Geophysical investigation of differences in weathering depths between the north and south facing slopes of a small catchment in the Reynolds Creek Critical Zone Observatory.

Abstract:

Geophysical and soil investigations of the Reynolds Creek Critical Zone Observatory (RCCZO) have revealed that the depth to unweathered bedrock is deeper on north facing than on south facing slopes. This asymmetry has been posited to be a function of greater soil moisture in the north facing aspects. The increased moisture input leads to higher rates of weathering and thus to deeper unweathered bedrock. However, how this asymmetry changes from the higher elevations to the lower elevations has not been examined in the RCCZO. To examine the catchment wide weathering trends, seismic refraction tomography (SRT) surveys will be conducted in Johnston Draw along the lines shown in Figure 1. Johnston Draw is a 1.5 km² east-west trending granitic catchment that ranges from 1400 m to 1900 m in elevation in the semi-arid Owyhee Range of southwest Idaho. The site has been instrumented with meteorological instruments on its north and south slopes. Because of its uniform lithology, easterly drainage direction, range of elevations, and instrumentation, it is a unique site for exploring aspect controlled weathering asymmetry. SRT provides a 2D profile of P-wave velocity from which the depth to unweathered bedrock can be inferred by the depth to the velocity of fresh bedrock which is about 4km/s at this site. Observations of snow accumulations and plant communities in Johnston Draw suggest that at the top of the draw the difference in the soil moisture between the south and north facing slopes is greater than at its bottom. Thus it is hypothesized that the difference between the rate of weathering of the north and south facing aspects will be greater at the top of the draw and this will be reflected in a greater asymmetry in weathering depths. This is diagramed in Figure 2 in which the blue arrows depict qualitatively the hydraulic flux and the vegetation the changes in ground cover through the catchment. A previous SRT survey conducted near the top of Johnston Draw by the WyCHEG group showed that significant north-south slope weathering depth asymmetry was present. Our SRT lines will spaced at progressively lower elevations down in the catchment (Figure 1). This work will enable us to examine how the weathering depths change from the higher elevations at the top of the draw to the lower elevations at the bottom of the draw.