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Mio-Pliocene sediments of the lower Colorado River area represent the northernmost well documented extent of the proto-Gulf of California, a tectonically enigmatic marine incursion that occupied much of what is now the Gulf of California region—including the lower Colorado River area and the Salton Trough—as much as 8 m.y. prior to the onset of spreading-and transform-related subsidence in that area. Fanglomerate and volcanic rocks informally referred to here as the Osborne Wash strata interfinger with the conformably overlying Bouse Formation. The Bouse Formation includes carbonate and coarse terrigenous-clastic basin margin deposits that interfinger laterally with basin fill material; basin fill strata comprise a basal estuarine carbonate unit overlain by fine-grained terrigenous-clastic deltaic deposits. The upper portion of the Bouse interfingers with overlying cobble conglomerates here referred to as the Colorado River gravels. Syndepositional folding of the Bouse Formation and bracketing units is believed to reflect slumping on oversteepened slopes, perhaps exacerbated by episodic tectonic activity. Syndepositional faults show both normal and reverse separation; outcrop relations allow but do not prove a strike-slip component of motion on some structures. Contemporaneous minor faults seem to reflect mutually incompatible stress orientations; this suggests that they record either localized stress fields or localized anomalous responses to regional stresses. Alternatively, they may reflect extension in two directions in the sediment pile overlying the subsiding basin floor. The Bouse Formation and bracketing units record four stages in the evolution of the northern proto-Gulf/lower Colorado River area: (1) dissection of preexisting, detachment fault-controlled topography and localized, interior-drainage alluvial deposition (about 14–9 Ma); (2) regional subsidence and proto-Gulf transgression (perhaps as early as about 8 Ma; not later than about 5.5 Ma); (3) progradation of ancestral Colorado River delta into the northern end of the proto-Gulf basin (prior to about 4.3 Ma); and (4) arrival of throughgoing Colorado fluvial channel (prior to about 3.5–4 Ma). Outcrop relations between the Osborne Wash strata and the Bouse Formation, and the relationship of these units to modern landforms, indicate that topography in the lower Colorado River area has not changed significantly since middle Miocene time. Subsidence of the Bouse basin is believed to have occurred via broad regional downwarping, which may have represented a sag formed as the locus of active proto-Gulf extension propagated northward into the lower Colorado River area from the block-faulted southern portion of the proto-Gulf. Timing suggests that incipient proto-Gulf extension in the lower Colorado River area was arrested by the approximately 5-Ma shift to the modern transtensional regime.