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Geology

The Influence of Normal Fault Geometry on Porous Sandstone Deformation: Insights from Mechanical Models

Kali Allison ('12); Dulcie Head ('14); Mentor: Linda Reinen

Abstract:  Slip on non-planar faults can produce secondary faults, too small to be resolved on seismic surveys, that may affect host rock fluid flow. Numerical modeling can yield insights into the role of fault geometry on regions of altered fluid flow. We model the system as a segmented fault in a linear elastic, homogeneous, whole or half space subjected to constant or gradient stress. Preliminary results indicate that the size of the damage zone increases in the footwall with increasing lower leg dip and remains constant in the hanging wall; the dip of the lower fault segment significantly affects the amount and style of deformation. We also investigate how vertical offset and dip of host rock beds respond to slip on a planar fault in the same system under a constant stress. We find that vertical offset increases with slip, which increases with size of the fault. Dip variations appear negligible.
Funding Provided by: Geology Faculty Research Fund (KA) Pomona College SURP (DH) 

Investigating Endolithic Algae Proliferation in Montastraea Faveolata, a Caribbean Reef Building Coral

Cornelia Clarke ('12); Karla Parsons-Hubbard*; Dennis Hubbard*; Mentor: Robert Gaines
*Oberlin College, Oberlin, Ohio

Abstract:  As a student of a Keck Geology Consortium project, I spent two weeks in the Virgin Islands collecting coral samples to study bioerosion of coral. Corals are host to a variety of organisms, including endolithic green algae that live inside corals. Cross sections of corals sometimes display vivid green bands distributed throughout the skeleton, indicating preserved dead algae. The cause of these bands is still disputed, but several techniques can help test potential theories of whether the bands are caused by differential preservation or differential growth. Scanning electron microscopy can be used to estimate the volume of algae in green bands versus non-green bands, while carbon and oxygen isotope studies can determine temperature and coral growth, teasing out the relationship between the coral and the alga. As corals are becoming increasingly threatened, understanding the interactions between coral and its microbiome is vital to understanding the effect of a changing environment.
Funding Provided by: Keck Geology Consortium  

Using Finite Element Flexural Models to Study the Effects of System Geometry on Venusian Magma Reservoir Failur

Lorelei Curtin ('13); Benjamin Murphy ('13); Shelley Chestler ('11); Gustavo Ruiz ('13); James Muller ('13); Kyle Metcalfe ('14);*Julita Perido ('12); Mentor: Eric Grosfils
*Mount Holyoke College South Hadley, Massachusetts

Abstract:  Using axisymmetric finite element models, we investigated factors controlling the failure of magma chambers beneath large Venusian volcanoes. Parameters examined include the depth and shape of the magma chamber, the weight of the overlying host rock and edifice, and the flexural response of the lithosphere—which is a function of the elastic thickness. We found that all of these variables play a role in the amount of pressure that can build up inside a chamber before it ruptures, and that their interplay controls the location at which the chamber ruptures. Initial failure near the crest of the reservoir can feed surface eruptions, enabling edifice growth. As more eruptions take place, however, mass is added to the edifice, and the corresponding flexural response of the underlying crust eventually forces the chamber to fail at the middle or bottom, terminating movement of magma to the surface and upward growth of the volcano.
Funding Provided by: NASA's Planetary Geology and Geophysics Program (EG) Pomona College SURP (JM, GR) Mount Holyoke Summer Research Fellowship (KM)

Silica Cementation of Lacustrine Sandstone, Long Valley Caldera, California

James Muller ('13); Mentor: Jade Star Lackey

Abstract:  The Long Valley Caldera is home to lacustrine sandstones silicified ~300,000 years ago by silica-saturated hydrothermal fluids, the source of which was a magnetically driven hydrothermal system.  Not buried under further sedimentary deposits due to the draining of the Long Valley Lake, these rocks are natural examples of silica cementation, a process also found in rift basins settings around the world, and accordingly provide an opportunity to study a well-preserved example of said process.  Through the use of δ18Ο analysis, specifically via ion microprobe, insight can be gained regarding the characteristics of sandstone reservoirs, hydrothermal fluid flow, and basin diagenesis, and shed light on how cementation affects permeability and porosity of volcaniclastic reservoirs, which has implications for the production of hydrothermal petroleum and assessing the longevity of geothermal systems.
Funding Provided by: Pomona College SURP (JM) Petroleum Research Fund (JSL) 

Structural Study of the Cheddar Dome, Grenville Province, Ontario, Canada

Callie Sendek ('12); Michelle Markley*; Steve Dunn*; William Peck†; Mentor: Linda Reinen
*Mount Holyoke College South Hadley, Massachusetts; †Colgate University, Hamilton, New York

Abstract:  The Cheddar dome in Southeastern Ontario is one in a series of granitic gneiss domes associated with the Grenville Orogeny of approximately one billion years ago. Gneiss domes and gneiss dome systems have been associated with orogenic events worldwide, and their formation attributed to a variety of tectonic environments. Three distinct mechanisms for the Cheddar dome’s formation have been proposed. Each has a unique implication for the tectonic stresses at work in the area at the time of orogeny. During the summer I spent two weeks in Ontario mapping field relations and collecting samples. Thin sections are being prepared from these samples for kinematic analysis. During the upcoming year, I will develop the project into my senior thesis. By coupling kinematic analysis with structural data from the area I will be able to evaluate the mechanisms of dome emplacement and contribute to the current understanding of regional deformation during orogeny.
Funding Provided by: Keck Geology Consortium  

Research at Pomona