Opal-CT is the only diagenetic silica phase present in the Santa Cruz Mudstone within a 16 Km long area extending from the city of Santa Cruz north to Davenport, California. The d(101)-spacing values of opal-CT range from 4.11 Angstroms in outcrops within the city of Santa Cruz area, to 4.06 Angstroms in outcrops north of Santa Cruz. Associated with the decrease in d-spacings, the morphology of crystallites making up rosettes of opal-CT (lepispheres) varies from feathery in high d-spacing samples to spiny in low d-spacing samples. The extent of silica diagenesis in the Santa Cruz Mudstone suggests the unit should have reached burial depths of a minimum of 1000 m, in comparison to data obtained from other biogenic siliceous units. Even though present thicknesses of the Santa Cruz Mudstone within the study area are only a few hundreds of meters, a rapid thickening of the Santa Cruz Mudstone northwest of Santa Cruz suggests that extensive erosion of the Santa Cruz Mudstone preceded the deposition of the overlying Purisima Formation and Pleistocene deposits.