Surficial geologic maps of the Tucson Metropolitan Area

Four geomorphically distinct ages of alluvial fan surfaces can be recognized on the piedmonts flanking the Catalina, Tortolita, and Tucson Mountains. Channel entrenchment of the upper piedmonts during the Quaternary has lead to the preservation of individual fan surfaces that become progressively younger and lower in relief toward the valley axes. Ongoing downcutting of Rillito Creek and Santa Cruz River as well as the Canada del Oro Wash has enhanced the incision of mountain-sourced drainages and further increased the relief separating alluvial surfaces of different ages. Thus, periods of climatically induced downcutting separated times of aggradation or equilibrium produce a sequence of discrete alluvial surfaces of different ages. Episodes of downcutting along the Santa Cruz and Rillito Rivers has further resulted in a series of fluvial terraces within the central Tucson Basin Area. Wide pediments flanking each of the mountain fronts in the map area indicate general tectonic quiescence during the Quaternary. (7 pages & 12 map sheets, 1:24,000 scale)