The Mio-Pliocene Fish Creek Gypsum (FCG) is a remarkable deposit of nearly pure calcium sulfate which attains a thickness of over 60 m. The estimated 46 square km deposit is located in the western Salton Trough, approximately 160 km east of metropolitan San Diego, California. The Fish Creek Gypsum and an underlying transitional marginal marine unit are sandwiched between nonmarine and marine strata. Past investigators have proposed both marine and nonmarine origins for the FCG, for no conclusive evidence of either origin had been disclosed. The first discovery of fossils within the FCG was made in this study and represents, perhaps, the most important finding of the project. The fossils found in the claystone partings within the FCG together with sulfate isotope ratios and stratigraphy provide unequivocal testimony of the marine origin of the FCG. Moreover, the similarities between the fossil assemblages within the FCG and the overlying Imperial Formation emphasize their close relationship and conformable contact. The FCG is now the lowest proven marine unit in the Fish Creek-Vallecito region and records the first incursion of the Gulf of California to the area. The marginal marine FCG is conformably underlain by a gradational sequence of terrigenous sedimentary facies. Alluvial fan conglomerates, which flanked the ancestral Vallecito Mountains to the north of the study area, grade southerly into transitional marine sandstones and thin-bedded siltstones and claystones. The finer clastic sediments grade upward into the FCG. The FCG laps out over the alluvial fan deposit to the north; a former landslide deposit to the northwest, west and southwest; and the Cretaceous crystalline basement to the southeast. Upsection, the FCG gives way to paralic clastic sediments, which in turn, grade upwards into full marine turbidites in response to the transgression of the Imperial Sea. The FCG has a multi-phase history. Gypsum precipitated from shallow-marine waters. During early diagenesis the gypsum was partially dehydrated to anhydrite. Complete anhydritization of the gypsum occurred during burial of the FCG as a result of increased temperatures associated with a 5 km burial load. Within the past one million years, the FCG anhydrite was uplifted, exhumed and partially gypsified by the downward percolation of meteoric waters. A variety of textures are displayed in the FCG. Most of the textures were created after deposition from early diagenesis, burial diagenesis, tectonism, gypsification, and weathering. Other minerals associated with the FCG are, in general, rare. Many of the FCG associated minerals are interpreted to have been transported with the clays into the FCG basin. The clay minerals in the FCG include predominately smectite, vermiculite with lesser amounts of mica/illite clays and chlorite, and a trace quantity of kaolinite. Minerals associated with the clays include quartz, biotite, feldspars, calcite, hematite, manganite and pyrolusite. Other salts, derived from residual brines, also occur in the FCG. Celestite and fluorite occur within the basal and upper layers in the FCG and are interpreted to be late diagenetic byproducts of gypsification. Pollen dominated fossil assemblages characteristize the strata below the FCG and the basal layers of the FCG. Phytoplankton dominated fossil assemblages characteristize the middle to upper FCG layers and the strata above the FCG. The change in the assemblage composition represents a change from a fluvial dominated to a sea water dominated depositional regime. The FCG calcareous nannoplankton coupled with the magnetic stratigraphic dating of Johnson et al. (1983) constrain the deposition of the FCG to between 4.3 and 6.3 ma. The FCG is one of several thick Neogene calcium sulfate deposits occurring between nonmarine and marine strata located along the eastern continental borderlands of Baja California. It appears that these evaporites formed in response to the Neogene extensional-rift tectonics associated with the opening of the Gulf of California.