Thesis Defense - Fall 2009 - Nicole Velasco

Bulk and Elemental Mass Change in Development of Gabbroic Corestone and Saprolite near the Elsinore

Nicole Velasco

B.S. Candidate

Department of Geological Sciences

San Diego State University

Advisor Dr. Gary Girty

Thesis Defense - Fall 2009 - Marck Maroun

Major and minor elemental analyses of distal channel deposits from the Late Devonian marine Alamo Impact Event

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



ABSTRACT
This study determined the geochemical signal—major and trace element composition as well as a potential extraterrestrial component, iridium—of distal, onshore channel deposits of the Alamo Breccia. The Breccia formed by the early Late Devonian—early Frasnian, ~382 Ma—Alamo Impact Event, which was centered in an oceanic off-carbonate platform site in south-central Nevada. Twenty-eight samples were analyzed—sixteen from the Alamo Breccia channel deposits and twelve from above and below—as a representative of the Alamo Impact Event stratigraphy. The Breccia shows a wide range of concentrations in major, trace and rare earth elemental compositions, yet this same range is present, but in significantly lower concentrations, in normally deposited pre- and post-event carbonate layers below and above the Breccia. Results clearly indicate an iridium (Ir) anomaly—peaking at 190 ppt—as well as relative enrichment in other siderophile elements (Co, Ni, Au and Fe), chromium, and rare earth elements from within the Breccia. For the most part, the enrichments within the Breccia can be attributed to its greater siliciclastic component—from greater mud/clay and quartz sand contents—compared to carbonate rocks below and above. However, the high relative abundance of Ir in the Breccia far exceeds Ir concentrations expected solely from increased siliciclastic input, and is consistent with Ir values previously reported from other proven impact-related deposits. The Alamo Ir data are therefore interpreted to give additional supporting evidence for the impact origin of the Alamo Breccia channel deposits.

Thesis Defense - Fall 2009 - Kieri Hutchins

Late Devonian Alamo Impact, Southern Nevada: Analysis of Hematite Inclusions within Quartz Grains

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



ABSTRACT
The early Late Devonian (~382 Ma) Alamo event was produced by a marine bolide impact near the offshore margin of a large Paleozoic carbonate platform fringing western North America, with an impact site located in the current area of south-central Nevada. Effects of the impact, including the Alamo Breccia and related deposits, are now found in numerous mountain ranges in south-central Nevada, western Utah, and southeastern California. A key feature characterizing the Alamo deposits is the common occurrence shocked-quartz grains from impact ejecta that contain abundant inclusions of hematite after pyrite and magnetite. The sizes of the hematite inclusions and containing quartz grains found across the Alamo impact site show evidence for size sorting and a direct correlation between proximity to the impact site and inclusion size. Nine slides from eight locations proximal-to-distal from the impact were analyzed. The average size of the proximal hematite inclusions is 15.98 μm and the average size of the distal inclusions is 8.07 μm. The exceptions are the Confusion Range (CON, western UT) and Bat Mountain (BAT, SE CA) data. The CON location appears to be distal, but has an average inclusion size of 15.4 μm. This anomaly is possibly due to irregular impact plume movement. The BAT data appear to be proximal, but have an average inclusion size of 8.3 μm. In the Late Devonian this area was distal, but post-impact strike-slip faulting has moved this impact breccia block to the northwest. The Alamo is one of the only known impact sites with such hematite inclusions in the shocked quartz grains. These inclusions are useful in supporting previous estimates of the impact size and may provide a valuable tool for tracking proximal-to-distal fallout from the event.