The Salton Basin of southern California is roughly triangular. It ranges from 5 to 130 km in width and is 200 km long. The fault-bounded trough has been filling with a complex series of continental clastic deposits in late Pleistocene and Holocene time. The Holocene deposits are estimated to be up to 60 to 100 m thick.
Two major sources supply sediment to the basin. The Colorado River, carrying eroded debris from the Colorado Plateau, has intermittently flowed into the southern part of the basin and deposited sand and mud in deltaic and lacustrine facies. Sediment derived from sources at basin margins is deposited in alluvial fans, braided streams, barrier beaches, and lacustrine beds. Sands from both sources have been reworked into extensive aeolian deposits.
The lacustrine muds and silts form an extensive central deposit with which all other facies interfinger or are deposited upon. Abrupt facies changes are common. One example is a 60 m vertical sequence of lacustrine muds overlain by coarse alluvial fan deposits upon which a barrier beach is developed. Lateral changes are also sharp; examples are lacustrine muds cut by meandering or braided channels filled with fine to coarse sand and gravel. Sand thicknesses change abruptly due to changes in transport directions and gradients.
The sands from the two major source areas are compositionally distinctive. Colorado River sands contain 60 to 70 percent quartz, 2 percent chert, and 20 percent feldspar, whereas most sands that are derived from the basin margins contain 40 to 50 percent quartz, 30 percent feldspar, and 1 percent chert. The remainder in both sand types is made up of clay, rock fragments, detrital carbonates, and heavy minerals. This information is particularly useful in determining the source of aeolian sands. It shows that the Algodones Dunes are derived from Colorado River sands rather than basin margin sources as suggested by other workers.
Faunal variations in the lacustrine sediments are related to salinity changes with changes in lake volume. The saline lacustrine fauna is a restricted type and is unlike the marine fauna of the nearby northern Gulf of California.