Browsing by Subject "geology"
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Item Antecedent Geologic Controls on the Distribution of Oyster Reefs in Copano Bay, Texas(2011-08-08) Piper, Erin AlynnCopano Bay is a shallow (< 2-3 m), microtidal estuary in south central Texas. In an effort to both determine the distribution as well as investigate the controls on the distribution of oyster reefs, a geophysical survey of Copano Bay was conducted in June and July 2007. Surficial sediment analysis confirms that the recent sedimentation in Copano Bay is comprised of mostly estuarine mud with little sand or shell, large extents of oyster reefs and smaller areas of sand. Seismic stratigraphy analyses verify that the first oyster reefs in Copano Bay formed atop topographic highs in the Pleistocene surface. About 6 ka, sea level rise slowed to near its present rate and sediment supply decreased tremendously to Copano Bay decreasing the amount of suspended sediment. The first oyster reefs began forming around this time using these fluvial terraces as suitable substrate. Once the initial reefs were established, additional reefs began forming atop these initial reefs, or on the eroded shell hash material from the initial reefs. During this time of slow sea level rise and low sediment input to the bay, oyster reefs thrived and reef and shell hash material covered a majority of the bay surface. Once climate change increased sediment input to the bay, the reefs began to decrease in size due to siltation. The reefs have continued to decrease in size causing a 64 percent reduction in oyster reef and shell hash area from approximately 4.8 ka to today.Item Geology of the McMillan Ranch in Mason, Texas: An Assessment of the Nature of Normal Faults in the Mason Area(2011-10-21) Harper, Rebecca AnneMason, Texas and the surrounding areas have been previously studied and mapped at small scales, showing the large normal faults that cut through the area. Many secondary faults exist close to the large faults, and are not mapped in previous studies because of the small scale of the maps. The large number of faults, when the smaller secondary faults are considered, makes Mason a good place for studying the nature of normal faults in this region and making generalizations about their nature. This thesis examines one of these faults, the McMillan Fault, and the secondary faults in its hanging wall at a large scale, in order to assess the nature of normal faults in the Mason area. The McMillan Ranch in Mason, Texas, was mapped at a scale of 1:7,000 using both traditional and digital mapping methods, to determine the lengths and displacements of each fault, and attempt to determine a length/displacement ratio which can be applied to all normal faults in this area. A single length/displacement ratio was not determined, just as in previous studies. This study determined that the normal faults in the area are planar, high angle normal faults with varying displacement amounts. As a result, observations determined that deformation in the hanging wall of normal faults exceeded the deformation in the footwalls of the same faults. The main fault on the McMillan Ranch is the McMillan Fault, and its shape is determined based upon the orientation of the subsidiary normal faults in its hanging wall. A detailed study of the geology of the McMillan Ranch and the surrounding area, including a geologic history of the area, geologic map and cross section, and stratigraphic descriptions including bed-by-bed descriptions, stratigraphic column, and thin sections of each unit was carried out as a preliminary step to perform analyses of the faults on the ranch. The presence of the McMillan Fault was already known, and the pasture that was chosen for this study was best represented at a scale of 1:7,000. At such a large scale, it was necessary to recognize precisely where in the stratigraphic section the mapper was located, as some subsidiary faults were recognized by beds missing, rather than entire units. The structural data gathered from the field convey the varying natures of faults, even within the same area, and support the conclusion that length alone is not sufficient to predict displacement value on a fault.Item Hillslope Hydrological Processes in a Costa Rican Rainforest: Water Supply Partitioning Using Isotope Tracers(2014-04-24) DuMont, Andrea LynCosta Rican tropical premontane rainforests are among the world's most valuable ecosystems in terms of diversity of animals, plants, and natural resources. These environments are dependent on water resources which fluctuate in quantity during the dry and wet seasons and which are significantly influenced by vegetation feedbacks. Currently, tropical premontane forest watersheds are insufficiently characterized in terms of groundwater and stream water interactions due to their limited accessibility and complex geological conditions. However, water produced from these watersheds is a critical renewable resource in Costa Rica. It plays a significant role in the production of downstream hydropower and acts as a supply for water distribution systems in many rural areas. In this study, stable isotope tracing of ?