Is San Diego Ready For The 'Big One'?

SAN DIEGO -- Could an earthquake as large as the one the devastated China hit San Diego, and if it did, would the city be prepared?

Experts said without a doubt, San Diego could be hit with a powerful earthquake at any time. But the threat might not be as ominous as you think.

"If there's any good news about the China earthquake for us it's we cannot have an earthquake that big and devastating in San Diego," said Dr. Pat Abbott, professor of geology at SDSU.

Abbot said that's because the faults in San Diego are not capable of producing the same dramatic movement. Abbot said the biggest threat is the San Andreas Fault, about 80 miles away, which is overdue for a "big one." The fault is capable of 7.9.

"The high-frequency shaking tends to die down with distance -- but the kind that doesn't is the longer period," Abbott said.

Those low-frequency movements that could sway and potentially topple high-rises and bridges are the situations for which ta special team of first-responders in San Diego County is trained.

"Our teams are most focused on individuals that are truly trapped in heavy structures that have collapsed, or entombed is how we say it," said Jeff Frazier of the Urban Search and Rescue Task Force.

Frazier is a firefighter and leader with the search and rescue task force. He worked recovery after the Oklahoma City bombing and Sept. 11.

"We've done a lot to be prepared and to be able to respond," he said.

Abbott said the largest local threat is the Rose Canyon Fault, which runs under the city and north along the Interstate 5 corridor. That fault could produce up to a magnitude 7.0 quake.
He said older brick buildings would likely crumble, especially those not upgraded to earthquake standards.

Abbott said the death toll from a Rose Canyon quake could range from zero to 30 -- not even close to the devastation seen in China.

Source: NBCSandiego.com

Video

Thesis Defense - Spring 2008 - Sarah Johnson

Physical, petrological, and chemical trends associated with the conversion of k-feldspar-absent quartz dioritic and tonalitic conestones to saprolite in a meditteranean (hot summer) climate, Santa Margarita Ecological Reserve (SMER), southern California, U.S.A

Sarah Johnson
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
W. Nesbitt and colleagues proposed that weathered plutonic material will spread linearly from parental fields subparallel to the A-CN join toward the A-K join on A-CN-K diagrams, and followed by a linear trend subparallel to the A-K join towards the A apex. Such linear trends are common because soil solutions are typically supersaturated with respect to K, but not CN. At SMER, our studies reveal a weathering trend that deviates from that proposed by W. Nesbitt and colleagues. SMER lies within a Mediterranean (hot summer) climate with an average precipitation of ~39.4 cm/yr and average temperature of ~16.6˚C. We sampled corestone and adjacent saprolite in an ~123 Ma tonalite, and at two sites lying within an ~107.5 Ma quartz diorite for thin section, physical properties, and chemical analyses. We used the chemical index of alteration (CIA) to determine the degree of weathering and used the transport function (τ) for assessing changes in elemental mass. Each variety of sampled corestone lacks modal K-feldspar but contains 4.50% to 12.0% biotite. Our thin section study of samples of saprolite suggests the following order, from most to least weathered; biotite, amphibole, plagioclase, and quartz. On A-CN-K diagrams saprolitic samples spread linearly from the parental material away from the K apex toward the A-CN join. XRD and microscope analyses indicate that this trend is due to the conversion of biotite to the mixed-layer expandable clays vermiculite-illite and vermiculite-smectite. The transport function, (τ), indicates that during this conversion K, Rb, and to a lesser extent Ba mass was consistently removed at the level of our sampling traverses by migrating paralithic fluids. In contrast, zero to only minor increases or decreases in the masses of most other elements reflect the spatially inconsistent and varied activity of eluviation and illuviation processes within the paralithic zone. Our data consistently suggest that in plutonic rocks lacking K-feldspar, biotite weathers more readily than plagioclase, and, as a result, it controls the direction of weathering trends in A-CN-K space. As noted above, the signature of this process is a trend in saprolitic samples extending from unweathered parental plutonic material away from the K apex and toward the A-CN join. Once biotite has been completely altered, then the weathering of plagioclase should produce a new trend extending subparallel to the A-CN join and toward the A apex. Under the Mediterranean (hot summer) conditions at SMER our data appear to have captured the first step in the initiation of this process.

