Mapping Variations in Bedrock Weathering With Slope Aspect Under a Sedimentary Ridge‐Valley System Using Near‐Surface Geophysics and Drilling.

Autor: Hudson Rasmussen, Berit M.1 (AUTHOR), Huang, Mong‐Han1 (AUTHOR) mhhuang@umd.edu, Hahm, W. Jesse2 (AUTHOR), Rempe, Daniella M.3 (AUTHOR), Dralle, David4 (AUTHOR), Nelson, Mariel D.3 (AUTHOR)
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Zdroj: Journal of Geophysical Research. Earth Surface. Jul2023, Vol. 128 Issue 7, p1-20. 20p.
Abstrakt: Understanding how soil thickness and bedrock weathering vary across ridge and valley topography is needed to constrain the flowpaths of water and sediment production within a landscape. Here, we investigate saprolite and weathered bedrock properties across a ridge‐valley system in the Northern California Coast Ranges, USA, where topography varies with slope aspect such that north‐facing slopes have thicker soils and are more densely vegetated than south‐facing slopes. We use active source seismic refraction surveys to extend observations made in boreholes to the hillslope scale. Seismic velocity models across several ridges capture a high velocity gradient zone (from 1,000 to 2,500 m/s) located ∼4–13 m below ridgetops that coincides with transitions in material strength and chemical depletion observed in boreholes. Comparing this transition depth across multiple north‐ and south‐facing slopes, we find that the thickness of saprolite does not vary with slope aspects. Additionally, seismic survey lines perpendicular and parallel to bedding planes reveal weathering profiles that thicken upslope and taper downslope to channels. Using a rock physics model incorporating seismic velocity, we estimate the total porosity of the saprolite and find that inherited fractures contribute a substantial amount of pore space in the upper 6 m, and the lateral porosity structure varies strongly with hillslope position. The aspect‐independent weathering structure suggests that the contemporary critical zone structure at Rancho Venada is a legacy of past climate and vegetation conditions. Plain Language Summary: Below Earth's ground surface, porous space within weathered bedrock can store a significant amount of water, which is essential for ecosystems, particularly during the growing seasons. Collecting hydrologic data and core samplings from boreholes provides direct measurements about how bedrock is weathered and broken down toward the earth surface. Our study site is located in a series of ridges and valleys in Northern California, USA, where the local Mediterranean climate has distinctive dry summers and wet winters. This site represents a common topography along the east side of the Coast Ranges. In addition to synthesizing borehole and hydrologic data, we conduct complementary seismic refraction surveys to image material strength in the subsurface in 2D. These images can better capture the lateral variation of weathering zone thickness from channels to ridgetops. Seismic velocity derived from seismic refraction data shows an increase of material strength at the transition zone between saprolite and bedrock that agrees with borehole observations. Although vegetation density is much higher in the north‐ than the south‐facing hills, the depth to fresh bedrock is roughly the same. Our results also indicate that porous spaces in the weathered bedrock have the potential to store more water than annual precipitation. Key Points: A combination of geophysics and borehole measurements allows us to characterize lateral critical zone structure in a ridge‐channel systemDespite a strong aspect‐dependent contrast in soil thickness, saprolite thickness does not vary with slope aspectRock physics modeling using seismic velocity suggests that inherited bedrock fractures substantially contribute to saprolite total porosity [ABSTRACT FROM AUTHOR]
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