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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.

Authors hold full copyright ownership of their original works. Please contact the repository manager at digital@sdsu.edu for any further questions.

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Locating the main central thrust of the Himalayan orogen: Results from the Melamchi River Traverse of Central Nepal
The Main Central thrust (MCT) is the major thrust fault of many east-west oriented faults in the Himalayan Orogen. These faults are responsible for over onehundred kilometers of crustal shortening as the Indian plate has pushed under the Eurasia plate to form the Himalaya. The MCT is a ductile shear zone marking the fault boundary between generally crystalline rocks of the Greater Himalayan sequence (GHS) to the north from generally low-grade to un-metamorphosed rocks of the Lesser Himalayan sequence (LHS) to the south. In many places however complications make this boundary hard to identify. The central Nepal Himalaya is one of these complicated areas because the rocks here do not contrast with one another strongly. This study focuses on detrital zircon U-Pb geochronology of three rock samples along a traverse of the Melamchi river valley that span the presumed location of the MCT in this area. The zircon U-Pb ages from these samples should be able to distinguish between Late Proterozoic to Paleozoic Greater Himalayan rocks from Proterozoic Lesser Himalayan rocks. Samples’ locations were determined by regional lithology, stratigraphy, and structural deformation indicative of a major thrust fault. Samples were collected from outcrops using sledgehammers and chisels, and were pulverized into powder by hand using steal pestle and mortar. Hand panning concentrated zircon in the heavy fraction. Zircons were then mounted with standards at SDSU and analyzed by Laser Ablation Multi-collector ICP Mass Spectrometry at the University of Arizona LaserChron Center. Zircon U-Pb ages from the three samples (MCT1, MCT2, and MCT3) were plotted on relative age-probability diagrams for visual comparison with one another. All three samples are similar to another with ageprobability peak ages between 1100 Ma - 750 Ma indicating an affinity with GHS rocks. The Kolmogorov-Smirnov statistic was used to compare the age distributions under the null hypothesis that the samples are drawn from the same distribution. Comparison of each pair of samples using the K-S test yields P-values that are all >0.5 indicating that these grains could have been randomly sampled from the same population. The results of this study indicate that the location of the MCT is to the south of the area where these samples were collected. Further sampling and mapping is necessary to accurately map the location of the MCT across the Melamchi river valley in central Nepal Himalaya., San Diego State University
Locating the source of the of the sacatone springs granite pegmatites by use of major and trace element chemical analysis
The Sacatone Springs gem-bearing granite pegmatites of eastern San Diego County form part of the Jacumba dike swarm which is an extensive swarm of subparallel dikes that strike approximately N-S and extend for nearly 10 kilometers along strike. The swarm is hosted by the amphibolite facies metasedimentary and metaigneous wallrock of the Carrizo Gorge inlier, which separates the La Posta pluton into western and eastern parts. The light­colored dikes of the Jacumba swarm contrast strongly with the dark metamorphic host rocks and hence are easily visible on aerial photographs. The garnetiferous 2-mica monzogranite Indian Hill pluton occurs to the NE of the main Jacumba swarm; the compositionally similarly Boulevard pluton 2-mica monzogranite occurs at the south edge of the swarm along Interstate-8. The Sacatone Springs pegmatites occur at the north end of the Jacumba dike swarm. Two main dikes, which are encompassed at their northern extent by the Pack Rat mining claim, were investigated in this study. The lowermost of the two dikes is famous for "Beebes Hole" at the northern tip of the dike where big kunzite crystals were discovered. The Sacatone Springs dikes exhibit strong internal textural variability and include aplite, pegmatite, and related textural variants. Pegmatite segregations typically exhibit a wall zone of graphic microcline ± biotite/muscovite and a core of megacrystic perthite and quartz ± muscovite ± garnet ± schorl. Pegmatitic segregations are bounded along sharp contacts by aplite and coarser granite. This granite "host" to the pegmatite ranges from a fine-grained biotite granite to a medium-grained 2-mica granite ± garnet. The source of the Jacumba dike swarm is unclear but the compositions of the dikes suggest it may have originated from one of the nearby