^(18)O and ?D was used to determine the source of water in a stream, and the relative contributions of water budget components (e.g., groundwater, soil water). Samples were collected beginning in the dry season and continuing through the wet season from 2013-2014 as the soil became progressively wetter. The ?^(18)O and ?D samples represent precipitation in the tropical forest, as well as groundwater, soil water, and stream water at several locations. This data is important to understanding the influence of vegetation and hydrogeological properties on groundwater and stream water in tropical headwater catchments. Streamflow averaged 0.06 m^(3)/min in baseflow and greater than 0.10 m^(3)/min during storms. Groundwater was seen to contribute to 80% of streamflow and was the main stream component even during storm events. A small proportion of the total amount of streamflow came from interflow and soil water (1%). Additional findings indicated that precipitation, about 4200 mm/yr, in the rainforest can be recycled source water. Storm tracks alternate from distribution starting in the Pacific Ocean to the Caribbean Sea over the course of the wet season. Overall precipitation was seen to be dominated by deep convection and enhanced during the wet season due to the North American Monsoon and the Intertropical Convergence Zone.Item Overprinting Deformations in Mantle Rocks, Dun Mountain, New Zealand(2014-04-25) Donnelly, SaraDun Mountain is the northern-most ultramafic massif of the Dun Mountain Ophiolite Belt, South Island, New Zealand, and is the type-locale of dunite. The dunites that are preserved at this location provide an opportunity for studying olivine rheology over a range of naturally-occurring mantle temperatures, pressures, stresses, and strain rates. Field observations include mesoscale dunite and harzburgite compositional bands ranging from 1 cm to 1 m in scale and oriented at 038?, 40? SE. Spinel lineations are oriented at 004?, 40? and lie approximately in the plane of the compositional bands. Thin sections oriented perpendicular to the compositional bands and parallel to the spinel lineations show olivine bi- and tri- modal grain size distributions and olivine exhibits undulose extinction, subgrains, and a shape-preferred orientation. Olivine lattice preferred orientation indicates ?pencil glide? or that the (010)[100] and (001)[100] slip systems were active. In harzburgite, straight and aligned phase boundaries occur between olivine and orthopyroxene grains and microscale olivine and orthopyroxene compositional bands. Olivine microstructures suggest deformation was accommodated by dislocation creep and, in harzburgite, by dislocation creep and phase boundary sliding. Olivine grain size and grain size distributions in dunite and harzburgite are similar, suggesting that phase boundary sliding in harzburgite occurred due to the presence of orthopyroxene, and not due to a finer olivine grain size. Grain size analyses of olivine indicate grain size distributions containing three populations of grain size with average chord lengths of 5, 1.8, and 0.4 mm. A stress piezometer empirically fit to the experimentally derived relationship between stress and dynamically recrystallized grain size suggests stresses of 2, 5 and 15 MPa, respectively, for these chord lengths. Two-pyroxene thermometry suggests that the intermediate grain size population deformed at 1100 ?C, while the finer grain size population deformed at lower temperatures, perhaps as high as 950 ?C. Mineral equilibria in plagioclase-bearing spinel peridotite yields pressures of 550 to 600 MPa at these temperatures. Deformation mechanism maps based on olivine flow laws at 950 and 1100 ?C both suggest strain rates of 10^(-13) s^(-1). Olivine flow law and stress piezometry indicate deformations at a constant strain rate while stresses increased in response to decreasing temperature conditions.Item Quantification of uncertainty in reservoir simulations influenced by varying input geological parameters, Maria Reservoir, CaHu Field(Texas A&M University, 2005-02-17) Schepers, Karine ChrystelFinding and developing oil and gas resources requires accurate geological information with which to formulate strategies for exploration and exploitation ventures. When data are scarce, statistical procedures are sometimes substituted to compensate for the lack of information about reservoir properties. The most modern methods incorporate geostatistics. Even the best geostatistical methods yield results with varying degrees of uncertainty in their solutions. Geological information is, by its nature, spatially limited and the geoscientist is handicapped in determining appropriate values for various geological parameters that affect the final reservoir model (Massonnat, 1999). This study focuses on reservoir models that depend on geostatistical methods. This is accomplished by quantifying the uncertainty in outcome of reservoir simulations as six different geological variables are changed during a succession of reservoir simulations. In this study, variations in total fluid produced are examined by numerical modeling. Causes of uncertainty in outcomes of the model runs are examined by changing one of six geological parameters for each run. The six geological parameters tested for their impact on reservoir performances include the following: 1) variogram range used to krig thickness layers, 2) morphology around