Thesis Defense - Spring 2008 - Wallace Sconiers

The feasibility of antipodal volcanism as a result of the K/T impact

Wallace Sconiers
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Jared Morrow

Friday, May 9th

ABSTRACT
Research regarding the impact event at the Cretaceous-Tertiary (K/T) boundary at Chicxulub, Yucatan Peninsula, and the Deccan flood basalts of Western India has shown both possibility and dismissal of the two as being a series of interrelated events, although most workers agree that both sides were relatively antipodal at the K/T boundary. Subsequent 2D & 3D computer models following the development of Simplified Arbitrary Lagrangian-Eulerian (SALE) and similar code have shown that axial focusing of seismic waves following an impact for a planet analogous to earth is most significant at ~100 km depth below the antipode surface. However, calculations of the initial kinetic energy, total seismic energy produced as a function of the seismic efficiency for a C2 Chondrite bolide impact, and total energy delivered to a basaltic volume near its melting point at a depth of ~100 km generate a thermal pulse, or sudden change in temperature, of 1 millikelvin. This temperature increase is not sufficient to create or enhance pre-existing melts at depth.
Estimates of the total volume of lavas produced at the Deccan traps range from 1 x 10^5 to 1 x 10^6 km^3 over a duration of ~1 m.y., with average intervals between eruptions of sub-groups within the traps of 2-10,000 years. A stratigraphic section composed of main eruptive units within the traps shows one sub-group, the Wai, which is responsible for 50% of the total eruption volume from 66 to 64.5 Ma, peaking with the Ambenali Formation within the sub-group producing 200,000 km^3 of basalt 66 to 65.5 Ma. Activity substantially drops during the last two formations within the sub-group, Panhala and Desur, producing 25,000 and 10,000 km^3, respectively. A ~65 Ma date for the K/T impact would have had no effect on the Deccan trap system whose eruptive volumes were dropping per successive formation at this time.

Thesis Defense - Spring 2008 - Angela Cavallini

Major element variations of Hawaiian parental magmas:
Mantle source or melting control?

Angela Cavallini
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Friday, May 9th

ABSTRACT
The abundances of the major element oxides (SiO2, TiO2, Fe2O3, MnO, MgO, CaO, NaO2, K2O, P2O5) in Hawaiian lavas are subject to change due to variation in (1) the amount of crystal fractionation or accumulation or (2) the pressure (depth) and degree of partial melting of the mantle, and (3) differences in the composition of the mantle source. Since isotope ratios (e.g. 206Pb/204Pb or 87Sr/ 86Sr) are not subject to the effects of crystal fractionation or partial melting, they are thought to be good indicators of the mantle source composition. Literature data shows that there is a correlation between the isotope ratios and major elements abundances of Hawaiian lavas when the latter are corrected for the effects of crystal fractionation. This suggests a relationship between the major element chemistry and the mantle source composition. However, another possibility is that the variations are related to changes in the depth of melting. Hawaiian volcanoes been categorized into two main geographic trends: the northeastern Kea trend (named after Mauna Kea) and the southwestern Loa trend (named after Mauna Loa). In this study, I summarized major element data for both Kea and Loa lavas from the scientific literature. The Kea trend lavas include Mauna Kea and Kilauea and the Loa trend lavas include Mauna Loa, Koolau, and Kahoolawe. Prior to this study, Loihi lavas were grouped with the Loa trend (based on geography), but I found that their chemistry is actually more similar to the Kea trend. To correct for the effect of crystal fractionation and accumulation, sample data were adjusted to a constant MgO value. This was achieved by running the data through a computer program which added or subtracted small increments of equilibrium olivine to each sample composition until 13 wt. % MgO was reached. My results show that there are significant variations in the major elements chemistry between Kilauea, Kahoolawe, Mauna Kea, Mauna Loa and Loihi lavas. The Kea trend lavas are relatively low in SiO2 and abundant in CaO. The Loa trend lavas have higher SiO2 and lower CaO. Some of the most significant variations are a range greater than 3% in both SiO2 and CaO. A 2% range in SiO2 can possibly be explained by variations in the pressure (depth) of melting. However, these same pressure differences cannot explain the 3% variation in CaO. Thus, these major elements variations are better explained by differences in the mantle source composition of these volcanoes. The lower SiO2 and higher CaO lavas are consistent with melting mantle peridotite, whereas the higher SiO2 and lower CaO lavas are not. These lavas are likely formed by melting of pyroxenite derived from ancient, recycled oceanic crust within the Hawaiian mantle plume.

