superfund site and used in batch culture studies to evaluate
the potential for aerobic reduction of Cr(VI) by the indigenous bacterial population. The concentration of Cr(VI) in the soil microcosms was
reduced as much as 33% (from 1,840 to 1,240 mg L-1 ) within 21 days under enrichment conditions. Reduction of Cr(VI) in this system was
shown to be biologically mediated and dependent on the availability of a usable energy source. The unenriched microbial consortium could
not use glucose as an electron donor for Cr(VI) reduction, even at much lower Cr(VI) concentrations (50 mg L-1). Reduction of Cr(VI) in this
system was not enhanced by increasing the microbial diversity nor the organic matter content via municipal sewage sludge amendments, and
it appears that biotic components in sludge inhibit aerobic bioreduction of Cr(VI). In addition, microbial consortia in soils with no history of
Cr(VI) contamination were found to be capable of mediating aerobic, chromate reduction. Results suggest that Cr(VI)-reducing aerobes are
ubiquitous in soil and thus may have a role in Cr(VI) bioremediation.
Extreme Tolerance to Hexavalent Chromium by a Fungal Isolate
Hexavalent chromium [Cr(VI)] is toxic and highly mobile in soil and water systems, whereas trivalent chromium [Cr(III)] is considered
relatively nontoxic and precipitates as almost insoluble oxides and hydroxides above pH 5.5. The objectives of this study were to determine
the Cr(VI) tolerance of a fungus isolated from a Cr(VI)-contaminated soil [25,100 mg L-1 total Cr, and 12,400 mg L-1 Cr(VI)], and to
determine the capacity of this isolate to remove Cr(VI) from solution when grown in a liquid culture. The fungal isolate exhibited growth in
flask cultures amended with 0, 500, 1,000, and 2,000 mg L-1 Cr(VI). After 31 days incubation, the fungal cultures spiked with 500 mg L-'
Cr(VI) reduced the solution Cr(VI) concentration 45% (from 529 to 293 mg L-1), while the cultures spiked with 1,000 mg L-1 Cr(VI) reduced
the solution concentration 19% (from 1,070 to 861 mg L-1). Although the fungus exhibited growth at 2,000 mg L-1 Cr(VI), at this
concentration Cr(VI) was not reduced. The extreme Cr(VI)tolerance of the fungal isolate combined with its ability to remove Cr(VI) from
aqueous solution suggest that fungi may be a critical component of indigenous microbial consortia capable of reducing Cr(VI) in polluted
soils.
Dissertation Abstract; 1998
Gamma-ray Characterization of Uranium-series Nuclides, and Its Application to the Study of the Pena Blanca Natural Analogue Site, Chihuahua, Mexico
Wong, Virgina
Department of Geological Sciences
Two natural analogue sites located in the Pena Blanca Uranium District, Chihuahua, Mexico were characterized for radionuclide mobility. Analogue I is used to assess the long-term behavior of uranium-series nuclides in a host rock and geochemical environment that is similar to the proposed high-level waste repository at Yucca Mountain, Nevada. Analogue 11 represents a former dump site to assess short-term radionuclide mobility. Gamma-ray spectroscopy analysis was used to measure radioactivity of the U-series nuclides.
Samples analyzed from Analogue I consist of: 1) fracture-infillings associated with different alteration assemblages collected within and outside the breccia pipe from various levels of the deposit and 2) fracture-infillings collected along an east-west trending fracture which intersects the breccia pipe and extends into the host rock. Alteration mineralogy, established via X-ray diffraction analysis, consists of pure kaolinite, a mixture of Fe-oxyhydroxide (goethite, hematite) with inclusions of jarosite and alunite, and carbonates.
Results from activity ratios of 23OTh/238U versus226Ra/23OTh indicate that the majority of the Fe-oxyhydroxides from the breccia zone show a slight disequilibrium with respect to Ra enrichment and U depletion. This observation is modeled as requiring a multiple-event history of U mobility. An amorphous Fe sample distal to the breccia zone shows similar behavior but to a greater extent. This extreme behavior is ascribed to initially low U content and greater late stage U removal. Two Fe-oxyhydroxide samples from within the breccia pipe also display multiple-event stages but exhibit both Ra and U leaching. This behavior is shared by Fe-oxyhydroxide samples collected inside and peripheral to the breccia zone from the east-west trending fracture. Finally, three samples, two Fe phase samples outside the breccia zone and a kaolinite inside the breccia zone, show Ra and U enrichment. Also, a distal Fe-oxyhydroxide sample from the east-west fracture shares this mobility trend. The enrichment event is modeled as a single-stage Ra and U precipitation event.
Activity results from surface samples collected from Analogue 11 strengthen the mobility trends documented from Analogue 1. Isotopic data reveal leaching of U and Ra from the stockpile into the alluvium and bedrock. Thorium, on the other hand, has not experienced mobility. Lead exhibits variable mobility behavior.
Correlation of isotope data with mineralogy indicates that the Fe-oxyhydroxide and clay minerals appear to be excellent scavengers of U. This
observation has been documented in other natural analogue studies, for example, the Oklo natural reactor, the Pocos de Caldas Natural
Analogue Project, the Koongarra U deposit, and the Shinkolobwe U deposit.
A Palynological Analysis of Upper Jurassic Dinoflagellate Cysts from the Chihuahua Trough and Bisbee Basin, Northern Mexico and Southeastern Arizona
Olmstead, Gary
Department of Geological Sciences
Two Upper Jurassic dinoflagellate cyst assemblages, including 25 genera and 17 species from the La Casita Formation in Cerro Los Panales, Chihuahua, Mexico, and 25 genera and 34 species from the Basin Trail Shale Member of the Crystal Cave Formation in the northeastern Chiricahua Mountains, southeastern Arizona, are defined in this study.
These dinoflagellate cyst assemblages suggest that an active exchange of dinoflagellate taxa was occurring from the western Tethyan, Boreal, and Euro-Atlantic paleo-oceanographic provinces into the Chihuahua trough and Bisbee basin during Late Jurassic time. These assemblages have direct affinities with dinoflagellate cyst taxa occurring in northwestern Europe, England, and southwestern Atlantic Ocean.
Concurrent age ranges of key dinoflagellate cysts in the Basin Trail Shale Member suggest marine incursion into the Bisbee basin by Middle Oxfordian time (base of the Tenuisseratum Zone) and extending to the Oxfordian-Kimmeridgian boundary (base of the Baylei Zone). Concurrent age range of dinocysts in the La Casita Formation suggest marine incursion into the region during Early Kimmeridgian time (Baylei Zone through Eudoxits Zone). Shifting shelf conditions during periods of transgressive-regressive sea level changes resulted in fluctuating dinoflagellate cyst populations. Proximate and proximochorate cysts dominate over cavate and chorate forms suggesting proximal to middle neritic zone environments. Upper Jurassic strata in the northeastern Chiricahua Mountains and Cerro Los Panales can be correlated with the Bossier and Haynesville formations and Cotton Valley Group of the United States Gulf Coast and with an unnamed unit and Cat Gap Formation in the Blake-Bahama Basin southwestern Atlantic. These correlations provide an important stratigraphic link between Upper Jurassic stratigraphy of northern Mexico, the United States Gulf Coast, and southwestern Atlantic Ocean.
An Integrated Geophysical Study of North African and Mediterranean Lithospheric Structure
Dial, Paul
Department of Geological Sciences
This dissertation utilizes gravity and seismic waveform modeling techniques to: 1) determine models of lithospheric structure across northern African through gravity modeling and 2) determine lithospheric and crustal structure and seismic wave propagation characteristics across northern Africa and the Mediterranean region.
The purpose of the gravity investigation was to construct models of lithospheric structure across northern Africa through the analysis of gravity data constrained by previous geological and geophysical studies. Three lithospheric models were constructed from Bouguer gravity data using computer modeling, and the gravity data was wavelength-filtered to investigate the relative depth and extent of the structures associated with the major anomalies. In the Atlas Mountains area, the resulting earth models showed slightly greater crustal thickness than those of previous studies if a low density mantle region is not included in the models. However, if a low density mantle region (density = 3.25 g/cm3) was included beneath the Atlas, the earth models showed little crustal thickening (38 km), in accord with previous seismic studies. Any mantle plume beneath the Hoggar, Darfur or Tibesti domal uplifts was found to be much less extensive than the mantle plume beneath the East African Rift system (EARS), because the negative Bouguer gravity anomalies associated with these features are only -50 mGal. Perhaps a plume head has dissipated leaving only a small plume tail beneath the Hoggar, although no hotspot track is readily evident across northern Africa. This study showed that the north African crust and mantle has densities (and most likely compositions, if the global relationships are accurate) similar to most continental shields and platforms. With the absence of seismic studies in northern Africa, the gravity data provided the best method for determining regional lithospheric structure and estimates of compositions.