Cretaceous plutons. This study had two main objectives: 1) to try and resolve the source of the Sacatone Springs pegmatite by comparison of whole rock X-ray fluorescence major and trace element analysis of the dikes with surrounding plutonic units, and 2) to investigate whether an individual dike shows any evidence for along strike compositional variation. Dike samples were taken from the aplitic parts of the dikes that are more representative of magma bulk compositions. The dike samples are monzogranites with SiO2 contents of~ 75 wt% and K/Na ratios of~ 1 and are similar in this respect to the Indian Hill and Boulevard plutons. The dikes on the other hand are compositionally very different from the La Posta pluton with the possible exception of one sample. Whole rock chemical analysis is therefore consistent with a monzogranite Indian Hills-type source for the Jacumba pegmatite swarm although none of the dikes have been traced back directly into this potential source. Internal along-strike variation within an individual dike was not detected at the sampling scale used., San Diego State University
Lower Triassic fossils from the Soda Mountains, San Bernardino County, California
A complexly faulted block of sedimentary rocks located at the northwestern edge of the Soda Mountains, San Bernardino County, California, is dated as middle Early Triassic based on the pres­ence of the cephalopod Flemingites cirratus. This general age also is supported by other longer ranging molluscs found as­sociated with the cephalopod. The Lower Triassic section of the Soda Mountains consists of brown and red shales interbedded with gray, argillaceous limestones and minor conglomerates and sandstones. Because of a similar age and lithology, the Soda Mountain section is correlated with the Timpoweap member of the Moenkopi Formation., San Diego State University
Lower cretaceous marine volcanics and sediments, alisitos group member, Arroyo San Jose, Baja California, Mexico
A 3000 m section of pre-batholithic marine volcanics and sediments are identified along the Pacific Coast at Arroyo San Jose, Baja California, Mexico. This analysis concerns the geology and paleontology of this rather large volcaniclastic section. During the past twenty years, opinions on the age of the Arroyo San Jose beds have varied, ranging from Jurassic to Lower cretaceous. In 1968, a pelgpo d otapiria sp. (Lower Jurassic) was identified, which indicated a Jurassic age for at least a portion of the section. This however, was not consistent with other fossils found in the section, which suggested a Lower Cretaceous age. Recent observations have indicated that the alleged otapiria was probably Aucellina, Lower cretaceous (Albian-Turonian). This study assigns a lower cretaceous age, possibly (Albian) for the entire section. The lithology, characterized by an abundance of calc-alkaline pyroclastic material appears to indicate an island arc depositional environment. Close relationships between the Arroyo San Jose deposits and the (Lower Cretaceous arc) Alisitos Group enable them to be considered of the same suite. The Arroyo San Jose sequence is therefore suggested part of the Alisitos Group. The appreciable amount of fine grained epiclastic material found at Arroyo San Jose indicates the sequence may have developed as a distal member of the Alisitos Group in a deeper marine environment. The fossil assemblage of Arroyo San Jose is indicative of deep-water habitats. It is suggested that the Arroyo San Jose sequence may have been deposited in an island-arc basin. The Arroyo San Jose sequence may provide an excellent model for intra-arc basinal settings., San Diego State University
Mafic injections and in situ hybridization of granodiorite, Isla Cerralvo, Baja California Sur, Mexico
Isla Cerralvo near the mouth of the Gulf of California is composed primarily of Cretaceous granodiorite plutons intruded into quartzofeldspathic schist. Metamorphic grade of wallrock schist is predominately greenschist facies but increases to amphibolite facies (andalusite+sillimanite) in close proximity to intrusive contacts. Mineral assemblages in the schist indicate shallow emplacement of plutons in the crust ( ~4-8 km depth), a conclusions supported by concordant zircon U/Pb ages and 40 Ar/39 Ar ages(hornblende and biotite) from plutons that indicate rapid crystallization and cooling to below ~300°C between 90 to 100 million years ago (Montrella, 2004). Hornblende-biotite granodiorite outcropping on the western coast of the Isla Cerralvo is characterized by abundant widely distributed mafic enclaves as well as younger cross-cutting greenschist facies mafic dikes. Coastal outcrops of this granodiorite map unit are broadly homogeneous with the exception of a distinctive zone of a strongly heterogeneous intermingled mafic, felsic, and hybrid