Thesis Defense - Spring 2008 - Aaron Hebeler

Elemental Transport and Volume Strain of Fault Core and Damage Zone, San Jacinto Fault Zone, California: Assessing The Influence of Weathering

Aaron Hebeler
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
The Clark segment of the San Jacinto Fault Zone is located in southern California. Our study was aimed at evaluating the chemical and physical properties of fault core and damage zone for the affects of chemical weathering. Fault zones are composed of distinct architectural zones that are based on texture, structural fabric, and grain size. Architectural zones commonly include a fault core, damage zone, and wall rocks. These architectural zones are well displayed in an exposure of the Clark segment of the San Jacinto fault zone. At this location measurements of bulk density progressively decrease from high values in the wall rocks to lower values in the fault core. In contrast, porosity trends show an inverse relation with regard to bulk density trends. In A-CN-K space samples from inner damage zone, outer damage zone, and fault core emanate from a clustering of points derived from the wall rocks toward the A-K join in a systematic progressive fashion. This relationship is consistent with petrological observations that suggest that grain size is systematically reduced within each architectural zone culminating in a generally black structureless aphanitic fault core. In A-CNK-FM space samples again spread in a linear fashion from wall rocks to the fault core toward the A-FM join. This spread again correlates well with each architectural zone and grain size. The transport function, τ, suggests that there is progressive loss of Al, Ca, Na, K, and Sr mass toward the fault core. In contrast, the masses of Fe, Mg, Mn are increased in the fault core relative to the adjacent damage zone and wall rock. The above relations are consistent with the idea that fault zones are greatly susceptible to chemical weathering because of their reduced grain size and hence greater surface area. Moreover, our data suggest that at or near the Earth’s surface they are also highly porous. Apparently chemical weathering under such conditions involves degradation of plagioclase, amphibole, and biotite by migrating soil and paralithic fluids. These fluids remove Al, Ca, Na, K, and Sr from degrading plagioclase, and convert amphiboles and biotites to mixed layer expandable clays such as vermiculite and mixed vermiculite and chlorite. The increase in Fe, Mg, and Mn mass within the fault core implies that the fluids were transporting such ions and that they accumulated within the fault core perhaps by some cation exchange process.

Thesis Defense - Spring 2008 - Jennifer Piper

Column Matrix Effects in the Mass Spectrometer and the Effects
on Isotope Analyses

Jennifer Piper
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Friday, May 9th

ABSTRACT
Mass spectrometry can be used to analyze radiogenic and stable isotopes in order to determine the concentrations of different isotopes in a sample, ages of rocks, and sample correlations. Multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has become the preferred method for isotope analysis compared to other methods because it is very precise, the plasma can ionize any element, and there is less time required for each analysis. One of the downfalls of isotope analysis using MC-ICP-MS is that it induces an instrumental mass-dependent fractionation (instrumental mass bias) that needs to be corrected for. This instrumental mass bias is primarily due to the extremely high temperatures of the plasma, which causes a spread in ion energy during ionization and transport within the mass spectrometer.
Instrumental mass bias in the MC-ICP-MS can be corrected using one of two main techniques (1) standard sample bracketing (SSB) or (2) double spiking. SSB is commonly favored, but it is more susceptible to matrix effects that result in inaccurate results. Many different types of matrix effects in the MC-ICP-MS have been identified and corrected for. Spectral matrix effects, or isobaric interferences, result from the occurrence of an element that overlaps in mass with the isotope of interest. Non-spectral matrix effects can create differences in the measured isotope ratios of a sample and the standard.
In a recent study, a new and possibly uncontrollable matrix effect was found that is thought to have come from the separation and purification of molybdenum using an anion exchange resin. It was shown that the collection of a pure Mo-free solution that had been passed through an anion exchange resin and subsequently added to a Mo standard appeared to be isotopically lighter than expected when compared to the same untreated Mo standard. Many tests were performed to try and correct for this “column matrix effect” but all have failed.
The purpose of this experiment is to test for the presence of this column matrix effect in measurements of radiogenic isotopes of U, Th and Pb to see if it affects elements other than molybdenum. In all of the experiments, the standards run with the addition of a column matrix appear to have a higher signal intensity than the untreated standards. However, this effect does not seem as significant when comparing the isotopic ratios of the treated and untreated standards. Based on the data obtained from this experiment, it can be said that there is a column matrix effect for U, Th, and Pb and it is produced from the resin that is used in the column chemistry.