The second portion of the research consisted of seismic waveform modeling of regional and teleseismic events to determine crustal and lithospheric structure across northern Africa and the Mediterranean. A total of 174 seismograms (145 at regional distances (200-1400 km) and 29 with epicentral distances exceeding 1900 km) were modeled using I-D velocity models and a reflectivity code. At regional distances from four stations surrounding the western Mediterranean basin (MAL, TOL, PTO and AQU) and one station near the Red Sea (HLW), I-D velocity models can satisfactorily model the relative amplitudes of both the Pn1 and surface wave portions of the seismograms. This method using simple 4-layer velocity models appears to break down at epicentral distances beyond 1400 km. In general, the western Mediterranean effectively blocks the propagation of surface waves (Lg), and thus, the surface wave amplitude and duration are generally overestimated by the I-D models. This blockage is not observed at station AQU on the Italian Peninsula. The western Mediterranean was found to be underlain by relatively low velocity mantle (Vp = 7.7-7.85 km/s). The Iberian Peninsula can be modeled with a crustal thickness between 30 and 35 km over propagation distances of at least 600 km.
Modeling of propagation paths greater than 1900 km was also conducted across northern Africa and the Mediterranean. The results indicate that the S-wave velocity model of Corchete et al. (1995) is more appropriate for the Iberian Peninsula, southwestern Mediterranean basin and northwest African coast than the other models tested. This model was better able to predict both the timing and amplitudes of the observed Sn and surface wave components on the observed seismograms. The model of Pino and Helmberger (1997) was derived for propagation paths primarily across the northern Mediterranean (north of 35oN latitude), while most of the propagation paths of this study cross the Mediterranean south of 35oN latitude. Thus, there appears to exist a significant change in upper mantle structure and/or seismic wave attenuation between the Mediterranean north and south of 35oN latitude. All of the propagation paths investigated across the Mediterranean and northern Africa showed strong Sn phases. In order to model the high amplitudes of these phases, it was necessary to introduce very high Q values (Qp > 1000 in the asthenosphere and Qp > 1400 below this) and/or high velocity layers (Vs > 5.0 km/s) at relatively shallow depths (less than 350 km). As the models are only I-D, perhaps part of the inability to model the Sn phase may be due to strong lateral (2-D) changes in crustal and upper mantle structure in the study area.
A Seismic Tomographic Study of the Mojave Region and Geophysical Constraints on Thrust Belt Structure in Eastern Tennessee
Whitelaw, Julia
Department of Geological Sciences
This dissertation uses geophysical methods to 1) determine the velocity structure of the Mojave region and 2) study thrust belt geometry in the Valley and Ridge province of eastern Tennessee.
The complex geologic and tectonic history of the Mojave region and southem Basin and Range suggest that the velocity structure may be complex. The Mojave block and the southern Basin and Range have had a similar tectonic history, however, geophysical properties and crustal extension of the two provinces are different. This study employs a tomographic inversion using P wave first arrival times from earthquakes and explosions recorded on the Southern California Seismic Network, the Northern California Seismic Network, and the Southern Great Basin Seismic Network to determine the velocity structure and its effect on wave propagation through the Mojave block and southern Basin and Range. Knowledge of the velocity structure should provide insights into the differences in the crustal structure of the Mojave and southern Basin and Range. In addition, the effects of anisotropy were examined.
The tomography algorithm used in this study incorporates a priori information to add stability to the inversion, accounts for anisotropy, and includes data and model uncertainties. The actual equation solved in this inversion is unique because it includes the actual data, the a pribrimodel, both data and model uncertainties, and the residual matrix between the observed and predicted travel times, which is minimized. Synthetic data investigations have shown overestimating the data and model uncertainties is better than underestimating them. The data uncertainty has a larger control on convergence criteria than model uncertainties. Interval and statistical methods were used to estimate errors associated with data and models, as well as determining the total solution uncertainties, for the a priori starting model. Only ten successive inversions were needed to compute both the information density and the total solution uncertainty. Estimates did not change significantly after iteration eight. The computer time used for this technique is significantly less than that of traditional error analysis methods used in geophysics. The information density coverage differs significantly from the hit density, indicating that the hit count does not provide a good estimate of solution uncertainty.
Anisotropy developed at depths shallower than 15 km using the one-dimensional model. However, no anisotropy developed at any level using the a prioristarting model. Neither model developed anisotropy at depths below 15 km. Crustal velocities of the western Mojave are similar to those of the southern Basin and Range, in areas resolved with raypath coverage (5-20 km depth for the one-dimensional model and 5-10 km depth for the a priori starting model). Basin and Range low velocities correlate well with areas of high heat flow; however, the Mojave Block has lower heat flow than the Basin and Range in areas with low velocities. High velocities in the eastern Mojave exist in an area containing the highest heat flow values (>100 mWm-1). The Garlock Fault appears to separate areas of lower velocities of the southern Basin and Range from higher velocitie s of the Mojave block at 35-40 km depth for the one-dimensional model and 30-40 km for the a priori model, suggesting that it may be a deep-seated feature. The distributions of velocities at these depths correlates well with heat flow. These velocity observations in the mantle reinforce the suggestions by other researchers that the Basin and Range is more active than the Mojave Block.
The second part of the dissertation integrates gravity data, seismic reflection profiles, and surface geology in order to study the thrust-sheet geometry in the Valley and Ridge Province of eastern Tennessee. These data are used as key constraints in the construction of balanced cross-sections in two locations across the province. The well-studied geology of eastern Tennessee provides an excellent opportunity to test the hypothesis that the integration of seismic reflection and gravity data will lead to a more reliable interpretation of the subsurface. This study also documents how each method affects and constrains an interpretation.
Balancing cross-sections adds subsurface constraints to an interpretation by limiting the suite of possible structures along the line of sections. The odds of a correct interpretation can only be increased by constructing the cross-sections using additional geologic and geophysical constraints.
Seismic reflection data provide excellent constraints in areas with good data quality. "Crooked line" thrust belt acquisition geometry often yields the best reflections in areas oblique to the dip direction, providing valuable constraints that can be interpolated into a straight-line profile to balance. Data acquired in this geometry may also allow a 3-dimensional look at the lateral characteristics and continuity of a thrust sheet. Pitfalls of this method include possible errors in the depth and thicknesses of the units interpreted from depth sections produced from inaccurate velocity models. In addition, velocity pull-ups and sags maybe difficult to distinguish from true geologic structure, without further constraints from other methods.
Gravity data/modeling was a significant asset in helping constrain thrust belt geometry and unit thicknesses, when used in conjunction with
balanced sections and seismic data. This method is strengthened when good density control is available in the area of interest. The distribution
of data along a profile may yield valuable insights into the third dimension; however, this study has shown that the data projected into a
profile must be carefully examined before use in modeling.
Dissertation Abstract; 1998
Evaluation of H2S formation in the Basal Zechstein
Steinhoff, Ingo
Department of Geological Sciences
This dissertation consists of an introductory chapter (1) and three following chapters (2, 3, and 4) which focus on problems concerning H2S (sourgas) encountered in the Basal Zechstein (Upper Permian) of northwest Germany.
Chapter 2 introduces a model of gas souring during migration, an alternative to the commonly reported model of in-place-souring after the gases have accumulated within deep-buried carbonate reservoirs.
H2S concentrations of more than 5% in deep carbonate gas reservoirs generally are attributed to thermochemical sulfate reduction (TSR). The Zechstein 2 Carbonate reservoir in NW Germany bears gas fields with H2S concentrations ranging from 0% to 40%. The reservoir is underlain by up to 400 m of sulfate of the Werra Anhydrite. Based on petrographic, chemical, and isotopic evidence, this served as the source rock for most of the H2S found in the overlying carbonate reservoir. Hydrogen sulfide and hydrocarbons that were not oxidized during sulfate reduction migrated up section out of the Werra Anhydrite to accumulate in the Zechstein 2 Carbonate reservoir. During migration up and through the Werra Anhydrite, sweet gases (preferably ethane and propane) contacted and reacted with the sulfates which comprised the force system to produce H2S.
