Expansive tidal flats and grassy salt marshes dominated the tide influenced shallow waters of the northern Gulf of California at ~6 Ma. Our work addressed the following questions: how far north did the Gulf of California extend and what depositional environments existed at the Gulf's northernmost arm? This work then informs tectonic reconstructions of the southwestern United States. We find evidence for high tides  (i.e., deposited at sea level) in sediment uplifted 100–200 m over the past 6 Ma. This work suggests activity of young crustal and upper-mantle deformation processes that remain incompletely understood.