Thesis Defense - Spring 2008 - Michelle Apon

Relationships between QuikSCAT Satellite Sea Surface Winds and Sea Surface Temperatures Off California and Northern Baja

Michelle Apon
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Clive Dorman

Friday, May 9th

ABSTRACT
QuikSCAT is an earth observing satellite that measures ten-meter wind speeds and wind direction over the sea surface. It is used to investigate sea surface wind patterns off the coastal waters of Southern California. A restricted data set is used with pixels approximately twelve kilometers by six kilometers. This small scale data set presents fine resolution of the winds. Wind driven coastal upwelling is expected where the wind blows parallel to the coast, heading from north to south, with the coast on the left, facing downwind. According to QuikSCAT data analysis, faster wind speeds will cause colder sea surface temperatures in the water. Mean QuikSCAT winds are compared to satellite derived sea surface temperatures for June 2001. Coldest sea surface temperatures are along the Central California coast where the sea surface winds are fastest in speed with a wind direction parallel to the coast. Warmest sea surface temperatures are in the Southern California Bight region where the coastal winds are both weakest and blowing across the coast due east. Cooler sea surface temperatures are along Northern Baja California, where wind speeds increase modestly and realign parallel to the coast.

CTL SEMINAR - R. Mark Leckie

Active Learning in Large GE Course: Examples from Oceanography

R. Mark Leckie
Department of Geosciences
University of Massachusetts, Amherst

Wednesday, May 8th, 2008

Sponsored by the
San Diego State University
Center for Teaching and Learning

Thesis Defense - Spring 2008 - Jon Sainsbury

The geology of Isla Natividad, Baja California Sur, Mexico

Jon Sainsbury
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. David Kimbrough

