EXPLORING HOW ROCK TURNS TO REGOLITH AT THE SUSQUEHANNA SHALE HILLS CRITICAL ZONE OBSERVATORY, CENTRAL PENNSYLVANIA

At the Susquehanna Shale Hills Critical Zone Observatory (CZO) we are investigating the physical and chemical characteristics of Rose Hill formation shale from tens of meters of depth to the land surface. The shale has disaggregated into a layer of augerable soil that mantles the landscape and varies in thickness from tens of centimeters to several meters throughout the watershed. At the ridgetops, this soil largely formed in place, but along hillslopes the soil creeps downslope. Water flow through the unsaturated soil along hillslopes is considered to be largely vertical but flow also occurs laterally at the interfaces of soil horizons marked by permeability contrasts. Water also infiltrates into the fractured bedrock underlying the soil, sometimes by flowing through macropores in the soil that allow water to bypass the porous soil matrix. The shale bedrock contains quartz plus illite, chlorite, and minor feldspar that weather to form secondary clays including kaolinite and interlayered vermiculites. Minor concentrations of pyrite and ankerite are observed at tens of meters of depth under the ridgetops. Depletion of sulfide and carbonate minerals in the upper layers may document reaction fronts that roughly correspond to the regional water table. In other words, infiltration of water may have caused oxidative dissolution of pyrite and ankerite (down to tens of meters of depth under ridgetops and meters beneath the valley floor). Porosity measured in rock chips and core from depth are generally lower than porosity measured in rock chips collected from the soil. Roots are observed to penetrate into the upper fractured layer of bedrock that underlies the augerable soil. The transformation of bedrock to soil in this watershed begins near the regional water table and is accelerated by biological processes at the land surface.