CEMENTATION AND GROUNDWATER CHEMISTRY IN PLEISTOCENE PALEODUNE DEPOSITS OF THE CENTRAL OREGON COAST

Pleistocene paleodune deposits occur along the Oregon coast, underlying major coastal towns, roadways, and associated power and water infrastructure(s). Secondary cementation within these deposits allows for near-vertical sea cliffs and roadcut outcrops, which fail episodically. Observation of stratigraphic profiles and penetrometer measurements show that these deposits are variably cemented, with more weakly cemented zones primarily associated with permeability boundaries along loess-paleosols or the underlying bedrock. Weakening of cements via changes to groundwater conditions could promote slope instability, threatening lives, and infrastructure.

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 optical microscopy, XRD, and SEM) to assess relationships between stratigraphy, soil moisture, and the type and degree of cementation. Thirty groundwater samples were analyzed to determine the chemistry of waters within these deposits. Cementing agents consist of hydrated minerals including Al-phases such as gibbsite, allophane, vermiculite, and halloysite, and Fe-phases such as goethite, ferrihydrite, and Fe-/Mn-oxides. The widest variety of cements is observed along permeability boundaries. The Fe- and Mn-rich cements are mobilized by fluctuating groundwater flow and completely fill pores, while the Al-rich cements are relatively immobile, and form a discontinuous coating on the sand grains.

Changes in groundwater flow from altered vegetative cover, drainage, and/or groundwater withdrawal might impact the hydration state and stability of the cements. Groundwater samples from the dune deposits were generally poorly buffered, with alkalinity as low as 4 mg/L as CaCO₃. Redox conditions were typically near the stability boundary of goethite and gibbsite, two of the cementing agents. We use reaction modeling to investigate how changes in groundwater parameters, such as pH or redox conditions, could affect the cements. This work can provide a framework to identify potentially weak stratigraphic horizons and assess how anthropogenic impacts, including climate change, might impact the stability of these paleodune deposits.