rocks along a~ 100 meter section of coast in the west-central part of the island. The occurrence of mixed/mingled mafic and felsic outcrops in this area poses questions relating to magma emplacement mechanisms, the origin of mafic enclaves, and processes of granitoid hybridization. The Cerralvo enclaves are fine-grained altered hornblende+plagioclase rock with igneous textures. Mafic-felsic contacts are variable; in some places mafic rocks are sharply chilled against felsic rocks; in other places contacts are gradational through hybrids to granodiorite. Whole rock major and trace element XRF data documents basaltic compositions for the enclaves. These observations indicate the enclaves represent basaltic magma injected into the granodiorite host before it was completely solidified. Convective stirring disaggregates the mafic injections and promotes hybridization of the granodiorite host as reflected by lower silica contents here relative to the "main phase" granodiorite. Mafic synplutonic injections on Cerralvo are similar in compositon to the ubiquitous fine-grained dikes that cut the granodiorite host. Based on this and age relationships (Montrella, 2004) I speculate that the dikes and mafic synplutonic injections may be derived from a common source., San Diego State University
Magnetic susceptibility as related to grain size
Three formations were studied during this project: (I) the Cretaceous Point Loma Formation, a coarse-grained sandstone (80% of rock had average grain size of 1. 5 phi), (2) the overlying Cretaceous Cabrillo Formation, a conglomerate with a clayey sandstone matrix (matrix averaged 44% 1.5 phi and 37% 2.5 phi), and (3) the Eocene Mt. Soledad Formation, a clayey medium-grained sandstone ( 60% of rock had average grain size of 1. 5 phi). Grain size and magnetic susceptibility of rock samples from each formation were analyzed. Magnetic susceptibility (MS) is defined as the extent to which a rock can be magnetized by an external field. Iron-bearing minerals like magnetite give high MS readings. However, rocks enriched with minerals like biotite and hornblende can give low MS readings. The sandstone matrix of the Cabrillo Formation conglomerate has high MS readings (avg. 119-159) and overlies the low MS (avg. 25-32) sandstones of the Point Loma Formation. The rock unit overlying the Cabrillo Formation is the Mt. Soledad Formation, a rock with low MS (avg. 32) readings. Provenance and depositional environment help determine the types of minerals in a rock, which in tum controls its MS. All three depositional environments that shaped the three formations had sufficient amounts of energy to transport magnetite grains. Furthermore, the provenance is similar for all the formations. The Point Loma Formation amples were fairly well sorted with very few fine grains (12% of rock <3.0 phi) whereas, the Mt. Soledad Formation, with slightly higher MS readings and a larger range of grain sizes, had 36% of the rock <3.0 phi. The Cabrillo Formation matrix, with the highest MS readings, is very poorly sorted, with close to a even percentage of coarse to very fine sand Figure 2 shows the rock samples collected form the Point Loma Formation, a moderately sorted biotite-rich sandstone. Of the three formations the Point Loma Formation was the best sorted, it had the least amount of grain-size distribution. Average grain size distribution is 1.5 phi with an average MS reading between 25 and 32. There were three abnormally high MS readings and if they are adjusted out the MS range drops to 14 and 25. A low percentage (12%) of the rock fell outside the 1.5 phi grain size and MS range. No free magnetite grains were extracted from the rock. grains. The magnetite grains removed with a magnet from the Cabrillo Formation matrix had a grain size oflower fine sand (2.5 - 3.0 phi). The dumping of magnetite grains within the poorly sorted Cabrillo Formation may be a result of sudden changes from high to low energy levels in the depositional environment. The degree of sorting appears to be one of the contributing factors in determining the amount of MS in a sedimentary rock, San Diego State University
Magnetic susceptibility of Upper Cretaceous strata in the Santa Ana Mountains, California
