Split Mountain is a northwest-trending asymmetric anticline located along the western margin of the Imperial Valley, California. Split Mountain Gorge breaches the anticline and provides an oblique cross-sectional view of the southwest limb. Two stratigraphic sequences are recognized in the Miocene to early Pliocene sediments exposed along the gorge: (1) a lower nonmarine sequence, and (2) an upper paralic and marine sequence. The upper sequence is described in this study. The stratigraphic nomenclature in the western Imperial Valley is confusing and controversial. As a consequence, contact mapping and section measuring were done in terms of lithofacies. Six lithofacies were recognized in the upper sequence: (1) lithofacies "A"--coarse nonmarine conglomerates; (2) lithofacies "B"--thickly bedded marine sandstones and siltstones; (3) lithofacies "C"--marine turbidite sandstones and siltstones; (4) lithofacies "D"--massive marine gypsum; (5) lithofacies "E"-- massive nonmarine and marine sedimentary megabreccia; and (6) lithofacies "F"--thickly bedded marine mudstones and fine-grained sandstones. Correlations between adjacent sections indicate that lithofacies “A," “B,” “C," and "D" are stratigraphically equivalent and represent contiguous depositional environments. Lithofacies "E" is present as an interbed in lithofacies "A," “B," and "C," suggesting a wide distribution for the landslide-debris flow unit. Emplacement of lithofacies "E" is thought to have been dramatic, and effectively, instantaneous. Together with a wide distribution, this type of emplacement establishes lithofacies "E" as a time-stratigraphic unit in the Split Mountain region. Lithofacies "F" represents the most distal portions of both the Split Mountain "A"-"E" sedimentary environment and the distant Colorado River delta. The vertical succession of environments at six locations indicates that an unconformity separates the lower nonmarine and the upper marine-related sequence in most areas. The absence of a well-defined paralic interval in all sections suggests a rapid marine transgression. Paleontologic age determinations indicate an early Pliocene age for the initial marine deposits. These sediments are thought to represent the earliest incursions of the Gulf of California into the Salton Trough.