The Huesos Member is proposed as a new lithostratigraphic unit for upper parts of the Palm Spring Formation in the Fish Creek-Vallecito Creek area of the Salton Trough, southern California. The Huesos Member is significant because it records the post-Colorado River history of this part of the Salton Trough, has produced the vast majority of vertebrate fossils in the Salton Trough, contains the Plio-Pleistocene boundary, and is a strong candidate for the Blancan-Irvingtonian boundary stratotype. Rocks of the Huesos Member are predominantly sandstone, silty sandstone, and siltstone. Texture, fabric, fluvial architecture, and stratigraphic relationships suggest that the Huesos Member represents deposits of a shallow, braided, bedload stream system that formed as the basinward extension of a conglomeratic, basin-margin bajada. Deposition was mainly by unchannelized sheetflow, rather than by active channel migration and reworking of older deposits. The stratigraphic pattern of the Huesos Member consists of three packages of strata: (1) sequences of alternating beds of sandstone and siltstone that represent "normal" deposition; (2) sequences of very coarse-grained sandstone that grade upward to siltstone, representing geologically instantaneous depositional events; and (3) thick beds of siltstone or claystone that represent primarily alluvial overbank deposits. Evaluation of stratigraphic patterns within paleomagnetic subchrons shows that "normal" sedimentation events dominated Huesos deposition, that sediment accumulation rates decreased with time, and that sediment accumulation rates resulted in continuous deposition.

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