Thesis Defense - Fall 2008 - Rumi Takedatsu

Study of crustal structure in the Caucasus and Caspian regions using receiver functions

Rumi Takedatsu
B.S. Candidate
Advisors Dr. Rob Mellors

Friday, December 12th, 2008 CSL 422, 9:00am

ABSTRACT
The Eurasia-Arabia collision has created complex tectonics and structure in the Caucasus and Caspian region. In particular, Caspian Basin has a very different lithospheric structure from the surrounding region and the origin of the basin and nearby structures, such as the Kura Depression, are not well understood. This study will map the main crustal structure and estimate Moho depth under each seismic station using receiver functions. Receiver functions are a well-established technique to determine the boundary between crust and upper mantle (Moho) using three-component broadband seismograms. Using seismic data from 14 broadband stations in Azerbaijan, we generate receiver functions for 90 events. The Moho depth beneath each is determined by forward modeling the receiver functions using a reflectivity method and a two-layer crust. The receiver functions show clear indications of dipping structure as well as thick sediments under many stations which make modeling difficult. The preferred results show Moho depths of 37 - 43 km under the Greater Caucasus, 46 km under the Lesser Caucasus, 34 – 42 km in the Kura Depression, and 31 – 35 km along the western boundary of the South Caspian Basin. Crustal thickness thins towards the Caspian

SEMINAR - Robert Gaines

Paleoenvironments, Paleoecology, and Exceptional Preservation in Burgess Shale-type Deposits

Robert Gaines
Department of Geology
Pomona College

Wednesday, December 10th, 2008

Cambrian Burgess Shale-type (BST) deposits occur worldwide and offer a remarkable window on the initial Phanerozoic radiation of the Metazoa. However, BST deposits also represent significant deviations from the constraints that govern the typical operation of the fossil record. These deviations remain to be adequately accounted for, hindering interpretations of the paleoecology of their exceptional biotas and the environments in which they lived. This talk will provide an overview of ongoing research towards developing a model for understanding paleoenvironments, paleoecology, and exceptional preservation in BST deposits, using data from 12 deposits of North America, Scandinavia and South China. Recently, elemental mapping of a survey of BST fossils demonstrated that the mode of fossilization is unique and common to all deposits. Thus, preservation of BST assemblages represents a single phenomenon that may share a common cause in all deposits in which it occurs. Sedimentologic and stratigraphic data indicate that BST deposits occur in a particular physical depositional window that acted as a first-order control, determining where exceptional fossilization occurred. Ichnologic and geochemical data indicate that, within the favorable physical depositional setting, benthic redox conditions exerted the next control. BST deposits occur at the transition from oxic to anoxic benthic environments and demonstrate dynamic redox conditions during deposition; exceptional fossils are confined to anoxic intervals. The chemistry of the early burial environment is interpreted to have exerted the final control. Ongoing work using the δ34S system from a core through the Chengjiang deposit extracted in July 2008 will provide a test of the hypothesis that sediment sealing by early carbonate cements promoted an early cessation of decay. Constraints at each tier, from the regional scale to the micron scale, were strongly favored by the prevalence of large epicratonic seaways in the Cambrian and by greenhouse conditions.

Thesis Defense - Fall 2008 - Matt Burgess

Characterizing an Artificial-Recharge Site in a Desert Environment Using Time-Domain Electromagnetics

Matt Burgess
M.S. Candidate
Advisors Dr. George Jiracek, Dr. Paul Bedrosian (USGS)

Friday, December 12th, 2008 CSL 422, 11:00am

ABSTRACT
Time-domain electromagnetics (TEM) were applied to characterize the
subsurface in the vicinity of a proposed artificial hydraulic recharge
site. Because of the investment involved, it is important to know the
possible barrier effect of a major fault situated near the proposed
recharge site. In this study, a series of TEM soundings were made to
estimate the depth to water on both the north and south sides of the
Pinto Mountain Fault. Results show strongly contrasting resistivity
structures across the fault. These findings are consistent with
expected water levels south of the Pinto Mountain Fault and suggest
that the fault does, in fact, act as a barrier to flow in the vicinity
of the proposed artificial recharge site.