May 2012 Meeting

Monday, May 14, 2012
6:00 - 7:30 pm

Pellissippi State Technical Community College
10915 Hardin Valley Road, Knoxville
J.L Goins Administration Building, Cafeteria Annex


Award-Winning Student Presentations

This month ETGS meeting will follow a slightly different format than is usual. Instead of having one speaker give a 50-minute-long presentation, we will be featuring three "mini" presentations, each approximately 15 minutes long, by graduate students from the Department of Earth and Planetary Sciences (EPS) at the University of Tennessee - Knoxville. Each year EPS offers a course on Professional Presentations (Geology 596) to provide a formal opportunity for students to develop their oral communication skills. This one-credit course involves writing an abstract and preparing, practicing, and delivering a professional presentation on any geological topic of interest, usually a portion of their dissertation/thesis research. The students present their talks at a departmental seminar and they are ranked by the seminar attendees and a five-person committee consisting of faculty, post-doc, and students. Once again, ETGS is partnering with EPS to further broaden this valuable experience by offering awards to the student presenters and inviting them to give their talks to a professional geology audience at the May ETGS meeting. We hope you can join us to support this new generation of geologists and see their presentations.

Microgravity and Micromagnetic Evidence for Shallow Subsurface
Termination of the Northeast Dike at Ship Rock, NM

Carolyn Tewksbury-Christle
Department of Earth & Planetary Sciences, University of Tennessee
Knoxville, Tennessee

The northeast dike at Ship Rock, NM is composed of en echelon segments whose offsets do not appear to be formed by post-emplacement deformation. Delaney and Pollard (1981) modeled the subsurface structure of the northeast dike based on the surface expression and concluded that the en echelon segments formed due to rotation of the maximum horizontal compressive stress direction as the dike propagated vertically. They proposed that a main dike is located at depth, connecting the en echelon segments.

I modeled the structure under the northeast dike based on gravity and magnetic field measurements I gathered over a pair of en echelon dike segments and used that modeling to constrain the emplacement mechanism. Though the reduced the gravity data appears to be random, suggesting that the actual gravity anomaly is within the error of the Lacoste and Romberg gravimeter used for this study, the magnetics show a distinct anomaly along the dike. For this reason, I developed several models of the subsurface using the University of British Columbia's Mag3D programs and determined a best fit between modeled and field data. The initial models, based off of Delaney and Pollard's research, produced anomalies that are both larger and wider than the anomalies in the measured field data. To produce a smaller and narrower anomaly, dike material must be removed from the model. In the best-fit model for the magnetic field data, the dike terminates at approximately six meters below the surface. This surprising result suggests that the northeast dike was originally emplaced above the present-day land surface and propagated both upwards and downwards. The remnants of the northeast dike appear to be the base, not the top, of the original dike, which has since been eroded away, and the en echelon segments formed due to rotation of the maximum horizontal compressive stress direction at the bottom edge of the dike.

Comparative Functional Analysis of Microbial Sulfide Oxidation
and its Impact on Environmental Geochemistry

Brendan Headd

Department of Earth & Planetary Sciences, University of Tennessee
Knoxville, Tennessee

Numerous metabolic strategies have evolved in Bacteria and Archaea to utilize reduced sulfur compounds as electron donors for energy transformation and the fixation of carbon dioxide for either chemolithoautotrophy or anoxygenic photosynthesis. As such, different enzymes, pathways, and mechanisms for the oxidation of reduced sulfur compounds are known and complicate detection and phylogenetic descriptions of sulfur-oxidizing microbes in natural systems. The extent to which gene nucleotide variation in any of the sulfur oxidation pathways affects enzyme and pathway function are not well understood. Gene variations among different sulfur bacteria in close proximity to one another, even if utilizing the same metabolic pathway, likely yield distinct and quantifiable metabolic potentials and byproducts. Because the sulfur oxidation (Sox) system is present in diverse guilds of sulfur bacteria, the purpose of this study was to examine the distribution of soxB genes along a geochemical gradient to identify how different variations of this particular gene are partitioned in a natural, flowing sulfidic spring-stream complex where H2S concentration decreased away from the spring orifice (from 188 to 0 Ámol/L), dissolved O2 increased (from 5.6 to 150 Ámol/L), and the pH increased (from 6.8 to 7.8) toward the end of the outflow channel 14 m downstream. soxB genes were retrieved from microbial mats and sediments in all areas of the spring outflow channel. However, soxB gene clusters were heterogeneously distributed within the spring-stream system. For instance, soxB OTUs related to the genus Chlorobium were found only in microbial mats at the spring orifice, but several soxB OTUs related to the genera Halothiobacillus and Paracoccus were retrieved from sediment microbial communities underlying the mats throughout the outflow channel. The upper and middle sections of the outflow channel, where sulfide concentrations were moderate, were dominated by a single soxB OTU related to the genus Thiothrix. Only where sulfide concentrations were low to undetectable were soxB OTUs distantly related (70-80% similarity) to the genera Bradyrhizobium, Rhodovulum, and Thiobacillus identified. The end of the outflow channel was dominated by soxB OTUs only found at the end of the outflow channel and distantly related to the genus Rhodovulum. The nucleotide sequence variations were specific to certain phylogenetic groups and metabolic pathways occuring within the spring outflow channel. These results suggest that the distribution of sulfur-oxidizing bacteria along this geochemical gradient is a function of substrate preference and availability in order to maintain optimal enzyme function. Consequently, efforts are underway to measure the concentrations of intermediate sulfur species and to express the different variations of the soxB gene clusters in the laboratory to test the currently hypothesized model for metabolic variability among the sulfur-oxidizing bacteria in this spring.

The Curecanti Pluton: A ~1.4 Ga Laccolith in the
Black Canyon of the Gunnison, CO

Donnie Hicks
Department of Earth & Planetary Sciences, University of Tennessee
Knoxville, Tennessee

The 1420▒15 Ma Curecanti pluton, exposed in the Black Canyon of the Gunnison, Colorado, is a 6-km-long discordant sheet of undeformed fine-grained quartz monzonite. The pluton was emplaced as a tabular body into amphibolite facies quartzofeldspathic migmatites, gneisses, and schists. Its floor dips 10-25║ to the northeast, its roof undulates between vertical and subhorizontal as it tapers towards the intrusion's margins, and all contacts are highly discordant with the subvertical foliation in the host rock. Floor and roof contacts are characterized by an agmatic zone where wall rocks are injected with leucosome along preexisting foliations, leading to paleosomes entirely surrounded by leucosome, some of which are then stoped into the main body of the Curecanti and digested. A pervasive set of east-west striking and vertically dipping pegmatites cross-cut the Curecanti pluton and are interpreted as recording the last stage of crystallization. The Curecanti pluton&rsquos geometry is interpreted as a laccolith with emplacement facilitated by extensive brecciation, assimilation, and lifting of roof rocks. Field observations suggest that the emplacement depth and geometry of the Curecanti pluton was likely a function of magma ascent being arrested in the mid-crust, perhaps at the brittle-ductile-transition, and forcibly lifting its roof rocks. Further investigation will help to demonstrate these emplacement mechanisms and determine how this fine-grained pluton relates to a global suite of megacrystic 1.4 Ga granites, providing insights into the tectonic setting of 1.4 Ga magmatism throughout southern Laurentia.


Page updated May 26, 2018