Chapter 3 concentrates on an ancient sulfate depositional system in the Upper Permian Zechstein 1 Cycle (Zl), the Werra Anhydrite, underlying the carbonate reservoir from which the sourgases are produced. It acts as a conduit for the migrating gases within the petroleum system of the Germanic Basin.
Well cores of seven wells from the Southern Zechstein Basin were examined for their structural and textural features. Facies interpretation was based on cut core material and thin sections.
Four main facies types, (I) supratidal, (II) intertidal, (III) shallow subtidal, and (IV) deeper marine, are further subdivided into 10 subfacies types: (1) karst and (2) sabkha within the supratidal environments, (3) algal tidal-flat, (4) tidal flat, and (5) transgressive deposit within the intertidal main facies type, (6) salina, and (7) sulfatic arenites within the shallow subtidal environment. Subfacies types of the (8) slope commonly associated with (9) turbidites and the (10) basin subfacies type subdivide the deeper marine depositional environment.
Vertical stacking patterns of these facies and subfacies types reveal the sequence stratigraphic development of the sulfatic system in response to sea-level fluctuations. The lower Werra Anhydrite (belonging to Zechstein Sequence ZS2) is characterized by a transgressive systems tract overlying the transgressive surface within the underlying upper Zechstein Limestone (Cal). It shows thicknesses of several tens of meters in platform areas and thins out to a few meters of thickness toward the condensed basinal section.
Most of the A1 succession is represented by three relatively thick parasequences belonging to the highstand systems tract that shows typical prograding sets. Enhanced platform buildup, and increased highstand shedding produced marginal "sulfate walls" up to more than 400 m thick as platform/slope portions of the Werra Anhydrite. Increasingly pronounced A1 bathymetries during highstand narrowed the slope subfacies belt aligned to the course of the platform margin. This contrasts with the broad but considerably thinner slope deposits of transgressive times with much shallower slopes.
The following sea-level lowstand is reflected by a type-I sequence boundary on top of the karstified A1 platform (Zechstein Sequence ZS3) and a lowstand wedge overlying portions of the slope and basinal subfacies of the Al highstand systems tract. Beyond the lateral limits of the lowstand wedge, the sequence boundary merges with the transgressive surface of ZS3, represented by the lithologic change from the Al to the overlying Stassfurt Carbonate (Main Dolomite, Ca2).
The Basal Anhydrite (A2), which overlies and seals the carbonate reservoir of the Ca2, can also be subdivided into sequence stratigraphic units by means of facies analysis. It is, however, much less complex than the Al and consists almost exclusively of a transgressive systems tract of Zechstein Sequence ZS4.
Chapter 4 focuses on pyrites as indicators of BSR and TSR in the Basal Zechstein. The Werra Anhydrite (A1), the sulfate member of the Zechstein I Cycle, yields a complex record of diagenetic alteration. Two generations of pyrite reflecting two episodes of sulfate reduction can be reported from the A1: (1) an early pulse of sulfate reduction which led to precipitation of framboidal aggregates of pyrite, up to 70 um in diameter, during eogenetic diagenesis; and (2) a later prismatic generation of pyrite which commonly replaces secondary anhydrite, with individual pentagon-dodecahedrons up to 40 um in diameter; it also occurs as thin overgrowth (a few microns) on the older framboidal pyrite.
These two generations of pyrite also are found within the overlying carbonate member of the Zechstein 2 Cycle. A third type of blocky-prismatic pyrite comprising aggregates of up to cm-length is found exclusively within the Zechstein 2 Carbonate (Ca2).
No significant differences in elemental composition among the three generations of pyrites could be detected by electron microprobe analysis.
Three distinctive ranges of delta34S values were acquired for the three pyrite phases by SIMS: (1) -25.8% to -10.8% CDT for the first, framboidal pyrite; (2) -43.2% to -34.9% CDT for the second, prismatic phase; and (3) +2.5% to +8.5% CDT for the third, blocky pyrite found only within the Ca2.
Since delta34S values of the parent sulfates in the Basal Zechstein range from +10% to +12% CDT (Claypool et al. 1980), the first two pyrite phases are interpreted to represent two distinct episodes of biological sulfate reduction (BSR), whereas the third generation of pyrite was derived from thermochemical sulfate reduction (TSR) (Machel et al. 1995).
The second generation of pyrite might be misinterpreted as a product of TSF, based on its prismatic appearance (Machel 1989) and its petrographic relationship to the surrounding matrix of secondary anhydrite. Its sulfur-isotopic composition, however, shows that this generation of pyrite is a precipitate of sulfur from a biologically induced sulfate reduction during a later stage of diagenesis within the Basal Zechstein.
These S-isotopic analyses suggest that the prismatic habit of pyrite alone is an unreliable indicator of origin.
Stratigraphy, Geochronology, Geochemistry, and Tectonic Setting of the Nazas Formation, North-central Mexico
Bartolini, Claudio
Department of Geological Sciences
Late Triassic to Middle Jurassic volcanic-sedimentary sequences that were part of the Mesozoic continental-margin of western North America are exposed in northern and central Mexico. These sequences have been grouped into the Nazas Formation and crop out in the states of Durango, Coahuila, Zacatecas, and San Luis Potosi. The Nazas Formation consists of 2,500 m or more of volcanic and pyroclastic rocks and interbedded clastic sedimentary rocks that were deposited in alluvial fan and fluvial depositional systems that developed in intra-arc basins, mainly fault-bound grabens and topographic depressions within an extending Mesozoic volcanic arc. Major and trace element geochemistry of volcanic rocks suggests that the volcanic suite is calc-alkaline and includes rhyolite, dacite, rhyodacite, andesite, trachyandesite and rare basalt. Pyroclastic rocks are basically air-fall tuffs and volcanic breccias. The sedimentary strata include conglomerate, sandstone, shale, and siltstone, locally red in color. Geochronology (Ar-Ar, K-Ar and Rb-Sr) and field evidence indicate that the age of the Nazas Formation ranges from Late Triassic to Middle Jurassic, but the peak of arc volcanism appears to be Early and Middle Jurassic. Petrographic studies of sandstone and conglomerate interbedded with the volcanic rocks indicate that the terrigenous particles were derived principally from the volcanic rocks of the arc, and to, a lesser extent, from the clastic strata interbedded with the volcanic rocks. Locally, the sandstones and conglomerates of the Nazas contain detrital grains that were derived from pre-Nazas source areas such as the Upper Triassic Zacatecas Formation, in Zacatecas, from Upper Paleozoic crystalline basement (Durango), and even from pre-Triassic sedimentary sources nowhere exposed on the surface.
Arc extension, volcanism and continental sedimentation were followed by a regional episode of structural deformation affecting the entire magmatic are in northern and central Mexico. This tectonic event is recorded by a general northwest-trending foliation imprinted on the Nazas Formation and older rocks. The timing of compressive deformation that produced this regional foliation can be bracketed to the early Oxfordian, apparently contemporaneous with the deposition of the basal part of the Zuloaga Limestone. This regional deformational event is presumably linked to northwest-trending transpressive structures that developed along a transpressive margin which resulted in the development of parallel transpressive structures along the entire length of the magmatic arc without offsetting it. The development of such a margin would eventually be produced by oblique subduction of the Farallon plate underneath the North American plate in early Late Jurassic. The construction of a tectonic model to explain the compressive deformation in the Mesozoic volcanic arc is not feasible at this time, however, it is reasonable to assume a linkage with transpressive activity.
