Abstract
To investigate how bedrock transforms to soil, we mapped the topography of the interface demarcating onset of weathering under an east-west trending shale watershed in the Valley and Ridge province in the USA Using wave equation travel-time tomography from a seismic array of >4,000 geophones, we obtained a 3D P-wave velocity (Vp) model that resolves structures ∼20 m below land surface (mbls). The depth of mobile soil and the onset of dissolution of chlorite roughly match Vp = 600 m/s and Vp = 2,700 m/s, respectively. Chlorite dissolution initiates porosity growth in the shale matrix. Depth to the 2,700 m/s contour is greater under the N- as compared to S-facing hillslopes and under sub-planar as compared to concave-up land surfaces. Broadly, the geometries of the ‘soil’ and ‘chlorite’ Vp contours are consistent with the calculated potential for shear fracture opening under weak regional compression. However, this calculated fracture potential does not consistently explain observations related to N- versus S-facing aspect nor fracture density observed by borehole televiewer. Apparently, regional compression is only a secondary influence on Vp: the primary driver of P-wave slowing in the upper layers of this catchment is topographic control of reactive water flowpaths and their integrated effects on weathering. The Vp result is best explained as the long-term integrated effect of groundwater flow-induced geochemical weathering of shale in response to climate-driven patterns of micro- and macro-topography.
| Original language | English |
|---|---|
| Article number | e2021JF006281 |
| Journal | Journal of Geophysical Research: Earth Surface |
| Volume | 126 |
| Issue number | 12 |
| DOIs | |
| State | Published - Dec 2021 |
| Externally published | Yes |
Funding
Funding is acknowledged from DOE OBES DE-FG02-05ER15675 and NSF Critical Zone Observatory grants EAR 12-39285 and 13-31726 to Susan L. Brantley and NSF EAR 1945431 and 2012073 to Seulgi Moon Work at Shale Hills is facilitated by the Penn State College of Agricultural Sciences and Department of Ecosystem Science and Management as part of Penn State's Stone Valley Forest. We thank the IRIS Passcal Instrument Center for providing seismic equipment and logistical and data management support for this study, and we thank many contributors who helped with the seismic survey and Brad Carr who collaborated on borehole logs. Funding is acknowledged from DOE OBES DE‐FG02‐05ER15675 and NSF Critical Zone Observatory grants EAR 12‐39285 and 13‐31726 to Susan L. Brantley and NSF EAR 1945431 and 2012073 to Seulgi Moon Work at Shale Hills is facilitated by the Penn State College of Agricultural Sciences and Department of Ecosystem Science and Management as part of Penn State's Stone Valley Forest. We thank the IRIS Passcal Instrument Center for providing seismic equipment and logistical and data management support for this study, and we thank many contributors who helped with the seismic survey and Brad Carr who collaborated on borehole logs.