Geophysical and Chemical Comparisons to Understand Structure in a Deeply Weathered Critical Zone

Abstract:

Geophysical imaging and logging are crucial in mapping the bottom boundary of the Critical Zone and provide insight into weathering at depth. Surface P-wave refraction velocity profiles in a deeply weathered, schist catchment in the Maryland Piedmont suggest an inverted structure, with deep weathering below the ridge tops and a comparatively shallow protolith under the valley. The maximum P-wave seismic velocities (4 km/sec) are interpreted as un-weathered rock from 2014 surface refraction profiling. These subsurface structural observations have been important in developing hypotheses around the importance of stress fracturing and weathering in Critical Zone evolution. We performed a field investigation to test the interpretation of the seismic velocity defined structures. A 45 m deep borehole was drilled in the ridge and borehole geophysics, including sonic and NMR measurements of porosity, were collected. In the valley, we acquired an additional down valley, surface seismic profile and used a backpack drill to bore to the interpreted 4 km/sec material. Rock cores were collected during both the ridge and valley drilling to depths of 45 m and 2.2 m respectively. Handheld laser induced breakdown spectroscopy (LIBS) was used to collect chemical composition data on the cores. Analysis of the LIBS data distinguishes fracture surfaces from rock matrix down to depths of ~ 44 meters. The LIBS data shows that fracture surfaces generally have lower sodium concentrations, likely due to plagioclase weathering. These results suggest that some weathering is present below the bottom boundary of the Critical Zone that was identified based upon seismic velocity . LIBS data were also used semi-quantitatively to examine changes in the rock chemistry that coincide with an abrupt collapse in porosity detected using borehole NMR from ~20% to %. This change in porosity occurs at the transition between saprolite and competent rock. Chemical composition of material with P-wave velocities of 4 km/sec from beneath the ridge and valley were also compared to test the similarity of the weathering . Linking chemical and geophysical data offers a more comprehensive understanding of Critical Zone structure, physical and chemical weathering effects, and tests our understanding of the CZ evolution.