The Mesozoic magmatic arc in Mexico has a northwest trend and extends from Sonora to Chiapas. The arc structure is more than 2,000 km long, and possibly up to 150 km wide. The width of the arc is uncertain due to the limited number of surface outcrops, however, it did not extend east into the western Gulf of Mexico. Arc-related magmatism began in latest Triassic time, but the peak of arc evolution occurred during the Early and Middle Jurassic. By Oxfordian time, the arc was deeply dissected and eroded, and magmatic activity had ceased. Transgressive marine waters from the Gulf of Mexico covered most of the Nazas arc, depositing the initial marine sediments of the Oxfordian Zuloaga Limestone in the Mexican Geosyncline. Jurassic crustal extension in the Gulf of Mexico was contemporaneous with extension along the continental margin volcanic arc in inland Mexico. Both extensional domains are the result of distinct tectonic mechanisms, evolved differently and have given rise to different rock sequences; volcanic-sedimentary sequences formed within the arc, and red beds and evaporite deposits formed in the Gulf domain. Whether the overlapping in space of Gulf-related extension and arc-related extension occurred can not be documented.
In eastern Mexico, Late Triassic red beds of the La Boca Formation were deposited in rift basins, whereas the La Joya and Mnas Viejas formations (Middle to Late Jurassic) record transitional depositional environments, basically lacustrine, alluvial and fluvial facies. The Late Triassic age of the La Boca suggests that rifting may be related to the break up of Pangea, whereas the Middle Jurassic strata of La Joya Formation represent rift basin deposits contemporaneous with the opening of the Gulf of Mexico. These strata are continental deposits with insignificant volcanic components. Despite the fact that the rocks of the Nazas Formation and Huizachal strata are partly contemporaneous, they represent two distinct tectonic settings: the Nazas volcanic-sedimentary sequences are part of the Cordilleran magmatic arc, and the red beds of the Huizachal Group are rift basin deposits.
Mapping of the Sierra de San Julian area was done utilizing Landsat thematic mapper images. This area has the largest outcrops of the
volcanic arc in central Mexico. The volcanic- sedimentary sequences in this particular locality are grouped into three formations that, from
older to younger, are the Caopas, the Rodeo, and the Nazas formations. These rocks record pre-Zuloaga and Laramide deformation. However,
reconstructing the structural deformation in the Sierra de San Julian is hampered by the extensive erosion that occurred in the Tertiary.
Dissertation Abstract; 1998
Leonardian (Permian) Bioherms of the Wilkie Ranch Formation, Finlay Mountains, Hudspeth County, Texas
Rathjen, James David
Department of Geological Sciences
Leonardian bioherms occur in the Wilkie Ranch Formation in the Finlay Mountains of West Texas. These bioherms are exposed at different levels within the 533-m-thick Wilkie Ranch Formation and are the only autochthonous Leonardian exposures of the western Diablo platform/eastern Marfa basin. Bioherms were sampled at three different stratigraphic horizons on the outcrop. Bioherm 'F' crops out at 183 m, Bioherm "D" at 297 m, and Bioherm "C" at 357 m; all are measured from the subsurface base of the section. These bioherms at the differing stratigraphic horizons have unique biotic assemblages which reflect changing environmental conditions within the Marfa basin.
Bioherm "F" was the lowest bioherm sampled in the section. The "F" bioherm is characterized as being the most diverse in terms of numbers and types of marine organisms, has the highest percentage of sponges, and is dominated by the problematic encrustor Tubiphytes. In addition, large numbers of silicified sponge root "tufts" referred to as Hydraductus were observed from this lower horizon.
Bioherm "D" was sampled approximately mid-section and is dominated by Tubiphytes encrustations associated with a stressed faunal assemblage. The " stressed assemblage" refers to reduction in numbers and types of organisms found within a bioherm. Hydraductus organisms were documented but are far less numerous than in the lower bioherms.
The highest bioherm in the section sampled was the "C" bioherm. It is Tubiphytes-encrusted with a "restricted" faunal assemblage. The dominant animals in this highest bioherm are crinoids.
At all three stratigraphic horizons investigated, initial bioherm development began on piles of bioclastic debris which were stabilized by the
problematic encrusting organisms Tubiphytes and Archaeolithoporella. Once the substrate was stabilized, subsequent ecological niches were
established that were inhabited by a normal marine fauna including sponges, brachiopods, echinoderms, and bryozoa. All three stratigraphic
levels where bioherms were investigated are interpreted to have developed in relatively deep-water, as evidenced by the absence of green algal
forms. Bioherms at all three levels did maintain stratigraphic relief above the sea floor bottom and were rigid enough to withstand high energy
events as is documented by their intimate association with Diablo platform conglomeratic debris that commonly surrounds and caps the
bioherms. Bioherm growth ceased with deepening of the basin waters and/or with an increased influx of mud-sized particles which fouled
filter-feeding organisms.Differences in the biotic make-up of the bioherms at the different stratigraphic levels reflect differing and changing
conditions within the Marfa basin. Optimal conditions existed low in the section as is reflected by the diverse make-up of the fauna.
Fluctuations in sea level (possibly third-order sequences) led to deposition of conglomeratic units followed by an overall deepening of the
basin in which bioherm development was re-established but with different environmental conditions which resulted in a "stressed" fauna
midsection ('D'). Repeated changes of sea level again resulted in an influx of siliciclastics followed by another deepening and renewal of
bioherm growth, but the uppermost bioherm ('C"). Leonardian bioherms of the Wilkie Ranch Formation in the Finlay Mountains of West
Texas represent a transitional phase of bioherm development from the shallow-water calcareous algal buildups of the Pennsylvanian to the
shallow-water sponge/calcareous algal forms from the Late Permian. The Middle Permian Wilkie Ranch bioherms are transitional in that they
were constructed primarily of encrustors such as Tubiphytes andArchaeolithoporella and sponges which are quite common in the Late
Permian bioherms, but they lack the shallow-water algae common in their predecessors.
Dissertation Abstract; 1999
Evolution of Quaternary Intraplate Mafic Lavas Detailed Using 3He Surface Exposure and 40Ar/39Ar Dating, and Elemental and He, Sr, Nd, and Pb Isotopic Signatures: Potrillo Volcanic Field, New Mexico, U.S.A., and San Quintin Volcanic Field, Baja California Norte, Mexico
Williams-W, Wendi J.
Department of Geological Sciences
Part I: Potrillo Volcanic Field, New Mexico, U.S.A.
The Pleistocene Potrillo volcanic field (PVF) resides within the southern axis of the Rio Grande Rift, New Mexico, U.S.A., near the eastern
extent of the Basin and Range Province. Its alkalic mafic volcanism has resulted in several hundred cones, flows and maars distributed over
approximately 4,600 km2. Alignments are segregated into two halves relative to the East Robledo fault system that dissects this field. Three of
the five maars have brought peridotitic and lower to upper crustal xenoliths to the surface; several older, non maar-related flows from the west
half of the field host ultramafic clots. Stratigraphic relationships, 3He surface exposure dating and 40Ar/39Ar methods confirm this area has
been active since 1 Ma ago to as recently as 20 ka ago. The Kilbourne Hole maar was dated at 28 ka by the acquisition of exhumation ages of
deposits using age differences between syn-maar and proximal non-maar deposits.
Elemental and isotopic signatures indicate source heterogeneity coupled with varying degrees of partial melting and polybaric crystal fractionation events. Melts underwent early clinopyroxene fractionation within the mantle, but then the easternmost volcanic complexes experienced a second, shallow-level olivine fractionation history. Fundamental differences existing between the east and west half of the field are further established by 87Sr/86Sr (0.703087 - 0.703917), 143Nd/144Nd (4.4 - 6.7 Nd) 206Pb/204Pb (18.363 - 19.081), 207Pb/204Pb (15.520 - 15.584), 208Pb/204Pb (38.192 - 38.699), and magmatic 3He/4He (5 - 15 R/Ra) isotopic data for ten lavas. Two isotopic groups are observed: Group I is characterized by higher Nd and Pb isotopes and lower 87Sr/86Sr. Group II melts display the opposite trends. At least three mantle reservoirs may have contributed to the melts: (1) PREMA, (2) HIMU, and (3) either EM1 or Lower Crust for Group II. The primitive (Mg # < 64) lavas have relatively high Al2O3 and Yb associated with low La/Yb and CaO/ Al2O3, similar to the San Quintín volcanic field in Baja California Norte, México (Luhr et al., 1995). These observations are consistent with progressive partial melting of spinel lherzolite at unusually shallow mantle levels.
