EAR-PF: Evolution of landscapes buried by Quaternary sediments

Kwang, Jeffrey S, Hadley MA

Investigators

Abstract

Dr. Jeffrey Kwang has been awarded an NSF EAR Postdoctoral Fellowship to carry out research under the mentorship of Dr. Andrew Wickert at the University of Minnesota (UMN) and Dr. Isaac Larsen at the University of Massachusetts-Amherst (UMass). This project aims to understand how landscapes buried by recent sediments evolve over time. Rivers and their networks drive change in landscape evolution; they reorganize in response to changing climate, tectonics, and rock type, drastically restructuring the Earth’s surface. Field observations have shown that a rock’s susceptibility to erosion exerts control in determining how rivers erode into landscapes, an assumption based in the idea that landscapes are made of a single material. Because natural rivers cut through multiple materials, it is important to study landscapes with contrasting erodibilities. This project focuses on regions that were previously glaciated where ancient river networks are buried under tens to hundreds of meters of sediments deposited by glaciers. After burial, a new river network is established on the surface that may have a geometry that is quite different from the ancient river network. As rivers erode into the glacial sediments, they re-expose the old landscape, triggering a competition between the ancient and new river networks. Using numerical models and field techniques, the PI will (1) determine important metrics that control the outcome of these competing networks and (2) predict timescales required for glacial sediments to be fully removed. Because contrasts in erodibility are common in other landforms, this research will provide important insight into how the structure of the rock below our feet determines rivers formation and landscape evolution in a broad spectrum of geologic settings. The PI also aims to promote Earth science education and diversity by designing interactive numerical models/games about river evolution. At UMN and UMass, Dr. Kwang will collaborate with local Girls Inc. organizations to develop summer workshops for Eureka!, a program that aims to close the gender-gap in STEM. In addition, the PI will co-mentor an undergraduate research student with Dr. Phillip Larson at the Minnesota State University. Landscapes dissected by dendritic networks are ubiquitous across the continental surfaces of Earth. Understanding how these geopatterns emerge and reorganize reveals how geomorphic processes create landforms. Drainage networks seldom change in landscapes that are in dynamic equilibrium, but drainage networks are actively forming and reorganizing in landscapes in disequilibrium. Regions in the American Midwest that were previously glaciated are classic examples of disequilibrium landscapes. Before Pleistocene glaciation, these regions generally contained sedimentary rock dissected by dendritic river networks. After glaciation, the sedimentary rock was buried by Quaternary sediments, i.e. till, and essentially, the landscape was reset. Such landscapes are ideal locations to study drainage networks because (1) new drainage networks are actively forming on the till-covered surfaces, and (2) old drainage networks from the buried topography are being exhumed. In this region, the PI will use numerical landscape evolution models to test how drainage network reorganization is affected by (1) rock erodibility contrasts between the glacial till and buried sedimentary rock and (2) the orientation of the surface drainage network with respect to the buried drainage network. The results will also detail a new mechanism for the formation of barbed tributaries, offering a new geologic interpretation for these river network features. Using field work and remote sensing, the PI will also calibrate a numerical model to predict timescales that are required for the full evacuation of glacial till from these landscapes. More broadly, the PI anticipates that this research will benefit our understanding of other geologic settings with contrasts in substrate erodibility. Field studies have found that variability in erodibility has a first-order control on landscape evolution and morphology, and moreover, it has been determined that substrates with contrasting erodibilities are the norm in terrestrial landforms, not the exception. Therefore, this research can provide a transformative insight into how lithologic variation and stratigraphy affect surface processes, drive drainage reorganization, and create geopatterns. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.