The Santa Cruz Mudstone is a Late Miocene geologic formation in California from the Tortonian and Messinian ages. Composed of siliceous, organic-rich mudstones laid down in deep-water and fluvial settings, it preserves evidence of a marine environment that later became part of the California Coast Ranges. In the local rock sequence, it lies above the Santa Margarita Sandstone and below the Purisima Formation. Once grouped within the broader Monterey Formation, it is now recognized as a separate unit. Its significance also rests in its fossil record, which includes bivalves, echinoids, Parabalaenoptera baulinensis, and the giant shark Otodus megalodon.