Recording Coastal Changes observed in Beach Ridges and Prograded Beach Stratigraphy using Ground-Penetrating Radar

• Main Author: Zurbuchen, Julie Marie
• Other Authors: Simms, Alexander R
• Format: Other/Unknown Material
• Language: English
• Published: eScholarship, University of California 2019
• Subjects: Sedimentary geology; Antarctica; Elwha River delta; Ground-penetrating radar; Oxnard Plain; stratigraphy; Joinville; Joinville Island; Swash; Antarc*; Ice Shelf
• Online Access: https://escholarship.org/uc/item/4qk4m46p

Beach ridges and other prograding beach deposits are important sedimentary archives of past floods, storms, and relative sea-level changes. Accurate interpretations of beach ridges requires an understanding of their formation and preservation through time. In the following studies, I use ground-penetrating radar to observe the stratigraphy of beach ridges and prograded beach deposits. Additionally, I employ the use of elevation surveys, aerial photographs, radiocarbon dating, and optically stimulated luminescence dating to understand the timing of events preserved in the sedimentary record. In Chapter 2, I explore the formation of swash bars on the Elwha River delta after the removal of two dams on the fluvial system simulated a large sediment pulse to the system, similar to a flood or landslide. I find that mouth bars form most often after higher than average discharge events in the fluvial system, and swash bars form soon after due to wave reworking of the mouth-bar sediments. However, only 10 of 37 swash bars that formed were preserved at the time of my GPR survey, five years after dam removal. Additionally, the swash bars that did survive amalgamated with one another, forming a large barrier at the delta front, indicating that in small mountainous river settings, beach ridges may be more indicative a large sediment pulse to the system, rather than a single flood. In Chapter 3, I examine the ~600-year sedimentary record of the coastal Oxnard Plain. Progradation on the Oxnard Plain has been relatively constant on centennial (150- to 200-year) timescales, prograding at rates of 0.3 to 1.4 m a-1. However, on shorter timescales, progradation is episodic, with greater progradation occurring after high discharge events along the Santa Clara River. Extended droughts remove up to 90 m of the beach, equivalent to ~5 to ~120 years of the sedimentary record. Additionally, I image beach cusps in shore parallel GPR profiles, which previously had not been recognized in GPR profiles.Lastly, in Chapter 4 I use gravel beach ridges to reconstruct the relative sea-level (RSL) record on Joinville Island, Antarctica. I find that RSL has fallen ~5 m over the last ~3000 years, at variable rates throughout the late Holocene. I interpret that ice mass loss, similar to the scale of ice mass loss after the 2002 Larsen B Ice shelf collapse, and ice mass growth caused by glacial advance, both occurred in the Late Holocene and were recorded in my RSL reconstruction. Therefore, global- and continental-scale global isostatic adjustment models, which currently only account for ice changes on thousand-year timescales, are missing crucial centennial-timescales ice mass changes.