Date of Graduation

12-2020

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

John B. Shaw

Committee Member

Ralph Davis

Second Committee Member

Michelle Bernhardt-Barry

Keywords

compaction, deltas, experiments, geomorphology, hydrogeology, subsidence

Abstract

Subsidence in low elevation coastal areas has been extensively researched through direct field measurement, numerical modelling, and stratigraphic reconstruction of ancient sediment deposits. Here I present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory scale river delta experiments. Compactional subsidence rates have never been thoroughly quantified in the experimental setting, though this mechanism is found to be a primary creator of total relative sea level rise which will likely cause coastlines to retreat in the coming years. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in a reduced scale model such as the ones studied. I compare subsidence between a control experiment using typical boundary conditions of standard laboratory fan-deltas with an experiment which has been treated with a proxy for highly compressible organic rich marsh or peat deposits. Both experiments have non-negligible compactional subsidence rates across the delta-top which are comparable to our boundary condition relative sea level rise of 250 μm/h. Subsidence in the control experiment, on average 54 μm/h across the low elevation areas of the subaerial delta, is concentrated in very low-elevation (level) areas near the coast and is likely due to creep induced by a rising water table near the shoreface. The marsh experiment exhibits larger (on average 126 μm/h) and more spatially variable subsidence rates which are controlled mostly by compaction of recent marsh deposits at or very near the sediment surface. These rates compare favorably with field and modelling based subsidence measurements when plotted in dimensionless space. By scaling these results to the field, we find that subsidence “hot spots” may be relatively ephemeral through longer timescales, but average subsidence across the entire low elevation region of a delta can be variable at century and millennial timescales. Subsidence rates in a given decade or century may exceed thresholds for marsh platform drowning, even in the absence of anthropogenic impacts.

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