A complex magmatic history for the Potrillo volcanic field is now elucidated from combining improved Quaternary dating methods with detailed geochemical studies and structural information. The magma dynamics model integrates temporal, spatial and chemical evidence in light of magma emplacement and neotectonic parameters. Five phases of volcanism are recognized. The punctuated volcanic activity, presence of both monogenetic and polygenetic centers, and evidence for shifting eruption foci across 30 km lateral distances during approximately 106 year time frames are explained in terms of a crack coalescence model by Takada. Changes in magma input rates and/or differential stress fields are invoked. These findings for the Potrillo volcanic field are all causes for concern with respect to prediction of future activity trends within small mafic fields in intraplate extensional terranes.
Part Ii: San Quintín Volcanic Field, Baja California Norte, México
A better understanding of eruption ages, episodicity, and spatial distribution of Quaternary volcanism can allow detailed petrogenetic
modeling and assessment of neotectonic events. Previous 3He surface exposure dating of young, mafic volcanism within the southwestern
United States has demonstrated the promising application of this method to pahoehoe surfaces located above 1000 meters elevation. We have
now gathered helium data for the San Quintín field in México to determine not only the timing of eruption events, but also to evaluate this
dating method for samples collected from both aa and pahoehoe surfaces located near sea level. Multiple lava flows from 9 of the 10 exposed
complexes were sampled for cosmogenic helium dating, yielding eruption dates from 165 ± 13 ka to 22 ± 5 ka (1 ). Two
major episodes of volcanic activity occurred along parallel lineations trending roughly N40oW. The younger trend of three centers (31
± 4 ka, 27 ± 5 ka and 22 ± 5 ka) steps outboard to the southwest from the older alignment. The older dated surfaces
range from 165 ± 13 ka to 84 ± 10 ka, where these bracketing dates happen to be for two distinct lavas associated with
nested cinder cones comprising the Woodford complex. The data provide quantitative geochronologic evidence for multiple eruptions (i.e.
polycyclicity) from a single mafic volcanic center which are separated by a time gap discernible within the resolution of the helium technique.
Comparison of 3He to 40Ar/39Ar dating for the field yields agreement within the uncertainties of the these dating methods: 1) a flow from the
Kenton complex yields a helium date of 145 ± 14 ka and argon date of 126 ± 8 ka and 2) for a Media Luna flow, 3He yields
106 ± 9 ka (weighted average of two surfaces from the same flow) whereas the step-heated argon analysis of groundmass results in
90 ± 10 ka. An interesting petrologic insight is provided by the helium trapped in the olivine and clinopyroxene mineral separates.
Geochemical and isotopic signatures documented by Luhr and others (1995) support incorporation of crustal components by ascending
magmas forming Ceniza and Monte Mazo centers. Lavas from those two centers and the other 7 centers we analyzed yield 3He/4He values
ranging from 4.9 ± 0.9 R/Ra to 7.4 ± 1.1 R/Ra, which may reflect addition of crustal helium (<0.1 R/Ra) to melts derived
from a MORB (8 ± 1 R/Ra) source. However, the San Quintín trapped component determinations do overlap within analytical error.
If the 5.52 ± 2.42 weighted average is considered instead of individual R/Ra values for this field, the trapped component composition
may represent the low end of the range reported by Graham and others (1992) for MORB rather than a crustal signature.
Studies of the Proterozoic Tectonic Evolution of the Southwestern United States
Seeley, John Martin
Department of Geological Sciences
This dissertation involves four separate investigations whose goal is to contribute to a better understanding of the Proterozoic evolution of southwestern North America. Chapter I provides an overall introduction; Chapters 2-4 present specific results from three different study areas; and Chapter 5 presents a synthesis and a tectonic model for the region.
Chapter 2 utilizes gravity and magnetic techniques in an integrated study to delineate subsurface occurrences and better define the
structural setting of Unkar and Chuar Group rocks in northern Arizona and southern Utah.
Mesoproterozoic Unkar Group (~1.25 to 1.1 Ga) and Neoproterozoic Chuar Group (0.9 to 0.7 Ga) sedimentary basin deposits east and
southeast of the Grand Canyon were delineated using Bouguer gravity and residual aeromagnetic anomaly data, in combination with gravity
profile modeling, seismic velocity analysis, preparation of geologic cross-sections, analysis of surface geologic structures, and analysis of
drilling data. Delineation of Proterozoic extensional trends within this region provides insights in to Proterozoic paleogeography along the
Southwestern margin of Laurentia.
Two phases of Proterozoic extension resulted in the formation of two structural trends: 1) a NW-SE Unkar-related trend indicating NE-SW extension throughout the area, and 2) a predominately N-S Chum-related trend indicating E-W extension, primarily in northern regions. All structures developed during these extensional cents were strongly influenced by a more ancient structural grain comprised of multiple and parallel linear NE-oriented trends. These basement zones are interpreted to be major deep-set faults of probable Paleoproterozoic strike-slip origin.
Unkar sediments were deposited on the Vishnu terrane from -1.25 to 1.1 Ga and were subsequently downdropped into grabens and half grabens during the first of the two extensional events. This first event was concurrent with extension of the Cardenas Basalt at ~1.1 to 1.0 Ga. Subsequently, long periods of erosion separated subsequent minor extensional events during afflicted the deposition of Nankoweap Formation rocks. The second major phase of extension region from ~0.9 to 0.7 Ga., during which time Chuar Group sediments were deposited in elongate basins created by their activation of older NE- and NW-trending faults. This protracted period of extension was eventually brought to a close by uplift and subsequent extension at ~0.7 Ga that resulted in the deposition of the overlying Sixtymile Formation.
The Proterozoic Chuar Group has gained attention as a potential hydrocarbon source due to high TOC (total organic carbon) concentration in the 3-9% range, seen primarily within the Walcott Member of the Kwagunt Formation. This potential source rock, combined with excellent reservoir potential of the overlying Cambrian Tapeats Sandstone, with the seal consisting of the overlying Cambrian Bright Angel Shale, comprise an intriguing three part petroleum system currently being explored within southern Utah. Prospective structures include anticlines of Laramide age that have not been drilled through the Tapeats interval. One result of this study establishes the presence of Chuar-age basins in regions south of the Utah state line, thereby redefining the southernmost boundary of this potential hydrocarbon play within the study area in north-central Arizona,
Chapter 3 focuses on the geologic mapping and areal distribution of the Mesoproterozoic Apache Group, Troy Quartzite and
associated coextensive diabase in south-central Arizona. Landsat Thematic Mapper imagery in combination with existing geologic
maps of the region were utilized for this study.
Middle Proterozoic sedimentary and intrusive igneous rocks are well represented in central and southern Arizona by the Apache Group, Troy
Quartzite, and coextensive diabase. The Proterozoic sedimentary rocks of the Apache Group and Troy Quartzite lie in structurally complex,
broken, and scattered outcrops intruded by diabase throughout the region. This study utilizes Landsat Thematic Mapper (TM data along the
existing geologic maps of the region, to define the areal distribution of these units. Results obtained were used to construct a concise,
regional-scale photo-geologic map of exposed Middle Proterozoic sedimentary rocks of the region. This map is not intended to provide
detailed outcrop information for these structurally complex rocks, but instead presents both a regional compilation of previous geologic
mapping efforts, and a summary of new interpretations of outcrop geology not previously accurately defined. The spectral capabilities of
Landsat TM data provided new information that could not be defined by conventional aerial photography or previous field studies within the
region.
Chapter 4 introduces a sequence stratigraphic model for siliciclastic rocks of the Mesoproterozoic Lanoria Formation of the
Franklin Mountains of westernmost Texas. Facies associations have been interpreted from previous studies, and a ew formation has
been tentatively described and named.
The Mesoproterozoic (Stenian) Lanoria Fomation located in the Franklin Mountains of westernmost Texas, is comprised of over 700 m of
metamorphosed marine shelf and marginal marine sandstone, siltstone and mudstone. The formation is divided into a sequence of six
alternating members (L1 through L6). An investigation of physical sedimentary structures and lithologies of the Lanoria Formation has
identified four primary facies associations 1) proximal and distal marine shelf, 2) lower, middle, and upper tidal flat; 3) incised valley fill;
and, 4) estuarine/deltaic.
