McGill, PaulaNicholson, UisdeanFrei, Dirk2023-04-122023-04-122023McGill, P. et al. (2023). Defining regional and local sediment sources in the ancestral Colorado River system: A heavy mineral study of a mixed provenance unit in the Fish creek-vallecito basin, Southern California. Geosciences ,13(2), 45. https://doi.org/10.3390/geosciences130200452076-3263https://doi.org/10.3390/geosciences13020045http://hdl.handle.net/10566/8736The Colorado River has flowed across the dextral strike-slip plate boundary between the North American and Pacific plates since the latest Miocene or earliest Pliocene. The Fish Creek- Vallecito Basin (FCVB) lies on the Pacific Plate in southern California, dextrally offset from the point where the modern Colorado river enters the Salton Trough; it contains a record of ancestral Colorado River sedimentation from 5.3–2.5 Ma. The basin stratigraphy exhibits a changing balance between locally derived (L-Suite) and Colorado River (C-Suite) sediments. This paper focuses on the Palm Springs Group (PSG), a thick fluvial and alluvial sequence deposited on the upper delta plain (between 4.2–2.5 Ma) when the Colorado was active in the area, allowing the detailed examination of the processes of sediment mixing from two distinct provenance areas. The PSG consists of three coeval formations: 1) Canebrake Conglomerate, a basin margin that has coarse alluvial fan deposits derived from surrounding igneous basement; 2) Olla Formation, fan-fringe sandstones containing L-Suite, C-Suite, and mixed units; and 3) Arroyo Diablo Formation, mineralogically mature C-Suite sandstones. Stratigraphic analysis demonstrates that the river flowed through a landscape with relief up to 2000 m.enPalaeotopographyClimate changeSouthern Californiasediment fluxGeoscienceArticle