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Collection Description

The Department of Geological Sciences has a long-standing Senior Thesis research option for the B.S. Degree which involves a written thesis, and a public oral presentation done under the supervision of a faculty member. These independent research projects typically involve field work and laboratory analyses of samples, but can also include laboratory-based experimental projects, numerical modeling of geologic phenomena and literature reviews. Senior theses are kept in the permanent collection of the Malcolm A. Love Library on the SDSU campus.

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The bootstrap technique: Estimating bulk mass change during development of La Posta soil profiles #2 and #3
Other investigators have shown that the development of weathering profiles on the small-biotite facies of the Cretaceous La Posta pluton in both arid and Mediterranean climates resulted in significant degradation and loss of plagioclase. Such results imply that a bulk mass loss may have occurred as the parent plutonic material was reduced to the relatively thin A-horizons typical of weathering profiles in both climatic belts. Here I report the results of work aimed at assessing how much, if any, bulk mass loss occurred during the development of the A-horizons in La Posta Soil Profiles #2 and #3. Bulk mass changes can be calculated from the relationship, T = C; / C 􀀡 -1 , where c; is the concentration of some immobile element in the original parent material and C􀀄 is the concentration of the same element in the altered material. If most elements are not affected by pedogenesis, then the ratio, c; / C􀀨 , for all such elements should be similar. In order to calculate the uncertainties in the ratios of c; / C􀀪 , the bootstrap algorithm developed by G. H. Girty and J. Lawrence was used. On a plot of c; / C! versus Ti, Fe, Mn, K, Rb, Zr, and Pb for soil profile #3, and AhO3, TiO2, FeO, MgO, Zr, Nb and Ga for soil profile #2 all data cluster about a similar value, thus suggesting that these elements are good candidates for a geochemical reference frame. The geometric mean of c;/c􀀁 for these elements is 0.85 for both sampling suites. Hence, in both La Posta Soil Profile #2 and #3 ~ 15% of the parent plutonic material was removed during development of the A-horizon., San Diego State University
The characteristics of a landslide in the Capistrano Formation San Clemente, California
The Capistrano Formation is a triangular shaped lithostratographic unit ranging from Dana Point, south to San Onofre and is bordered by the Christianitos fault on the east side. The Capistrano Formation consists of a notoriously landslide prone homogeneous siltstone which was deposited in the Miocene-Pliocene age Capistrano Embayment (Terres, 1992). The main cause for the landslides is occasional thin weak clay seams interbedded in the siltstone. Landslides in the Capistrano Formation are prominently block-glide slides. They have relatively flat dipping basal ruptures and steeply dipping shear surfaces that create the main back scarp. Slides in this area have been the cause of much property damage. The landslide chosen for this paper is part of a much larger landslide that previously slid either approximately 30,000 years or 15,000 years earlier. Due to this, the landslide is highly fractured and weathered, turning portions of the slide into highly expansive clays, determined through laboratory tests such as x-ray diffraction, Atterberg limits, hydrometer and sieve analyses. The landslide is approximately 40 m by 25 m with a maximum back scarp height of approximately 3.5 m. Its toe is continuously being eroded away by a stream channel and will continue to slide to maintain its stability., San Diego State University
The chemistry and mineralogy of mafic dikes in the Lookout Mountain – West Point area, Skamania County, Washington: Feeders for the basaltic andesite of Elkhorn Mountain?