Six stratigraphic sequences, each containing multiple parasequences, have been interpreted in the Lanoria Formation. Boundaries between individual sequences are marked by basinward shifts in facies and/or regional truncation. Lanoria sequences LS1, LS2, LS4 and LS6 represent marine shelf deposits. Sequence LS3 consists of incised valley fill and overlying tidal flat deposits. Sequence LS5 comprises incised valley fill overlain by estuarine/deltaic sediments.
Rocks of the Lanoria Fomation represent a series of transgressive and regressive pulses across a gently subsiding platform. Stratigraphic sequences, facies distribution, and paleocurrent data suggest that the depositional platform sloped northward, indicating a possible southern source of sediment, The shelf was probably embayed or protected from the encroachment of major waves and the shelf gradient was relatively low. Tidal processes dominated on proximal portions of the shelf, while storm processes dominated on distal portions. Extensive estuaries and tidal flats formed during lowstand landward of the shelf.
Two newly discovered bodies of rock are also described as part of this study. Approximately 20 to 60 m of distinctive, purple siltstone with minor pebbles and conglomerates that has been tentatively assigned to the overlying Coronado Hills Conglomerate of the Thunderbird Group. Up to 132 m of yellow-tan, sandstone, siltstone and mudstone, previously included with the L6 member of the Lanoria Formation, has been tentatively designated as the East Cottonwood Spring Formation. Future work will address the formal designation of this unit.
Chapter 5 of this manuscript presents a hypothesis for the Proterozoic tectonic evolution of southwestern Laurentia during the
Mesoproterozoic and Neoproterozoic.
The Proterozoic tectonic history of the southwestern United States is known only from scattered and widespread outcrops throughout this
region. As such, a convincing regional synthesis for Proterozoic rocks has not been presented to date. This paper proposes a tectonic model of
the geologic evolution of the southwestern margin of Laurentia during the Mesoproterozoic (1500 to 1000 Ma) and early-to-middle
Neoproterozoic (1000 to-700 Ma). Recent tectonic models of- Grenville-age rocks along the southern margin of Laurentia document
arc-continent and continent-continent collision ca. 1150-1120 Ma. Current interpretations, however, do not take into account the prior, and
coeval tectonic history of Proterozoic rocks of southwestern regions of Laurentia that currently comprise the southwestern United States.
This paper relies heavily on sedimentologic, structural, and geophysical evidence from Proterozoic rocks throughout the region, and on recent isotopic dating of many of these rocks. Proterozoic geologic events are divided into seven broad tectonic regimes. Mesoproterozoic events representing Calymian to Stenian periods are divided as follows regime 1) abduction along the southern margin of Laurentia resulting in felsic plutoniasm and volcanism (1500-1300 Ma); regime 2) NW-SE oriented failed intracratonic rifting across the southern margin of Laurentia (1300-1260 Ma); regime 3) minor regional compression and uplift (ca. ~1260 Ma), regime 4) rift subsidence and preferential marine flooding of previous rift basins (~1210 to 1150 MA); regime 5) multiple events related to continental collision-layered mafic intrusion, bimodal volcanism, and transpressional deformation (1163 to 1186 Ma); and finally, regime 6) regional mafic plutonism with minor volcanism and NE-SW extension in present day central and northern Arizona and southern Utah (~1086-1000 Ma). Tectonic regime 7, of early-to-middle Neoproterozoic time (Tonim to Cryogenian), is dominated by ~200 million years of gradual and punctuated E-W directed extension prior to the break-up of the Rodinian supercontinent. Major geologic events of this time are dominated by multistage extensional processes and are represented by the inboard syntectonic extensional deposition of sedimentary rocks in northern Arizona and north-central Utah. As such, the actual separation of fragments of Rodinia, the opening of the proto-Pacific Ocean, and the subsequent development of the Paleozoic passive margin, likely took place considerably later during the Eocambrian rather than at ~750-725 Ma, as most current tectonic models predict.
Selected Studies of Cretaceous Mixed Siliciclastic and Carbonate Sedimentation Patterns in Northern Mexico, Texas, and Montana
Steinhoff, Doerte
Department of Geological Sciences
Four study areas along North American Cretaceous basin margins were selected to study siliciclastic and carbonate sedimentation, fossil assemblages, facies associations, and their distribution. The Middle Albian Fredericksburg Group in northeastern Texas was deposited on a stable cratonal area along the northern rim of the Gulf of Mexico when it was influenced by the tropical waters of the Tethys ocean. The Finlay Formation in Trans-Pecos Texas is of the same age as the Fredericksburg Group but was deposited on the northeastern side of the subsiding Chihuahua trough in Trans-Pecos Texas, whereas the middle Aptian Agua Salada Formation in central Sonora developed on the western margin of the trough. The Campanian Virgelle Formation in northwestern Montana developed along the western margin of the subsiding Western Interior seaway.
The entire Fredericksburg Group (Middle Albian) is exposed at low water level conditions in an outcrop at Lake Texoma, Grayson County, Texas. The profile includes the upper Paluxy Sandstone, Walnut Clay, and Goodland Limestone. All major features of the Fredericksburg Group are exposed in a condensed section of 10 m of profile. The general depositional trend in the studied section ranges from older deltaic to younger shallow open marine conditions. The development of a terrestrial hard ground at the top of the Paluxy Sand Formation points to the presence of a sequence boundary. This sequence boundary can be correlated to the one between Upper Zuni A 1.2 and 1.3 in the Mesozoic and Cenozoic Cycle Chart of Haq et al. (1988).
The Middle Albian Finlay Formation was studied in the Rimrock escarpment in Hudspeth County, Texas. A stratotype profile for the Finlay Formation was established in addition to Brunson's (1954) unpublished, informally proposed stratotype profile at the Finlay Mountains. The new stratotype profile was stratigraphically documented and compared to other areas along the escarpment. The Lower Finlay Formation contains a succession of nodular weathering limestone layers intercalated with marls, which were deposited in a ramp depositional system. The Upper Finlay Formation includes massive, cliff-forming biowackestones to biopackstones, which contain caprinid nudists in the upper part and deposited on a carbonate platform. Two sequence boundaries can be recognized that are correlative with the major AS and minor A6 sequence boundary of the new chronostratigraphy chart of Hardenbol et al. (1998). The major sequence boundary AS is equivalent with UPPER ZUNI A 1.2 and 1.3 in the Mesozoic and Cenozoic cycle chart of Haq et al. (1988) and therefore correlates to the Paluxy Sandstone-Walnut Clay boundary within the Fredericksburg Group in northeastern Texas.
An outcrop of Late Aptian age was sampled for bivalves and gastropods in the Lampazos Sequence, which is situated in east-central Sonora, Mexico. The fossils were collected near the small village of Tepache from the upper black shale beds of the Agua Salada Formation. In this area, the Agua Salada Formation transitionally overlies the older El Aliso Formation. The lower part of the formation includes thick chert beds, which are rich in ammonites, whereas the upper part mainly consists of medium gray to dark black shale beds, which differ in resistance to erosion. The black shale is intercalated with light carbonate banks. An open shelf environment is proposed for the Agua Salada Formation.
The massive black shale in the lower Upper Agua Salada Formation is particularly rich in fossils. Six different gastropod species can be recognized, mainly belonging to the mesogastropod subclass. In addition, six different bivalve species were observed. The shallow marine bivalve and gastropod faunas show similarities to the Lower Cretaceous of Northeast Texas, Arizona, and California and point to a connection between the Gulf of Mexico and the Pacific Ocean at this time.
The lower Campanian (Upper Cretaceous) Virgelle Formation was studied in northwestern Montana in front of the Rocky Mountain foothills near the Augusta syncline. Data, gathered from the field and from thin sections, show that the Virgelle Formation includes 84 m (276 ft) of coarsening upward siltstone to sandstones, deposited during an overall regressive phase along the western coast of the Western Interior seaway.
Measured profiles helped to correlate data along a cross-section from south to north, which is about 100 km (70 mi) long. Data from the siliciclastic deposits point to nearshore deposition of predominantly immature sediments. While lower shoreface deposits dominate the lower part of the Virgelle Formation, middle to upper shoreface sandstones are recorded from the upper part of the Virgelle Formation. Within deposits of middle and upper shoreface facies to foreshore sediments, porosity increases toward the top. After deposition, the facies associations were transported to the east during thinskinned thrusting. During tectonic activity, post-depositionally cemented sandstone layers were pervasively fractured. Quartz grains in almost all deposits show a variety of microfractures.
