Abstract

The spatial distribution of weathered material across actively eroding landscapes partly determines how water and solutes are routed through the landscape. Generally, geochemical measurements are made on individual specimens and it is hard to upscale from the scale of minerals or clasts to pedons to catchments. In this study, we made dense geochemical measurements (bulk chemistry, minerology, bulk density and porosity) on drill cuttings and rock fragments from two boreholes (one under a ridge and one under the channel) at the Shale Hills subcatchment of the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA and are relating these observations to geophysical logs collected on the boreholes.

The Shale Hills catchment is entirely developed on Rose Hill shale. Weathering is initiated by dissolution of carbonate and oxidation of pyrite. Under the ridges, most of the pyrite has weathered away in the upper tens of meters above the water table. Downhole logs show that most of the fracturing in the shale occurs in this pyrite-depleted zone, but layers of closely spaced fractures are also present below the water table as indicated by lower density, higher neutron porosity, and higher sonic velocity. Near a few fractures in layers below the water table under the ridge, rock fragments demonstrate the same weathering processes observed above the water table -- dissolution of carbonate and plagioclase, oxidation of pyrite, vermiculitization of chlorite, and generation of porosity. In this zone below the water table beneath the ridge, the spacing of fractures is large and weathering is limited mostly to the matrix.

Under the stream channel, pyrite is depleted 6-7 m below the water table. Also, the zone of reaction is thicker than the reaction zone under the ridge. In contrast to the ridge, the spacing of fractures under the channel is smaller, and both the matrix and fractures are weathered to a greater depth with respective to the water table. We are working on a “seismic rock model” based on geophysical theory to predict the seismic velocity based on mineral composition, texture, matrix porosity and fracture density to extend these observations across the catchment.

Citation

Xin Gu*, Gary Mavko, Natalie J Accardo, Andrew Nyblade, Susan L Brantley (2018): Mapping geochemistry onto geophysics to understand the architecture of shale weathering in the shallow subsurface. Abstract NS43A-06 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.

This Paper/Book acknowledges NSF CZO grant support.