Critical Zone Structure Under a Granite Ridge Inferred From Drilling and Three‐Dimensional Seismic Refraction Data

Abstract

Observing the critical zone (CZ) below the top few meters of readily excavated soil is challenging yet crucial to understanding Earth surface processes. Near‐surface geophysical methods can overcome this challenge by imaging the CZ in three dimensions (3‐D) over hundreds of meters, thus revealing lateral heterogeneity in subsurface properties across scales relevant to understanding hillslope erosion, weathering, and biogeochemical cycling. We imaged the CZ under a soil‐mantled ridge developed in granitic terrain of the Laramie Range, Wyoming, using data from five boreholes and a 3‐D volume (970 by 600 by 80 m) of seismic velocities generated by ordinary kriging of 25 two‐dimensional seismic refraction transects. The observed CZ structure under the ridge broadly matches predictions of two recently proposed hypotheses: the uppermost surface of weathered bedrock is consistent with subsurface weathering driven by bedrock drainage and subsurface topography defining the top of unweathered protolith is consistent with fracturing predicted from topographic and regional stresses. In contrast, differences in slope aspect along the ridge are too subtle to explain observed variations in regolith structure. Our observations suggest that multiple processes, each of which may dominate at different depths, work in concert to regulate deep CZ structure.

Topographic Controls on the Extension and Retraction of Flowing Streams

“…The burial of valley bottoms with sediment, for example, will tend to tend to reduce cross-valley curvature and locally increase transmissivity. In addition, both numerical simulations and field measurements have shown that the layer of permeable fractured bedrock becomes thinner (smaller a) as topographic curvature increases (Flinchum et al, 2018;Moon et al, 2017). Near drainage divides, valley heads tend to be relatively smooth and un-incised, but farther downstream localized stream incision can narrow valley widths and confine flow laterally (Figure 2c).…”

Characterizing hydrological processes in a semiarid rangeland watershed: A hydrogeophysical approach

“…The range in seismic velocity within this layer (0.5–1.2 km s −1 ) corresponds to varying degrees of weathering. At the soil/saprolite boundary, the saprolite is mainly composed of loose, decomposed granite, whereas the bottom of the layer retains the original rock fabric (Flinchum et al, ). A prominent dipping structure, shown by high resistivity and strong GPR reflections, appears within the saprolite layer on the NE side of the profile (Figure a).…”

“…Large corestones are distributed throughout the area and can be seen partially buried close to the location of the anomaly (Figure a). An anomalously high velocity greater than 3.0 km s −1 at this location further supports the fact that this feature is much closer in structure to solid granite bedrock (~4.0 km s −1 ; Flinchum et al, ) than surrounding saprolite. Another region with velocities greater than 3.0 km s −1 can be seen around 40 m. In general, the large velocity gradient supports the interpretation of a subsurface in which fractured and solid bedrock is reached at a shallow depth.…”

“…Previous seismic refraction surveys in the UCCW have shown that a velocity of 1.2 km s −1 is a good estimate for the boundary between the saprolite layer and fractured bedrock (Flinchum et al, ; Kotikian et al, ). Surveys performed directly over granite outcrops in this watershed and work by a number of authors in similar crystalline bedrock settings (Befus, Sheehan, Leopold, Anderson, & Anderson, ; Holbrook et al, ) have identified 4.0 km s −1 as a reasonable estimate for unweathered bedrock with few to no fractures.…”

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

“…Saprolite is a material that has been significantly chemically altered but still retains the physical structure of the parent material (Lebedeva et al, 2007; Riebe and Granger, 2013; Langston et al, 2015). The fractured rock layer has the highest fracture density near the saprolite boundary and decreases away from this boundary (Wyns et al, 2004; Ayraud et al, 2008; Flinchum et al, 2018b). Unaltered bedrock occurs at a depth where the fracture density is low enough to prevent meteoric water from being circulated, limiting chemical reaction rates.…”