Macrotidal flats and grassy salt marshes dominated the shallow waters of the northern Gulf of California at ~6 Ma. My work addressed the following questions: how far north did the Gulf of California extend and which depositional environments existed at the Gulf's northernmost arm? Sedimentology informs tectonic reconstructions of the southwestern United States: a tidal influence (i.e., deposited at sea level) is found in sediment uplifted 100–200 m in only 6 Ma. Work suggests activity of young crustal and upper-mantle deformation processes that remain incompletely understood. This work is published in Geology and Sedimentology.