A chemically coherent swarm of mafic dikes occurs in the area between Lookout Mountain and West Point in Skamania County of southwestern Washington, ~ 35 km south of Mount St. Helens. The dikes range in composition from basalt to andesite, are uniformly tholeiitic, and have K2O contents between 0.1 to 0.8 weight percent. They are moderately to highly porphyritic, with phenocrysts of plagioclase and olivine; many also contain phenocrysts of augite and (or) hypersthene. Groundmass ranges from microcrystalline to medium-grained; is intergranular, pilotaxitic, or trachytic in texture; and consists of plagioclase, augite, and Fe-Ti oxide. Primary mineralogy is variably replaced by minerals such as quartz, calcite, zeolites, and clays; typical of the local character of zeolite-facies burial metamorphism. An average magnesium number of 45 suggests a moderately evolved magma, while incompatible-element ratios indicate that the dikes are probably related by fractional crystallization from a common parental source. Chemical and mineralogical similarities suggest that these dikes may have been feeders for a nearby package of Oligocene lava flows: the basaltic andesite of Elkhorn Mountain. Feeder dikes for this extensive unit have not been previously identified. These flows have been removed from the study area due to an apparent local uplift of greater than 1,000 meters since their emplacement between 27 to 25.5 Ma. The distribution of the feeder dikes thus exposed by this exhumation indicates that the original extent of the basaltic andesite of Elkhorn Mountain was in excess of 1,200 km2 and 530 km3, and that it was erupted from a multitude of coalesced vents; herein informally named the Elkhorn Mountain Volcanic Field. Contemporary Cascadian volcanic fields of similar size and chemistry, such as the Medicine Lake Volcanic Field, are thought to form in extensional environments due to decompression melting of shallow MORB-source mantle, rather than by slabfluid- flux melting that is typical of continental volcanic arcs., San Diego State University
The components of a typical geotechnical investigation
The purpose of this thesis is two-fold. One is to provide an understanding of a typical geotechnical investigation. The other is to perform an actual field investigation for a local geotechnical firm. Many people outside the field of geotechnical engineering are unaware of the principle roles of an engineering geologist. The geologist performs the field reconnaissance, research, subsurface exploration, geologic analysis of site conditions, and, along with the geotechnical engineer, project analysis. Before and during site reconnaissance, the geologist makes observations and assumptions about on-site soil conditions. During subsurface exploration, the geologist evaluates the soils in different locations by either a) stepping into a trench, b) watching spoils come up from an auger, or c) by going "down hole." He/she also collects samples for future lab analysis. During project analysis, the geotechnical engineer and the geologist discuss a) potential hazards, b) site conditions, c) excavation equipment and locations, and d) excavation logs and drilling complications ( such as anticipated depth of drilling not reached due to rock or caving). With laboratory results available, the engineer and geologist then write the geotechnical report. field study was conducted by a Southern California Soil & Testing (SCS&T) geotechnical engineer, engineering geologist, and myself in the vicinity of Escondido, just north of Lake Hodges. Hallmark Communities plans to develop two adjacent tracts ofland into a 17-lot subdivision and an 11-lot subdivision, Tracts 710 and 711, respectively. Field-testing included 16 air-track borings of Tract 710 and 7 backhoe trenches of Tract 711. Previous work in 1987 by SCS&T included both backhoe trenching and seismic traverses of Tract 710. Bulk & chunk samples were acquired from the trenches of Tract 711 in 1999. Trench logs from the subsurface investigation, design specifications, and discussion between the geotechnical engineer, engineering geologist, and myself helped determine the needed laboratory tests. The tests included sieve analysis (for grain size distribution), direct shear (for lateral strength of soil), expansion index (for ground swell potential), and collapse tests (for potential settlement). The final laboratory tests helped to make the evaluations and recommendations that in Tract 710 there is adequate rippability for all sites except lot 17. In Tract 711 compressible alluvial soils underlie lots 1, 10, & 11, and relatively loose fill underlies lots 6 & 7. These soils will require removal and replacement as recompacted fill., San Diego State University
The cumulate mystery wihtin a mafic intrusion: Petrology and geochemistry of basic and ultrabasic rocks of Los Pinos Mountain in the peninsular ranges batholith, southern California
