Cenozoic strata in the southern San Diego region have historically been difficult to subdivide and interpret. A heavy mineral petrological study of sandstones assigned to the Mission Valley, Sweetwater, Otay and San Diego formations has revealed a number of distinguishing factors that can help discriminate between strata and provide insight into changing tectonic styles. A relatively tight clustering of sample age dates provides temporal representations of distinct heavy mineral populations. Discrimination between the upper middle Eocene Mission Valley Formation and the uppermost middle to middle upper Eocene Sweetwater Formation may be optimized by noting the presence of occasional allanite, glaucophane and monazite exclusively within the former unit as well as limited barite and hornblende within the latter unit. A distinction can be made between the Sweetwater Formation and the lowest upper Oligocene Otay Formation by noting the presence of rare piedmontite within the former unit and uncommon actinolite, allanite, anthophyllite, brown hornblende and tremolite solely within the latter unit. A profound contrast is evident between the Otay Formation and the upper Pliocene to lowest Pleistocene San Diego Formation when the existence of allanite, barite and tremolite are recognized only from the former unit and andalusite, clinopyroxene, cummingtonite, glaucophane, kyanite, orthopyroxene, sillimanite and topaz are seen only within the latter unit. Examples of tectonic influences on heavy mineral assemblages are numerous. In the Mission Valley Formation high concentrations of epidote and a complete lack of hornblende attest to the dominance of the mature Poway fan delta during the latest middle to earliest late Eocene time. In the lower Otay Formation, a characteristic suite of heavy minerals associated with ash falls includes eubedral apatite, zircon and occasional hornblende, while the youngest Otay samples contain only relatively rare euhedral zircon and anhedral apatite and hornblende. This finding may correlate with a transition from subduction-controlled tectonics to rift-controlled tectonism between the late Oligocene and early Miocene. One of the youngest Oligocene samples includes an unexpected heavy mineral assemblage of actinolite, anthophyllite, thulite and tremolite found locally usually only in Miocene stratigraphy. A proposed source for this material could have been a previously unrecognized late Oligocene uplift of Catalina Schist terrane, or detrital sediments shed from Santa Catalina Island as it was strike-slip faulted northward from offshore Baja California. The San Diego Formation incorporates 14 heavy mineral species commonly included in reports of Catalina Schist petrography. These species are believed to originate from Plio-Pleistocene faulting of the Rosarito Beach Formation, a San Onofre Breccia correlative exposed from 10 to 50 km south of San Diego. One sample collected from near the base of the San Diego Formation contains sparse kyanite, a very rare component of Catalina Schist. Since this unusual finding occurs just north of the International Border, it evidently reinforces a proposed southerly source for Franciscan-type sediments.