Folded Lower Cretaceous formations (Colorado Group), underlie the Virgelle Formation. The lower Cretaceous rocks are mainly shaly and form thick, stacked packages developed by faulting and thrusting. The shales, namely the Maria River Shale, Blackleaf Formation, and Kootenai Formation, are known for their potential to generate hydrocarbons. Thus, they may have acted as source rocks, producing hydrocarbons that migrated through the fracture system of the thrusted area and accumulated in the proposed Virgelle reservoir rock.
Facies stacking patterns were used to establish a sequence stratigraphic framework in the study area. Since the spatial arrangement of facies types and thus, reservoir properties within the geologic setting of the Virgelle Sandstone depends on paleo-sea level fluctuations, sequence stratigraphic analysis of the Virgelle Sandstone is a powerful tool to predict the reservoir properties in the subsurface of potential exploration areas.
The four study areas along North American Cretaceous basin margins were selected to study siliciclastic and carbonate sedimentation, fossil assemblages, facies associations and distribution in a sequence stratigraphic framework. The results of the studies contribute relevant paleontologic and geologic data for interpretation of Cretaceous depositional environments in North America.
A Geophysical, Geochemical, and Remote Sensing Investigation of the Water Resources at the City of Chihuahua, Mexico
Rodríguez-Pineda, Jose Alfredo
Department of Geological Sciences
This doctoral dissertation is an investigation of the underground water resources of a typical urban center in arid northern Mexico, the City of Chihuahua. In the first part of the investigation I analyzed by remote sensing the urban growth and the consequent water demand and supply of the city of Chihuahua over a period of 22 years (1975-1997). A linear and a regression model was successfully fit to the data and can be used to forecast future water needs based on urban growth.
The second study was a gravity survey of about 200 stations and a reprocessing of about 800 earlier gravity readings in the City of Chihuahua area to define the local geologic structures. The findings were used as a first exploration step to assess groundwater resources in the area. Landsat TM imagery, 30-m resolution, was applied in conjunction with Bouguer gravity anomalies to understand and define the lateral extension of the Chihuahua, Tabalaopa Basins, and the western side of the Aldama Basin. Depth and strata thickness of these three alluvial basins were defined by gravity modeling and constrained by exploration oil well data and published stratigraphic columns.
The gravity results showed that the western side of the Chihuahua Basin could be the site of potential aquifers. Also, the gravity results suggested an structural gap between the southern side of the Tabalaopa Basin and the Horcasitas Basin, likely a former course of the Chuviscar River. This gap exposes the Horcasitas Basin as another basin with promise as future groundwater source.
The last part of this research focused on the groundwater pollution generated by the discharge of sewage into the Rio Chuviscar and Sacramento for decades by the City of Chihuahua. Based on a groundwater sampling during three consecutive years (from 1993 to 1995) the elements derived mainly from the urban sewage were identified in the Tabalaopa Aquifer. A multivariable statistical evaluation supported by a geologic and geochemical analysis helped to discriminate natural from urban sources. Natural pollution by arsenic was perceived in the Tabalaopa aquifer, potentially threatening the municipal water wells located in the area.
Receiver Function Studies in the Southwestern United States, and Correlation between Stratigraphy and Poisson's Ratio in Southwest Washington State
Kilbride, Fiona
Department of Geological Sciences
This dissertation consists of two separate lines of research. The first uses the receiver function technique to estimate crustal thickness and Poisson's ratio for three receiver stations in the southwestern United States. One station is located in El Paso because relatively few geophysical experiments have been conducted in the southern Rio Grande rift. Two stations are located on the Colorado Plateau, in an attempt to resolve an on going dispute concerning the crustal thickness of this province. Interpretation of two layer models at each of these locations yields an upper crustal layer with an average thickness of 17.6 km and a Poisson's ratio of 0.24 for the Rio Grande rift. The lower crust has an average thickness of 15.3 km and has a Poisson's ratio of 0.30. For the Colorado Plateau, the upper crustal layer is approximately 24 km thick with a VP/VS ratio of 1.85 (which corresponds to a Poisson's ratio of 0.29). The lower crust is about 23 km thick with a VPNS ratio of 1.88 (corresponding to a Poisson's ratio of 0.3). The thickness of the whole crust is approximately 47 km. Errors on these whole crust thickness estimates vary between ± 1.5 to ±5 km.
The results of the receiver functions studies are used as additional constraints for gravity models along two regional profiles coincident with the much shorter profiles of the Pacific to Arizona Crustal Experiment (PACE) that was led by the U. S. Geological Survey on the Colorado Plateau. Because the profiles extend into adjacent provinces, these models are balanced for isostatic equilibrium and are consistent with elevations predicted by buoyancy calculations. The results are most consistent with a thick (= 50 km) crust for the Colorado Plateau and do not support the presence of large lateral thickness variations within the plateau.
The second line of research presented also derives Poisson's ratio, in this case from seismic refraction data. The results are used to interpret a structural cross-section in southwest Washington State and to shed light on a feature of low resistivity (1-5 Qm) located in the High Cascades (the Southern Washington Cascades Conductor or SWCC). This feature is delineated by the interpretation of magnetotelluric and seismic reflection profiles and has been interpreted to be largely composed of Lower Eocene marine sedimentary rocks. Results from a wide-angle seismic refraction profile, collected in 1995, indicate a strong correlation between velocity and lithology, which can be placed in a structural framework, based on Poisson's ratio. Surficial deposits are indicated by Poisson's ratios in the range of 0.23 to 0.24, and are interpreted to represent sedimentary and volcanic units of the Puget Group and younger deposits. Beneath this lies a zone exhibiting a Poisson's ratio between 0.3 and 0.32, which may represent an extension of the Summit Creek basalt. Beneath this lies the top of the SWCC, the interpretation of which precludes the presence of large thicknesses of Lower Eocene marine sedimentary rocks within the SWCC.
Both lines of research estimate Poisson's ratio using dissimilar techniques, but have produced results consistent with one another. Poisson's ratio for quartz-rich rocks (such as sandstones and granites) generally lies between 0.23 and 0.26, as exemplified by the upper crust of the Rio Grande rift, and by sedimentary rocks in southwest Washington. In comparison quartz-poor rocks generally exhibit Poisson's ratios between 0.28 and 0.32, typical of the lower crust and the many basaltic formations that comprise the upper crust in southwest Washington.
Geology and Geochemistry of Newly Discovered Tertiary Carbonatite Occurrences near Villa Ahumada Area, Basin and Range Province, Chihuahua, Northern Mexico
Nandigam, Ravindra
Department of Geological Sciences
This study targets some newly discovered carbonatite occurrences located in the eastern Mexican Basin and Range province, a few kilometers to the east of Villa Ahumada, Chihuahua. The region containing these occurrences experienced compression related to subduction of the Farallon plate until about 32 Ma that was followed by Basin and Range extension. Geological mapping (1:5,000 scale), petrography, study of drill hole cuttings and satellite images, and major and trace element chemical analyses were utilized to understand the intrusive style of the carbonatites, their mineralogy and petrogenesis.
The carbonatites, named Yuca, Mariana and El Indio, collectively intrude limestones, granitic intrusives and subduction-related tuffs and lavas mainly as a stock, breccias and dikes. The Yuca carbonatite was emplaced as a 900-m diameter stock, 500050 m breccia body, numerous dikes and networks of fracture fillings. Crosscutting field relationships at Yuca suggest at least two stages of carbonatite emplacement. At Mariana, carbonatite was emplaced as a 750050 m breccia. Four out of nine reverse circulation drill holes penetrated and bottomed in the breccia at an average depth of about 300 m. At El Indio, carbonatite was emplaced as a 20 m diameter breccia pipe and a lm thick sill.
Major minerals present are calcite, Fe-rich calcite and hematite. Sporadic presence of fluorite is common. At Mariana, two generations of grossular-rich garnets associated with limestones and granite porphyry respectively are recognized. It is inferred that garnets in granite porphyry represent metasomatic alteration due to the emplacement of carbonatite breccia.
