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