Magnetic susceptibility was measured on conglomerate clasts and associated matrix along with medium-grained sandstone beds of the Upper Cretaceous strata within Silverado Canyon, Santa Ana, California. A hand-held magnetic susceptibility meter was used on conglomerate clasts and sandstone beds of the Trabuco Formation, Baker Canyon Member and Mustang Spring Conglomerate Lens of the Ladd Formation, and the Schulz Ranch and Pleasants Sandstone members of the Williams Formation. The upper 10 meters of the Trabuco Formation are the starting point, and the lower constraint, of this study. Average magnetic susceptibility (AMS) was determined from counts of 100 clasts within the Trabuco Formation, Baker Canyon conglomerate, and Mustang Spring Conglomerate Lens. AMS was determined for medium-grained sandstone beds throughout the stratigraphic section. Where outcrops permitted, sandstone beds were measured at Jacob Staff intervals. Results of data are as follows: Trabuco Fm. has an AMS of 58 for clasts, and 19 for sandstone matrix. The Baker Canyon conglomerate has an AMS of 27 for clasts and 15 for matrix. The Baker Canyon sandstone beds have an AMS of 6. Clasts in a conglomerate lens within the Baker Canyon sandstone yield an AMS of 195 due to the high magnetic susceptibility of aphanitic volcanic clasts. Within this conglomeratic lens the sandstone matrix has an AMS of 30. The Mustang Spring Conglomerate Lens gives AMS readings of 36 for clasts and 4 for sandstone matrix. AMS for the Schulz Ranch Sandstone Member is 0, and AMS for the Pleasants Sandstone Member is 8. Sandstone beds and sandstone matrix appear to closely follow patterns of magnetic susceptibility associated with the corresponding conglomerate clasts. Overall values of low magnetic susceptibility are consistent with readings of the currently exposed eastern side of the Peninsular Ranges Batholith., San Diego State University
Magnetic susceptibility of Upper Cretaceous strata, El Rosario, Baja California
Magnetic susceptibility was measured on medium-grained sandstone beds and on conglomerate clasts in the Upper Cretaceous strata near El Rosario, Baja California. A hand-held susceptibility meter was used on both the sandstone beds and clasts in the conglomerate beds of the Punta Baja, El Gallo, and Rosario formations. Sandstone beds were measured using a Jacob Staff and range in thickness from 0.5 to 3 meters in the upper Punta Baja Fonnation. The average magnetic susceptibility (AMS) of the beds was 35. A 100 clast count in the upper Punta Baja Formation yielded an AMS of 29 with the clasts consisting predominantly of quartzites and cherts with =some volcanics (AMS 97). Sandstone beds in the lower El Gallo Formation (La Escarpa Member) range from 2 to 6 meters thick and yield an AMS of 13.5. A 100 clast count displayed an AMS of 16 with the clasts also consisting mainly of quartzites and cherts with some volcanics (AMS = 73). The upper El Gallo Formation (El Disecado Member) sandstone beds have thicknesses ranging from 1 to 6 meters and yield an AMS of 584. Magnetic susceptibility readings on 100 clasts produced an AMS of 582. The clasts were generally plutonics with an AMS of 619 and volcanics with an AMS of 895. Quartzite and metamorphic clasts were also present but no cherts were found. The presence of the quartzite clasts effectively lowered the overall AMS in the El Disecado Member. The sandstone beds in the Rosario Formation range in thickness between 1 and 2 meters and yield an AMS of 56. The increase in the levels of magnetite in the clasts as well as the sandstone beds between the upper Punta Baja, lower El Gallo, upper El Gallo, and Rosario formations suggests a change in provenance. The similarity of the AMS readings between the sandstones and the conglomerates suggests that disintegration of the source rock yielded the sand for the sandstones. The data indicate that the magnetite-rich sandstones and conglomerates may have been derived from the western zone of the Peninsular Ranges Batholith (PRB). The magnetite-depleted sandstones and conglomerates indicate the eastern zone of the PRB as a possible source region. Alternatively, other basement-rock masses ( e.g. chert) may have been exposed for a time due to the active and changing tectonics., San Diego State University
Magnetic susceptibility of Upper Cretaceous strata, Santa Ana Mountains, Southern California
Magnetic susceptibility was measured on conglomerate clasts and associated matrix along with medium-grained sandstone beds of the Upper Cretaceous strata within Silverado Canyon, Santa Ana, California. A hand-held magnetic susceptibility meter was used on conglomerate clasts and sandstone beds of the Trabuco Formation, Baker Canyon Member and Mustang Spring Conglomerate Lens of the Ladd Formation, and the Schulz Ranch and Pleasants Sandstone members of the Williams Formation. The upper 10 meters of the Trabuco Formation are the starting point, and the lower constraint, of this study. Average magnetic susceptibility (AMS) was determined from counts of 100 clasts within the Trabuco Formation, Baker Canyon conglomerate, and Mustang Spring Conglomerate Lens. AMS was determined for medium-grained sandstone beds throughout the stratigraphic section. Where outcrops permitted, sandstone beds were measured at Jacob Staff intervals. Results of data are as follows: Trabuco Fm. has an AMS of 58 for clasts, and 19 for sandstone matrix. The