Los Pinos Mountain is located within the eastern boundary of the gabbro sub-belt in the western zone of the Peninsular Ranges batholith, southern California. It was studied in 1976 by Dr. M.J. Walawender of San Diego State University and hisl 976 paper provides the basis for this project. Los Pinos Mountain has a sub-rounded pyramid shape formed from weathering of a complex assemblage of cumulate layered ultramafic rocks within a host of hornblende gabbro. There are two distinct rock groups, the inner and outer group, which form the Los Pinos pluton. These groups are separated based primarily on mineralogy and texture. The inner group (peridotite, olivine gabbronorite, and anorthosite) is heterogeneous and consists of euhedral to subhedral cumulus of plagioclase and olivine with amphibole, ± cpx, ± opx forming the dominant post-cumulus minerals. The outer group is a more homogeneous hornblende gabbro, with hornblende typically found interstitial to euhedral plagioclase. There are also pegmatitic hornblende gab bro dikes ("comb layers") of the outer group near the contact zones with the inner group. Whole-rock geochemical analyses yield a range of SiO2 and MgO compositions from 40 to 46 wt.% and 0.14 to 27.70 wt.% for the inner group, and 43 to 49 wt.% and 4.32 to 7.24 wt.% for the outer group, respectively. Thus, this study of rocks of Los Pinos Mountain do not represent true igneous melt compositions, but are remnants of a cumulate network of deep-seated intrusions emplaced within the upper crust. A possible mechanism of emplacement is underplating of the continental arc in an extensional environment, with magma produced by melting of mafic crust and upper mantle. Differentiation of this melt by crystal fractionation in the upper crust may have produced the inner group and outer group rocks. More detailed geochemical analysis and extensive field mapping is required to better understand the spatial and temporal relationships between the Los Pinos Mountain rocks, the regional tectonics, and the Cretaceous intrusions of the Peninsular Ranges batholith., San Diego State University
The fission track method of age dating rocks
Four lepidolites of known age and location were etched in acid and observed microscopically for fission tracks. Only one had naturally occurring fission tracks; the rest had none. The ability to see fission tracks seems to depend upon telling them apart from other naturally occurring phenomena, plus the facility to etch them to the proper size for recognition. After etching, the lepidolite containing fission tracks was observed to have approximately 100 tracks per cm2., San Diego State University
The genesis of a highly vesticulated lava found in the San Quintin Volcanic Field, Mexico
Vesicle cylinders and pipe vesicles are two different features found in basalt lavas. Vesicle cylinders are associated with highly vesiculated basalt lavas which indicates that there must have been a high water content prior to the solidification of the melt. The source of this water is normally attributed to the degasing of magmatic (internal) volatiles. However, some workers suggest that an "external" water source may be involved in the genesis of some highly vesiculated lavas. Pipe vesicles that form near the base of many flows have been used as evidence for surface­water vaporization into the overlying flow. However, pipe vesicles are also present in the interior of basalt pillows where they appear to terminate against the glassy selvages along the exterior of the pillows. This has been used as an argument against the incorporation of the "external" water into the pillow. In the attempt to resolve the respective roles of "internal" and "external" water in the vesiculation of a basaltic lavas, we examine the nature of an unusual basaltic lava flow in the San Quintin Volcanic Field (SQVF), Mexico. This flow was chosen because (1) it is highly vesiculated, (2) it contains vesicle cylinders, and (3) it overlies a thick pillow­ palagonite/hyaloclastite zone, indicating a potential source of "external" water. The flow has a diktytaxitic texture, which is unique when compared with the other SQVF flows which have intergranular textures and lack of vesicle cylinders. The diktytaxitic flow contains vertical vesicle cylinders that are evenly spaced and separated by an average of <20 cm. The cylinders have an average length of 25.3 cm and the average width of about 4.5 cm. The flow also contains horizontal vesicle "sheets" and vesicle "pods", both of which indicate the ponding of rising bubbles within the flow. The mineralogy of the vesicle cylinders differs slightly from that of the diktytaxitic host. The cylinders contain higher proportions of red-altered iddingsite, iron-oxide, and apatite. This mineralogy is consistent with the vesicle cylinders having crystallized from a volatile-rich fluid produced by differentiation of the host basalt. As the enriched fluid was drained from the host basalt, it left behind angular open spaces between previously crystallized plagioclase laths. This is the hallmark of diktytaxitic texture. There is no evidence that the vesicle cylinders are more abundant in the lower portion of the flow. Th.e field evidence and mineralogical evidence is consistent with the "internal" process of differentiation, unrelated to the "external" source of water that created the pillow­palagonite/hyaloclastite zone at the base of the diktytaxitic flow., San Diego State University
The geology of a portion of the Rancho Santa Fe quadrangle, San Diego County, California
The area east of Rancho Santa Fe, California, is characterized by the presence of a series of slightly metamorphosed andesitic volcanic and volcaniclastic rocks which are interbedded with sedimentary rocks of Late Jurassic age. These volcanic units are the result of explosive andesitic vulcanism in the area. Erosional remnants of the volcanic centers are preserved in the stratal record and indicate a local source for some of the volcanic rocks. The Southern California Batholith intrudes the Santiago Peak Volcanics and is the source of the meta­morphism of the area. Metamorphism and intrusion were followed by deposition of a Cretaceous conglomerate on the erosion surface of the basement complex. Subsequent submergence produced the rocks of the La Jolla Formation. Post Eocene uplift of the area has produced the present topography., San Diego State University
