Interbedded mudrocks and fine-grained sandstones from three wells in the Plio-Pleistocene Colorado River delta in the Imperial Valley, southern California, were examined by X-ray powder-diffraction methods, chemical analyses, and scanning electron microscopy (SEM) to determine the extent and course of low-temperature metamorphic reactions. This sequence was compared to Tertiary rocks from the U.S. Gulf Coast in an effort to explore the kinetics of low-temperature reactions in natural systems. Mineralogical changes observed are 1) the progressive transformation of mixed-layer illite-smectite to pure illite over the temperature range 70 degrees C to 210 degrees C; 2) the apparent appearance of chlorite as a diagenetic mineral at 180 degrees C to 194 degrees C coincident with the onset of R (symbol) 3 ordering in the illite-smectites; 3) the loss of kaolinite above approximately 210 degrees C; and 4) a progressive decrease in the amount of detrital potassium feldspar over the same temperature interval that brackets the illitization reaction. Comparison of bulk chemical analyses with analyses of the <0.1-mu m fraction indicates that the clay mineral reactions are associated with redistribution of components from the coarse fraction to the fine, clay-rich fraction. Comparison of Tertiary sequences with this Plio-Pleistocene sequence indicates that kinetics significantly influence the appearance of clay mineral assemblages at temperatures below approximately 175 degrees C. At temperatures above 175 degrees C, temperature, rather than time, appears to govern the reactions.