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

Pleistocene dune deposits occur along the central Oregon coast, underlaying many coastal towns. Secondary cementation allows cliff formation along the edges of these deposits, however, stratigraphic profiles and penetrometer measurements indicate these deposits are variably cemented, with more weakly cemented zones associated with permeability boundaries along paleosols or underlying bedrock. Poorly cemented zones and potential weakening of cements via changes to soil moisture or groundwater quality may promote slope instability, threatening lives, property, and infrastructure. The relationship between stratigraphy, the type and degree of cementation, and groundwater flow and chemistry is established by determining bulk sediment properties (e.g., density, compaction, porosity, water content), mineralogies, cement characteristics, and groundwater chemistries at various depths within a series of vertical profiles within the Newport dune sheet. These profiles represent a range of dune ages from circa 20 ka to > 100 ka. X-ray diffraction analyses indicate the cementing agents include hydrated minerals such as gibbsite, allophane, vermiculite, goethite and ferrihydrite. Zones rich in gibbsite or allophane occur in association with paleosols or loess layers,

Changes to soil moisture and groundwater flow from climate change, over-pumping and drainage may impact the hydration state of cements. Groundwaters within these dune deposits are also typically poorly buffered and so water quality may be readily altered, for example, from septic systems. We are studying how changes in groundwater parameters, such as pH or redox conditions, may either trigger dissolution or induce precipitation of the cementing agents by reaction modeling in conjunction with analyses of natural waters from the deposits and column experiment solutions. A better understanding of the groundwater conditions that induce dissolution of the cementing agents, as well as the role that soil horizons and paleosols play in these processes, can provide a framework to assess the potential hazard posed by the variably cemented paleodune deposits.