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Redefining the age of the lower Colorado River, southwestern United States: Reply

September 1, 2021

Crow et al. (2021) report new geochronologic and paleomagnetic data indicating that the lower Colorado River (CR) became integrated to the proto–Gulf of California (GOC) between 4.8 and 4.62 Ma instead of at ca. 5.3 Ma, as suggested by Dorsey et al. (2007, 2018). Dorsey et al. (2021) dispute this new chronology but offer no alternative explanation for one of the key data sets requiring it, new detrital sanidine (DS) geochronology. This accurate and precise constraint agrees with detrital zircon results on separate samples (Cloos, 2014) and is tied through magnetostratigraphy to the first known CR sands in the GOC.