Cementation and Groundwater Chemistry in Pleistocene Paleodune Deposits of the Central Oregon Coast

• Main Author: Stephens, Adrienne Lynn
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2023
• Subjects: Environmental Geology; Geochemistry; Geology
• ISBN: 9798377662082

Pleistocene paleodune deposits occur along the Oregon coast, underlying coastal towns, roadways, and associated power and water infrastructure(s). Secondary cementation within these deposits provides some stability, allowing for near-vertical sea cliffs and roadcut outcrops. Yet, slope instability is a prevalent hazard observed within the paleodune deposits. Weakening of cementing agents via changes to groundwater conditions due to altered vegetation, climate change, or contamination, for example, could promote slope instability, threatening lives and infrastructure. This study aims to investigate the variability in the type and degree of cementation and to determine how they are affected by changes in groundwater conditions.To assess relationships between stratigraphy, soil moisture, and the type and degree of cementation fifty-six soil samples from four profiles within the Newport paleodune sheet were analyzed for bulk properties (grain size, density, porosity, and moisture content), mineralogy, and cement characteristics via X-ray diffraction, scanning electron microscopy, and optical microscopy. To determine the chemistry of waters associated with these deposits thirty groundwater samples and seven porewater samples were analyzed via ion chromatography, inductively coupled plasma mass spectrometry, and isotope analysis. Results of these analyses were used in geochemical modeling using PhreeqC and Geochemist’s Workbench to better understand how cementing agents may be affected by changing groundwater conditions and chemistries.Stratigraphic profiles and penetrometer measurements confirm these deposits are variably cemented, and reveal more weakly cemented zones are primarily associated with permeability boundaries along loess-paleosols, accumulations of Fe- and Mn-rich laminae, or the underlying bedrock. Results of mineralogical analyses reveal cementing agents consist of hydrated minerals including Al-phases such as gibbsite, allophane, vermiculite, and halloysite, which typically form a discontinuous coating on the sand grains, and Fephases such as goethite, ferrihydrite, and Fe-/Mn-oxides, which typically completely fill pores between sand grains and provide exceptional cohesion. Results of groundwater analyses and modeling reveal the Fe- and Mn-rich cementing agents are mobilized by fluctuating groundwater flow, particularly under low redox conditions characteristic of deeper flow systems, while the Al-rich cementing agents are relatively immobile, particularly under higher pH conditions, although the range of favorable pH is narrow. Furthermore, waters associated with the deposits are generally poorly buffered, and thus are susceptible to significant changes in pH. This study provides a more thorough understanding of the type and degree of cementing agents, their relationship with groundwater and stratigraphy, and the soil formation processes involved. The results can be used as a framework to evaluate the potential hazard posed by the variably cemented paleodune deposits, particularly slope stability given site-specific characteristics. Additionally, this framework can guide planners and regulators overseeing coastal development in assessing how anthropogenic impacts, including climate change, could potentially impact the stability of the paleodune deposits.