Friday, May 9th

ABSTRACT
Isla Natividad is a Pacific island that sits just offshore to the west of Punta Eugenia on the Vizcaino Peninsula of Baja California. The geology of Baja California here preserves remnants of a Mesozoic Subduction zone-magmatic arc as well as Cenozoic rocks and structures that record the evolution of the region from a convergent margin to a strike slip dominated plate boundary.
Isla Natividad is underlain almost entirely by sedimentary rocks of the Late Jurassic-Early Cretaceous Eugenia Formation which comprises part of the thick forearc basin marine stratigraphic assesmblages in the Vizcaino region. The type locality of the Eugenia Formation is at the fault bounded western tip of the Vizcaino Peninsula (Punta Eugenia), and it has also mapped on Cedros Island and recognized in reconnaissance on Isla Natividad. The goal of this study is to provide basic mapping and descriptions of the geology of Isla Natividad.
Geologic mapping conducted in this study reveals that the main part of the island is dominated by gently southwest dipping strata that include conglomerate, turbidite and mudstone units.
Conglomerate outcrops mainly on the NE coast of the island. It is dominated by cobble-boulder clast sizes that range from 0.1-0.5 meters. The maximum clast size observed is 0.75 meters. Conglomerates are clast-supported, massive, well-indurated and weathered to a dark grey color. Clast imbrication is locally present but there is no internal bedding or size grading. More than 50% of the clasts are rounded quartzite clasts. Other clast compositions include andesite, chert and subangular intraformational sandstone and mudstone rip-up clasts.
Turbidite beds are well exposed along the coast in places and have an average bedding strike and dip of N50W 30SW. They are characterized by laterally continuous uniform thickness beds with classic sediment gravity flow structures such as graded bedding and parallel laminations. They comprise 5-8 cm thick beds of fine-grained feldspathic-lithic sandstone and 2-4 cm beds of grayish siltstone to mudstone. Sandstone and siltstone weathers to a green color.
The mudstone unit makes up the higher elevations along the crest of the island and have an average orientation of N40W 20SW. It outcrops poorly except along the coast but consists of dark grayish-green siltstone, brownish heavily fractured mudstone, blackish shale, and occasional light gray claystone. Beds range in thickness from several millimeter thick laminae up to 1-4 centimeters beds. Desert vegetation including jumping choya cactus prefer this fine grained unit so that the distribution of the units can be traced using Google Earth images.
Massive poorly bedded sandy breccia occurs throughout the island. It consists of 50 percent angular gravel clasts in a muddy-fine grained sandy matrix. Occasional size grading was noted with large 1 meter diameter gray shale and sandstone lense rip-up boulders concentrated near tops of beds. This unit has occasional interbeds of sandstone with 4-6 centimeter thick beds and 1 centimeter laminations. The average orientations of the interbeds were N50W 40SW. This massive unit was interpreted as debris flows and near-shore sediment slumps.
Other map units on the island include a whitish shell dominated beach/channel sands. This unit was classified as a shallow dipping well-sorted feldspathic shelly arenite with cross-bedded laminations and poor to absent bedding planes. It was bounded by erosional unconformity contacts. A fine-grained resistant lithic sandstone to siltstone was found on the southern tip of the island and bounded by coast and a southern fault zone. These units may be correlative to the Pliocene Almejas Formation which is widespread through the Vizcaino region.
In the southern part of the island a major fault that strikes N70E cuts off the southern tip of the island. This fault is well exposed on the beach at the SW corner of the island where is comprises a broad shear zone (~20 m wide) with folded, faulted and disrupted strata. Strike and dips in units immediately to the north side of the fault are discordant to regional bedding strike and dip on the island. Two other potential faults with NE strikes were also mapped farther north on the island. The NE strike of this faulting is highly discordant to the strike of major northwest trending Cenozoic faults along this part of the Pacific margin. These include the San Benito and Tosco-Abreojos Faults that are believed to have accommodated Pacific-North America relative plate motion from ~12-6 Ma before the plate boundary jumped eastwards into its present position in the Gulf of California.
The strong discordance of the Natividad structures to the NW trend of the Baja margin here suggests these faults may be conjugate shear zones within this system as R’ left lateral faults. Sub-horizontal slickensides were observed in the fault-breccia zone. These faults could also be reverse-thrust faults as the fault zone shows evidence of compression and the dip of the fault was measured to be 66 degrees. Using an average N32-42W trend of the transform boundary this puts these faults striking almost perpendicular to the principle stress direction of the Pacific plate sliding north-westward. This study is only a reconnaissance survey of the island geology. However it confirms the presence of the Eugenia Formation and documents abundant quartzite which indicates a continental provenance for the Eugenia basin. The N70E trending faults are interpreted as Cenozoic faults related to major NW trending structures along this part of the Pacific margin.

Thesis Defense - Spring 2008 - Ryan Sholty

Petrochemical Stratigraphy and Corrleation of Steens Basalt between Catlow Peak and Steens Mountain, Oregon

Ryan Sholty
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Vic Camp

Friday, May 9th

ABSTRACT
Catlow Peak is located in southeastern Oregon, near the McDermitt caldera which is the first caldera to form along the Yellowstone hotspot track. The Catlow Peak section includes 421 m of Steens Basalt, which represents the oldest basalt unit of the Columbia River Basalt Group (CRBG). Most workers believe that the CRBG and the Yellowstone hotspot track are genetically related, with the former forming above the Yellowstone mantle plume head and the latter above its connecting plume tail. The most complete section of Steens Basalt was collected at Steens Mountain, ~77 km north of Catlow Peak. Here, I examine the chemical stratigraphy of Steens Basalt at Catlow Peak with the intent of determining if flow correlations can be made between Catlow Peak and Steens Mountain. Knowing these correlations will result in a further understanding of the processes that develop this major basaltic group.
Samples from Catlow Peak were collected by Nick Jarboe, a Ph.D. candidate at UC Santa Cruz, for a detailed paleomagnetic study. I have obtained his complete section of 70 samples, each of which I have analyzed for major and trace elements on the MagixPro XRF spectrometer at San Diego State Universtity. Here, I combine my data on rock chemistry with the paleomagnetic data of Jarboe for Catlow Peak, the paleomagnetic data of Mankinin et al. (1987) for Steens Mountain, and the geochemical data of Johnson et al. (1998) for Steens Mountain in an attempt to correlate flows or groups of flows between these two stratigraphic sections.
The Steens Basalt has been divided into two stratigraphic and chemical groups: Upper Steens and Lower Steens by Camp et.al (2003). The Lower Steens lavas typically have Ba contents < 290 ppm and K2O contents < 1.00 wt.%, with Upper Steens lavas typically being higher than these values. The Upper Steens shows more variability in Ba and K2O while the Lower Steens shows more homogeneity. The Catlow Peak lavas show more variability so it is assumed that the chemistry is somewhat similar to the Upper Steens section. This is consistent with the paleomagnetic stratigraphy in that the Catlow Peak lavas occur across a well-established paleomagnetic transition in Upper Steens basalt which is present in the upper third of the Steens Mountain section. However, there are slight differences in the chemical pattern of the Catlow peak lavas, indicating that they were derived from different eruptive centers than the lavas at Steens Mountain. Incompatible elements such as Nb and Zr were plotted together showing three visible trend lines: Upper Steens, Lower Steens, and Catlow Peak. The Steens trend lines appear to pass through the origin, which indicates trends of constant Zr/Nb ratio. The Catlow Peak trend line appears to have a slightly different origin. These trends with subtly different ratios indicate that the Steens section (upper and lower), and Catlow Peak lavas may have been derived from subtly but different mantle sources.