The geology of the Rosarito Beach Area
In the Rosarito Beach area, tilted Early Cretaceous metavolcanic rocks of the Alisitos Formation are overlain with angular unconformity by essentially flat-lying mudstones and sandstones of the Rosario Formation. Faunal analysis indicates that the Rosario Formation is of Upper Cretaceous age (Companion to Early Maestrlchtian) and was deposited in a cold, deep­water marine environment. Disconformity separates the Rosario Formation from Later Cretaceous-Early Tertiary conglomerate of Formation A. A paleosol developed on Formation A is overlain by Eocene(?) sand­stone and conglomerate of Formation B. A series of interbedded Tertiary basalts and sandstones overlie Formation Band and are in turn unconformably overlain by Pleistocene terrace deposits., San Diego State University
The geophysics of a geothermal area in Punta Banda, Baja California
The Punta Banda Peninsula is. located on the west coast of Mexico near Ensenada and is believed to have formed as a direct result of right-lateral movement along the Agua Blanca fault system. Geothermal activity along the southwest shore at Bahia Papelote includes multiple hot water emissions and a single gas emission. This study of the geothermal area involves the use of two geophysical techniques: a Very Low Frequency (V.L.F.) electromagnetic survey and seismic refraction. The V.L.F. method was used first as a reconnaissance technique to delineate faults in the study area. Next, refraction surveys were made along the same lines where the V.L.F. method indicated faults. Both techniques showed four faults (in the same areas). In addition to the right-lateral separation found in previous studies, these faults were found to have a reverse separa­tion and a throw of 22-150 feet. Refraction profiles seldom reached the basement. However, offset of the layers above the basement indicates more recent faulting. Finally, the Punta Banda geothermal area was found to be at the intersection of two of these cross faults., San Diego State University
The lithology and structure of a gabbro plutton, Pine Valley, San Diego County
The Laguna Junction pluton is three kilometers east of Pine Valley, San Diego County. The Chariot Canyon fault zone runs adjacent to the northern edge of the pluton. The pluton is horseshoe shaped and contains many faults. The purpose of this study is to determine if the pluton's shape is a result of folding by the: Chariot Canyon fault. The pluton has two lithologically distinct groups of rocks. The northern group consists of a hornblende gabbro whose texture varies from very fine grained to very coarse grained. The texture changes gradationally and in no particular pattern. The southern group of rocks contains olivine-hornblende gabbro, olivine gabbronorite, anorthosite, and peridotite. While results of this study suggest that the faults in the pluton are related to movement of the Chariot Canyon fault, the pattern of the faults does not support the idea that the pluton was folded by the fault. Foliations found in the pluton give more evidence against the idea of folding by the Chariot Canyon fault zone., San Diego State University
The paleomagnetism of some plio-pleistocene siltstones in the Ventura Basin, California
Three lines of evidence from schists of the Great Smoky Mountains, NC, indicate that isogradic monazite growth occurred at the staurolite-in isograd at 600°C: (1) Monazite is virtually absent below the staurolite-in isograd, but is ubiquitous (several hundred grains per thin section) in staurolite- and kyanite-grade rocks. (2) Many monazite grains are spatially associated with biotite coronas around garnets, formed via the reaction Garnet Chlorite Muscovite Biotite Plagioclase Staurolite H2O. (3) Garnets contain high-Y annuli that result from prograde dissolution of garnet via the staurolite-in reaction, followed by regrowth, and rare monazite inclusions occur immediately outside the annulus and in the matrix, but not in the garnet core. Larger monazite grains also exhibit quasi-continuous Th zoning with high Th cores and low Th rims, consistent with monazite growth via a single reaction and fractional crystallization during prograde growth. Common silicates may host sufficient P and LREEs that reactions among them can produce observable LREE phosphate. Specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million, and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section. LREEs may be more limiting, but sheet silicates and plagioclase can contain tens to 100 (?) ppm LREE, so recrystallization of these silicates to lower LREE contents could produce hundreds of grains of monazite per thin section. Monazite ages, determined via electron and ion microprobes, are 400 Ma, directly linking prograde Barrovian metamorphism of the Western Blue Ridge with the “Acadian” orogeny, in contrast to previous interpretations that metamorphism was “Taconian” (450 Ma). Interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite-forming reactions, including reactions previously viewed as involving solely common silicates. Copyright © 2004 Elsevier Ltd, San Diego State University