well 14, 3) shelf edge orientation, 4) bathymetry ranges attributed for each facies, 5) variogram range used to simulate facies distribution, 6) extension of the erosion at top of the reservoir. The parameters were assigned values that varied from a minimum to a maximum quantity, determined from petrophysical and core analysis. After simulation runs had been completed, a realistic, 3-dimensional reservoir model was developed that revealed a range of reservoir production data. The parameters that had the most impact on reservoir performance were: 1) the amount of rock eroded at the top of the reservoir zone and 2) the bathymetry assigned to the reservoir facies. This study demonstrates how interaction between geological parameters influence reservoir fluid production, how variations in those parameters influence uncertainties in reservoir simulations, and it highlights the interdependencies between geological variables. The analysis of variance method used to quantify uncertainty in this study was found to be rapid, accurate, and highly satisfactory for this type of study. It is recommended for future applications in the petroleum industry.Item Structural and stratigraphic evolution of the central Mississippi Canyon Area: interaction of salt tectonics and slope processes in the formation of engineering and geologic hazards(Texas A&M University, 2006-04-12) Brand, John RichardApproximately 720 square miles of digital 3-dimensional seismic data covering the eastern Mississippi Canyon area, Gulf of Mexico, continental shelf was used to examine the structural and stratigraphic evolution of the geology in the study area. The analysis focused on salt tectonics and sequence stratigraphy to develop a geologic model for the study area and its potential impact on engineering and geologic hazards. Salt in the study area was found to be established structural end-members derived from shallow-emplaced salt sheets. The transition from regional to local salt tectonics was identified through structural deformation of the stratigraphic section on the seismic data and occurred no later than ~450,000 years ago. From ~450,000 years to present, slope depositional processes have become the dominant geologic process in the study area. Six stratigraphic sequences (I-VI) were identified in the study area and found to correlate with sequences previously defined for the Eastern Mississippi Fan. Condensed sections were the key to the correlation. The sequence stratigraphy for the Eastern Mississippi Fan can be extended ~28 miles west, adding another ~720 square miles to the interpreted Fan. A previously defined channel within the Eastern Fan was identified in the study area and extended the channel ~28 miles west. Previous work on the Eastern Fan identified the source of the Fan to be the Mobile River; however, extending the channel west suggests the sediment source to be from the Mississippi River, not the Mobile River. Further evidence for this was found in ponded turbidites whose source has been previously established as the Mississippi River. Ages of the stratigraphic sequences were compared to changes in eustatic sea level. The formation stratigraphic sequences appear decoupled from sea level change with ?pseudo-highstands? forming condensed sections during pronounced Pleistocene sea level lowstands. Miocene and Pleistocene depositional analogues suggest the location of the shifting Mississippi River Pleistocene depocenter is a more dominant influence on sequence formation. Thus, the application of traditional sequence interpretation with respect to sea level change should be reconsidered to also account for the shifting depocenter for both the study area as well as the broader Eastern Mississippi Fan.Item Transforming Native American Youths' Concepts of Geoscience Through a Connection to Culture, Nature and Community(2014-05-07) Ricci, Jamie LeighThis qualitative study examines the experience of twelve Native American youth who participated in culturally appropriate geoscience summer programs throughout California. These programs have been shown to change participating youths? perceptions of science. After the programs, the youth are more likely to describe science as something tribes use to manage natural resources and have been using for a long time, something that is not only learned in classrooms, that they like science and they can live a cultural way of life and still be scientists. Hermeneutic phenomenology is used to understand the experience of the youth participating in the program. Semi-structured, life-world, pre- and post- interviews were designed to elucidate participants? program experience, conceptions of science and home life. From these, salient themes were found and organized into meaning units. It is suggested that having a supportive community, which youth have identified as a group of people described as familial, supportive and empowering, where youth can express their culture while enjoying outdoor programming provides the foundation and safe space to approach program science. Moreover, positive connections between nature and program science are made in this context. This provides scaffolding where these new conceptions of science as nature, and nature as science, can be applied to participants? lives outside of the program.