Thesis Defense - Spring 2008 - Melanie Biggs

Miocene volcanic rocks and conglomerates, SE California: Evidence for Neogene reactivation of the Chocolate Mountains anticlinorium

Melanie Biggs
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
In and around Picacho State Recreation Area, early Miocene volcanic rocks are folded about the Paleogene Chocolate Mountains anticlinorium. New laser ablation U-Pb zircon geochronology reveals that the Walker andesite, one of the youngest mapped volcanic units lying on the northern limb of the anticlinorium, is 23.5 ± 1.0 m.y. old. The early Miocene volcanics, including the 23.2 ± 0.2 Ma ignimbrite of Ferguson Wash, also are displaced in a dextral sense ~1 km by the Taylor Lake fault system. About 11-13 km west of Picacho, flat-lying gravels of the upper member of the Bear Canyon conglomerate are capped by the ~13 - 9 Ma basalt of Black Mountain. In contrast, the lower and upper members of the Bear Canyon conglomerate at Picacho are tilted ~21o and ~15o respectively on the northern limb of the anticlinorium, while flat lying gravels of the upper member are not folded nor transected by the Taylor Lake fault system. Geochemical data indicate that the clasts in the lower member of the conglomerate were derived from the progressive unroofing of the underlying early Miocene volcanics. These observations suggest that the Chocolate Mountains anticlinorium is a long-lived feature that may have been reactivated during formation of the Taylor Lake fault system sometime between ~23 Ma and ~13-9 Ma. If this interpretation is correct, then the Chocolate Mountains anticlinorium and associated structures would have accommodated some Neogene displacement, and, as a result, the discrepancy between paleoseismic and plate tectonic estimates of the displacement history between the Pacific and North American plates would be mitigated to some extent.

Thesis Defense - Spring 2008 - Lauren Palazzolo

How useful is incomplete information for inferring predation intensity?

Lauren Palazzolo
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Lindsey Leighton

Friday, May 9th

Thesis Defense - Spring 2008 - Cynthia Earl

Three dimensional elemental mass transfer during development of saprolite from corestone: Santa Margarita Ecological Reserve, Temecula, California

