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Geophysical and geological studies of margin evolution: impacts of sea level fluctuations, tectonic deformation, and climate


Understanding modern process that shape continental margins has both scientific and societal relevance. By studying modern marine deposits, where the stratigraphy can be imaged at unprecedented scales and accurately dated with radiocarbon techniques, we can assess the link between the forcing factors and the consequent deposit. Toward this goal, this thesis presents geological and geophysical data that provide new insights into how different forcing mechanisms (e.g., sea level fluctuations, tectonic deformation, climate variability, glacial lake drainage) shaped continental margins. Three different margins were examined: San Onofre, CA, Block Island Sound, RI, and the Beaufort Margin, Arctic Ocean. In San Onofre, CHIRP data on the shelf imaged multiple transgressive deposits that gave insights into their controlling processes. Although numerous faults dissect the area, there is no evidence of recent activity or deformation on the fault systems offshore San Onofre. Instead, the rate of sea level rise, sediment supply, and preexisting morphology were found to be the controlling factors in sediment dispersal. Compressional features were imaged along the Cristianitos fault that runs from onshore to offshore in the study area. This suggests that the fault is actually a strike-slip fault with a down-to-the-northwest dip-slip component, versus a simple normal fault as purported.

In Block Island Sound, CHIRP data, scanned 3.5 kHz seismic profiles, and bathymetry data provide important insights into the morphologic evolution of the sound and the draining manner of glacial lakes that formerly occupied Block Island Sound and neighboring Long Island Sound. Architecture of sediment units imaged in the seismic data and erosive features imaged by bathymetry data suggest a rapid draining of the glacial lakes. Only partial infill of erosive features in Block Island Sound suggests a rapid transgression and/or a lack of modern sediment deposition.

Along the Beaufort Margin, seismic data and sediment cores were used to constrain the deglacial history of the area. Oxygen isotopes document a large input of freshwater that entered the Arctic via the Mackenzie River. Timing of this event correlates with the onset of the Younger Dryas cold period, suggesting that this flood may have triggered the attendant climate cool period. This study reveals that the slope west of the Mackenzie River has higher rates of Holocene sedimentation, suggesting it is influenced more by Barrow Canyon and continental runoff. The slope east of the Mackenzie River has a much stronger record of Mackenzie input, including a rapid depositional event. Ice rafted debris from the Amundsen Gulf is observed throughout most of the margin, but more prevalent in the eastern Beaufort.

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