Abstract

More than 600 paleocurrent measurements from fluvial paleochannels of the early Colorado River delta plain, preserved in the Palm Spring Formation in the Fish Creek–Vallecito (FCV) section of southern California, indicate that the FCV section was on the Gulf of California (southeastern) flank of the delta plain as recently as 2.8 Ma. These measurements imply ∼130 km of tectonic translation to the northwest since 2.8 Ma, which can be accounted for by the displacement histories of Baja California and component faults of the San Andreas system. Stratigraphic data from the FCV section and other Neogene localities, when plotted on a palinspastic base, indicate (1) marine transgression of a narrow Gulf of California rift basin and Bouse embayment (site of modern lower Colorado River) by 5.5 Ma, and (2) uplift of the Bouse embayment, initiation of the: lower Colorado River, and southward progradation of the delta to essentially its present position by 4 Ma.

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