Parental magmas of Yuca carbonatites have undergone differentiation under low f02 conditions during which they were progressively enriched in iron. The carbonatite compositional types recognized based on major element data, in the sequence of least to most highly differentiated, are 1) magnesio-, 2) calcio- and 3) ferrocarbonatites. The concentrations of thorium, uranium, niobium, yttrium and rare earth elements increase with differentiation. Carbonatites are associated with up to 20000 times enrichment of lanthanum and cerium with respect to chondrite. Finally, the Mariana carbonatite breccia is associated with about 7,928,550 short tons of rock with a grade of 0.14% REE, and primitive mantle normalized trace element plots of carbonatites indicate a mantle origin of their parental melts.
Investigating Crustal Structure in Western Washington and in the Rocky Mountains: Implications for Seismic Hazards and Crustal Growth
Snelson, Catherine M.
Department of Geological Sciences
This dissertation consists of two seismic studies, one in western Washington Stat& and one in the Rocky Mountains. The study in western Washington State is one component of the SHIPS (Seismic Hazards Investigations of Puget Sound) experiments, a continuing effort to define Cenozoic basin and fault geometry beneath the densely populated Puget Lowland. In September 1999, the U.S. Geological Survey and a number of university collaborators collected the "Dry" SHIPS seismic profile across the Seattle basin of western Washington State. The objectives of the "Dry" SHIPS study were to define the geometry of the Seattle basin in an E-W direction and to determine the structure of the eastern and western boundaries of the basin. In addition, the experiment was designed to test the hypothesis that N-S trending faults lie beneath Puget Sound or the adjacent Lowland. One of these faults may form the eastern boundary of the Siletz terrane. The "Dry" SHIPS data are characterized by travel time advances associated with the Siletz terrane to the west and the Cascades to the east and by delays of as much as 2 s in the Seattle basin.
P-wave 3-D tomographic results show that the basin is about 70 km wide and contains sedimentary strata with velocities increasing gradually from 1.8-4.5 km/s. The contact with underlying basement rocks is characterized by a rapid increase in velocity from 4.5 to 5.0 km/s. At its center, the basin is 6-7 km deep along this profile. This result is consistent with results from a N-S trending reflection line collected in 1998 during the "Wet" SHIPS phase of the project that is tied to well control. The symmetry of the Seattle basin is consistent with thrust loading as the major contributor to the formation of the basin. The lower velocities within the upper part of the basement found east of the Puget Sound may be indicative of pre-Tertiary basement rocks of the Cascades. This change is probably an expression of the Coast Range Boundary fault, which has previously been interpreted from gravity and magnetic data. Density modeling tied to the velocity model shows that the Olympic accretionary wedge is indistinguishable from surrounding rocks below a depth of about 20 km. The contact between the Siletz and Pre Tertiary basement rocks is a subtle contact as inferred from the velocity and gravity models.
The study in the Rocky Mountains is one component of the Continental Dynamics-Rocky Mountains Project (CD-RoM '99), a collaborative interdisciplinary study involving 14 American universities and the University of Karlsruhe, Germany that focuses on Precambrian features and their effects on Phanerozoic deformation. One of the major field efforts in the CD-RoM project took place during August, 1999. The University of Texas at El Paso and the University of Karlsruhe, with the assistance of several other institutions, collected data along a ~950 km long seismic refraction/wide-angle reflection pro file extending from Fort Sumner, New Mexico to the Gas Hills, Wyoming. Station spacing was nominally 800 m using ~600 instruments during two deployments. Eleven shots were fired ranging in size from 167.2-4540.9 kg and were nominally spaced at ~100 km intervals along the profile. The profile crosses major structural features of the continent including the Jemez lineament, the Colorado mineral belt, and the Cheyenne belt (a prominent Proterozoic suture).
Velocity modeling, employing several techniques, indicates that crustal thicknesses ranges from ~45 to 55 km in New Mexico and Colorado. In northern Colorado, the crust begins to thin from ~50 and reaches ~40 km in Wyoming, north of the Cheyenne belt. A mid-crustal interface is very prominent within the data and can be thought of as the Conrad discontinuity. This interface falls at depths of about 25 to 30 km and is a discontinuity below which velocities increase to about 6.8 km/s. A high-velocity lowermost crustal layer with a thickness ranging from 5 to 10 km is evident in the Southern Rocky Mountains-Great Plains (SRM-GP) portion of the model. The velocity of this layer ranges from 7.0 to 7.4 km/s, a value that is consistent with a composition of mafic garnet granulite. One interpretation of this high-velocity lower crustal layer is that it originally formed during assembly of the Proterozoic terranes. Magmatic underplating at 1.4 Ga may have increased the thickness of this layer beneath the SRM-GP. This is not to say that the depth to the Moho has not been locally modified during Phanerozoic events, but that major modification took place during the Precambrian.
The Deep Structure of Northeastern New Mexico from Deep Seismic Reflection Profiles: Implications for Proterozoic Tectonic Evolution of Southwestern North America
Eshete, Tevera Gashu
Department of Geological Sciences
Previous geologic, geochronologic, structural, isotope, and xenolith studies have shown that the Precambrian rocks of northern New Mexico belong to the Yavapai and Mazatzal provinces. The boundary between the provinces is a wide zone defined on its northern edge by the northern extent of 1.65 Ga deformation and southern edge by the southern most extent of Yavapai crust (pre-1.7 Ga). However, the nature of the Precambrian province boundary at depth, its evolution through time, and the tectonic processes that affected the interior of these provinces, are not well understood. In order to obtain new information concerning these problems, processing and interpretation of reflection seismic data was conducted on data collected during the 1999 Continental Dynamics-Rocky Mountain (CD-ROM) project and data obtained from industry.
In this study I present new information on the crustal structure of northern New Mexico provided by processing and interpretation of three seismic reflection profiles (NM-i, TB-i and TB-2). These profiles, with a total length of ~300 kin, are located in the Yavapai-Mazatzal provinces of southwestern North America. The data were acquired using Vibroseis trucks at source intervals of 100 m (NM-1) and 134 m (TB-1 and TB-2) receiver spreads of 96 to 1002 channels spaced at 25-67 m, to give 24-125 fold stacked data. The seismic data present evidence for Precambrian crustal growth and amalgamation, followed by subsequent reactivation of Precambrian structures. The interpretation of the seismic observations is fully integrated with gravity modeling, magnetic, geologic, and borehole data.
A seismic profile and gravity modeling across the NM-1 show a strongly reflective high-density (2850 kg-m3) dome-shaped body in the middle to lower crust. On the basis of the absence of a hanging-wall antiform, the occurrence of normal sense of deflection of reflectors in the footwall, possibly Moho pullup, and geological information such as an exposed Proterozoic extensional shear zone in the Sandia Mountains, this feature is interpreted to represent a 1.4 Ga? extensional shear zone which resulted in rotation of ~1.65 Ga imbricate thrust zones. Reflectors related to the domal structure terminate at the northern and southern part of the profile. This abrupt termination is attributed to vertical structural boundaries that may be related to the inferred locations of Jemez and Morenci lineaments, respectively.
Layered reflectivity directly below the top of Precambrian basement on profiles TB-1 and the eastern part of TB-2, based on geophysical and geological information from nearby areas is interpreted as a sequence of 1.4 Ga volcanic and sedimentary rocks within the Proterozoic Granite-Rhyolite terrane. A mafic intrusion probably related to northward extension of the Pecos mafic complex may be present within the layered assemblages. A band of prominent reflections is also observed in the western part of TB-2 that is interpreted to be another Precambrian layered sequence comprised of sedimentary and volcanic rocks of the 1.26 Ga Debacca-Swisher terrane. Sub-horizontal to slightly dipping, discontinuous, high amplitude reflectors are observed from ~3 s down to 8 s under profiles TB-l and TB-2. These reflectors, constrained by well data and synthetic seismograms from previous studies, are interpreted to represent Proterozoic mafic sills (1.35 or 1.1 Ga?) related to extension.
Most of the interpreted structures and lithologies on profile NM-1 are related to ~1.65 Ga and ~1.4 Ga tectonic events, whereas the geologic features on TB-1 and TB-2 are interpreted to be mainly related to ~1.1, 1.26, and 1.3 Ga extensional event. These significant differences in structure and lithology between the two regions may be related either to more pervasive modification of the crust to the east of the region imaged under NM-1 during ~1.36, ~1.26 and ~1 .1 Ga tectonic and magmatic events, or to profound north-south trending boundary at or near the Rocky Mountain front.
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