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Thick loess of the central Great Plains, USA, preserves a detailed record of Holocene climate change, extremely thick loess accumulated during and just after the last glacial maximum, and a sequence of older loess units and paleosols. This loess sequence is only well-preserved beneath the summits of tablelands, plateau-like landforms with flat to undulating summits and steep gully-dissected marginal slopes. These loess tablelands are also a key setting for preservation of organic carbon in buried soils and for long-term storage of sediment in the form of loess. Even under tableland summits, however, at some sites interbedded or surficial sand aeolian sand interrupts the loess sequence and/or parts of the loess sequence are missing. These are interpreted as the result of downwind/upwind shifts in the boundary between thick loess and the dune fields or bedrock surfaces of sand transport that occur upwind of the loess. We are testing a set of hypotheses on how landscape evolution through aeolian, hillslope, and fluvial processes controls the development and long-term persistence of loess tablelands. Here we focus on three of those hypotheses: 1) closed depressions on tableland summits, produced by aeolian erosion, disconnect runoff on the summits from the drainage network on marginal slopes, enhancing tableland preservation; 2) episodic migration of aeolian sand into the loess region has truncated the loess record locally, but in the long term the sands enhance tableland persistence through effects on infiltration and runoff; and 3) loess tablelands in the region all developed on older bedrock tablelands that were preserved by similar processes including formation of closed depressions and mantles of aeolian sand or fluvial sand and gravel. The first hypothesis is supported by analysis of surface flowpaths and by landscape evolution modeling using the Landlab toolkit (Hobley et al., 2017; Barnhart et al., 2020). The second is tentatively supported by field and lab measurements of the hydraulic properties of aeolian sand, loess, and loess-derived soils in the study area. The third hypothesis is supported in local areas by reconstruction of the underlying surface using subsurface data and outcrops, as well as observations of nearby bedrock tablelands that are not loess covered. Interesting questions arising from these hypotheses include: 1) Is the destruction of tablelands essentially irreversible or can additional loess “smooth out” dissected surfaces? 2) Are all the loess tablelands relatively old (Middle Pleistocene or older) or did some form more recently? Hobley, D. E. J., Adams, J. M., Nudurupati, S. S., Hutton, E. W. H., Gasparini, N. M., Istanbulluoglu, E. and Tucker, G. E., 2017, Creative computing with Landlab: an open-source toolkit for building, coupling, and exploring two-dimensional numerical models of Earth-surface dynamics, Earth Surface Dynamics, 5(1), p 21-46, 10.5194/esurf-5-21-2017.Barnhart, K. R., Hutton, E. W. H., Tucker, G. E., Gasparini, N. M., Istanbulluoglu, E., Hobley, D. E. J., Lyons, N. J., Mouchene, M., Nudurupati, S. S., Adams, J. M., and Bandaragoda, C., 2020, Short communication: Landlab v2.0: A software package for Earth surface dynamics, Earth Surf. Dynam., 8(2), p 379-397, doi:10.5194/esurf-8-379-2020. |