Baker Canyon conglomerate has an AMS of 27 for clasts and 15 for sandstone matrix. The Baker Canyon sandstone beds have an AMS of 6. Clasts in a conglomerate lens within the Baker Canyon sandstone yield an AMS of 195 due to the high magnetic susceptibility of aphanitic volcanic clasts. Within this conglomeratic lens the sandstone matrix has an AMS of 30. The Mustang Spring Conglomerate Lens gives AMS readings of36 for clasts and 4 for sandstone matrix. AMS for the Schulz Ranch Sandstone Member is 0, and AMS for the Pleasants Sandstone Member is 8. Sandstone beds and sandstone matrix appear to closely follow patterns of magnetic susceptibility associated with the corresponding conglomerate clasts. Overall values of low magnetic susceptibility are consistent with readings of the currently exposed eastern side of the Peninsular Ranges Batholith., San Diego State University
Maintaining erosion control at the Torrey Pines reserve extension
In the 1960s, a series of gabions were set up along a drainage channel in the Torrey Pines Reserve Extension to prevent sediments from eroding and flooding the newly built homes below. Today, these gabions are all breached and the once straight channel is now meandering and eroding its banks. What caused these gabions to fail and what affect do they have on the drainage during a heavy rainstorm today?, San Diego State University
Major- and trace-element geochemistry and petrogenesis of the North-Central Pisgah Lava Field, Mojave Desert, California
Pisgah Crater is a Pleistocene (perhaps Holocene) scoria cone located approximately 30 miles east of Barstow, California. This large basaltic flow field shows distinct flow fronts within the lava field with unknown associated time intervals. The main purpose of this thesis is to determine whether or not Pisgah lavas are composed of chemically distinct f1ows, and to describe the petrogenesis of the lava flow field. Major­ and trace-element data from 18 samples were used to classify rock types and to distinguish similarities and/or differences among the collected samples. The collected data was then compared with averaged analyses of Basin-and-Range basalt, oceanic­ island basalt, and MORB. The geochemical data was then used to draw conclusions on petrogenesis, largely by means of graphical interpretation, using incompatible-element ratios, spidergrams, and normative-mineral data. The chemical data illustrated that (l) all Pisgah lavas were derived from a similar mantle source, consistent with the sources for both oceanic-island basalt and for the average Basin-and Range basalt, (2) a lava lake present at Pisgah crater was derived from larger degrees of partial melting than the remaining lavas of the flow field, (3) the depth of melting for all lavas was greater than 70 km, and (4) none of the Pisgah lavas are primary magmas, but instead have undergone variable degrees of crystal fractionation. These petrogenetic conclusions are consistent with regional tectonic interpretations that suggest that Basin-and-Range volcanism is the result of asthenospheric upwelling and decompressional melting through a “slab window" beneath the western U.S., San Diego State University
Mass-dependent isotopic fractionation of molybdenum in the MC-ICP-MS: an evaluation of correction methods and an application to MoS2-bearing ores
The stable isotope geochemistry of molybdenum, a highly redox-sensitive element, is poorly known due to the lack of a suitable method to correct for instrumental mass-dependent isotopic fractionation. I worked with Mo isotopic data retrieved from a multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS) to find a precise and accurate method to correct for the effects of instrumental mass-dependant isotopic fractionation. Several correction methods used on the same data from Mo isotopic standards were compared (inter-element, sample-standard bracketing, and graphical). The sample-standard bracketing method is shown to provide a more precise correction than either the inter-element or graphical methods. To further test these correction methods, I applied each method to Mo isotopic data collected from MoS2-bearing ores that formed under a range of redox conditions. Overall, the sample-standard bracketing method of correction also improved the precision of replicate MoS2 analyses compared to the other methods. These MoS2 samples display a 1.4%0 range of Mo/95Mo, which is larger than the 0.4%0 range found in previous studies. These preliminary results suggest that Mo may be isotopically heavier in reducing ore systems compared to oxidizing ore systems, but more data is needed to test this hypothesis. The results of this project will improve our ability to study mass-dependant Mo isotopic variations in nature., San Diego State University

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