Chemical weathering in a region of active orogeny: Pescadero Creek Watershed, California

Base flow chemical signals were used to determine the weathering reactions that control the groundwater chemistry of a geologically heterogeneous, mou ntainous watershed. Major ion signatures result from cyclic salt and format ion water inputs and from the weathering of easily oxidized or highly solub le minerals, such as pyrite, calcite, and dolomite. If montmorillonite is t he dominant secondary mineral product, then the bulk of silicate weathering probably involves volcanic rocks. Spatial rates of base flow chemical denu dation range from 0.0004 to 0.02 mm yr(-1), and temporal rates range from 0 .0006 to 0.6 mm yr(-1). The mean chemical denudation rate for the watershed is 0.03 mm yr(-1), which is comparable to some of the world's most rapidly weathering large drainage basins. Because highly soluble or easily oxidize d minerals contribute the bulk of the chemical signal to basin waters, spat ial and temporal rates of chemical denudation are constrained largely by re charge and discharge rather than local variations in lithology.