Cynthia Earl
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
Our understanding of the characteristics of chemical weathering in a Mediterranean (hot summer) climate has increased significantly over the last few years. For example, we now know that as quartz dioritic corestone is converted to saprolite in this climate, biotite expands and weathers to mixed-layer clays while plagioclase and amphibole play much less significant roles. However, there have been no studies that have investigated the three dimensional aspect of this weathering trend. Hence, I undertook a study that utilized the previous work of Chris Martinez to constrain the horizontal aspects while I focused on assessing the relevance of this trend along a vertical sampling traverse through the same corestone-saprolite pair. I collected 9 specimens, 5 from the corestone and 4 from the saprolite, spaced approximately equally, along a 1.2 m vertical traverse. Each sample was thin sectioned for petrological work and was analyzed for major and trace elements using standard XRF methods. Samples of the corestone are hypidomorphic granular quartz diorites that exhibit little evidence of chemical alteration. In thin sections, euhedral to subhedral biotite exhibits deep brown pleochroism, sharp traces of the {001} cleavage, and typical “birds eye” extinction. Similarly subhedral plagioclase exhibits well developed polysynthetic twinning and concentric compositional zones, while amphibole commonly displays cleavage parallel to {110} at angles of 56o and 124o. In contrast, in saprolite overlying the corestone, the {001} cleavage in biotite becomes less distinct while pleochroism takes on pale orange hues. S. Johnson showed that these characteristics reflect the transformation of biotite to mixed layer expandable clays. Unlike biotite, both plagioclase and amphibole exhibit only mild indications of weathering, while quartz remains unweathered. The average Chemical Index of Alteration (CIA) for corestone samples analyzed during this study is 60.0 ± 0.1 (95% confidence interval) and for saprolite is 60.9 ± 0.5 (95% confidence interval). In A-CN-K ternary space, samples of the corestone plot below and more toward the K-apex than do specimens of saprolite which plot close to the A-CN join. Moreover, τ, the transport function, suggests that the masses of Si, Fe, Ca, Na, Mg, and Mn changed little relative to their masses in the average corestone. In contrast, the masses of K, Rb, and Ba are depleted significantly in the saprolite. Volumetric strain, ε, for the four samples of saprolite ranges from 16% to 21% while porosity ranges from 14.5 % to 16.5%. In contrast, there is little significant change in bulk mass in going from the corestone to the saprolite. The above weathering trends are similar to those documented by C. Martinez, and thus suggest that saprolite development from corestone is a three dimensional process that in Mediterranean (hot summer) climates depends primarily on the expansion and alteration of biotite to mixed layer expandable clay. Apparently, during this process stresses are produced that are sufficiently large enough to break or weaken adjacent intercrystalline bonds. Given sufficient time, this process will render solid and hard quartz diorite to saprolite, a weak material that breaks easily under moderate finger pressure.

Thesis Defense - Spring 2008 - Amelinda Webb

Quantifying the Ecological Response of Brachiopods during the Ordovician Extinction

Amelinda Webb
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Lindsey Leighton

Friday, May 9th

ABSTRACT
Extinction events are morbidly fascinating to both scientists and the public alike, with catastrophic events and death occurring on vast scales. The current biodiversity crisis is being hailed as the next mass extinction, and the need to understand the processes and patterns of extinction grow as the rate of species loss rises. Extinction events are recognized by taxonomic loss; however the ecological impact of these events is more difficult to quantify. This study applies rank-abundance curves (RACs), a modern ecological tool used in measuring ecosystem health, to quantify the changing community structure of brachiopods during the two pulses of the end Ordovician mass extinction. By applying RACs and measuring the shape with kurtosis, communities can be compared before and after the extinction event to recognize any early warning signs as well as the point of ecological recovery for communities which may be offset from the recovery of taxonomic diversity.
Brachiopod communities were sampled from Paul Copper’s collection from Anticosti Island located at the Geological Survey of Canada’s Ottawa office. Anticosti Island is the most complete, shallow water, fossiliferous record of the Ordovician- Silurian boundary in North America. Upper Ordovician through lowermost Silurian samples were collected from the surface of limestone slabs that showed no evidence of taphonomic overprinting. Stratigraphic position (within 10 members), lithology (Dunham carbonate classification scheme), and surface area of all samples were noted. Individuals were identified to the lowest taxonomic level (species in most cases), and both abundance and biovolume data were collected. RACs were generated for every sample with taxonomic richness greater than two. Samples dominated by one taxon were noted, and the frequency of these communities was compared to the kurtosis of the RACs from that member.
The average kurtosis for each member shows a distinct pattern through both pulses of the extinction. The members directly below each extinction pulse show increasing stress, and the members following the extinction pulse have significantly lower stress (t-test, p<0.05). The frequency of communities dominated by one taxon shows the same pattern as the average kurtosis. While the taxonomic recovery has not been identified until the Middle Silurian, there is an ecological recovery relatively quickly after the increased stress levels before each extinction pulse. These findings support previous qualitative work that suggests that the ecological impact of the end Ordovician extinction is not as severe as the taxonomic impact which ranks this extinction as the second worst in the history of life on Earth. RACs have been successfully used in modern systems, and now this technique can be applied to the fossil record to better understand the progression of and recovery from extinction events.