In the Pacific Northwest, diagenesis of Cretaceous strata mantled by extrusive volcanic rocks occurred at temperatures of 150-200 degrees C due to heat conducted downward from a perched, confined geothermal aquifer. This aquifer produced a homogeneous temperature field within the underlying sedimentary rock. Authigenic minerals, such as sphene, chlorite, illite, and quartz, developed throughout a vertical section composed of andesites, basalts, and both silicic and mafic sedimentary rocks. The geologically rapid heating also resulted in nearly constant organic maturation profiles within the sedimentary rock and produced unknown quantities of hydrocarbons. The contemporaneous diagenesis and maturation took place before extrusion of the Columbia River Basalt Group and was p rhaps associated with earlier intrusive activity. Such hydrothermal maturation accounts for both local diagenesis and other organic maturation anomalies in the Pacific Northwest. The mechanism has ramifications for the petroleum potential of geologically active basins because it provides a means by which volcanism and associated geothermal activity can increase maturation and generate hydrocarbons.