PALAEO-DRAINAGE OF THE SW US: THE EVOLUTION OF THE COLORADO-GILA-SALT RIVER SYSTEM

Since drainage networks can cover up to 35% of a continent and feed several basins, sand being delivered to a basin by a large river system is independent of intra-basinal processes and depends on the geology and scale of the catchment. This study starts from the observation that although the mineral piemontite is extremely rare in most sediments, it is present as a detrital phase in the Sespe (Oligocene-Early Miocene) and Topanga (Middle Miocene) formations of the Los Angeles Basin (LAB), where palaeo-Colorado deposits have been postulated. Potential sources of this mineral are both local to the LAB (Sierra Pelona, San Gabriel Mountains and Devil’s Punchbowl region) and exotic (Old Woman, Turtle, Mazatzal, Cargo Muchacho, Chocolate, Trigo and Orocopia mountains).

We used heavy mineral analysis: first, to deduce the origin of various sediment families in the lower Colorado River area and translated regions west of the San Andreas Fault; and, second, to map connectivity of regional drainage in the period from Oligocene-present. The study focused on the modern Colorado, Gila and Salt rivers; the Pliocene palaeo-delta of the Colorado in the Fish Creek-Vallecito Basin (southern California); and the LAB.

Mineral assemblages suggest that LAB sediment came from a catchment similar to today’s lower Colorado River, with sediment from the Cargo Muchacho, Chocolate and Trigo mountains. The lack of topaz in the LAB makes it unlikely that the upper Colorado River was connected at this time. The presence of Type Cii garnets in Salt River samples and the lack of these in the LAB suggest that the Salt River was not part of this early Neogene catchment. Stable mineral ratios show that the Gila River upstream of the confluence with the Salt River has an assemblage similar to Fish Creek-Vallecito samples, while all Gila samples downstream of the confluence are similar to the Salt River catchment sediment. The combination of these data indicate that although the Gila River is likely to have been a part of the Colorado River system when feeding the Salton Trough, the Salt River was not part of this system until more recently, after the Colorado River entered the northern Gulf of California.