The Sonoyta quadrangle is characterized by northwest-trending ranges of pre-Cretaceous(?) igneous and metamorphic rocks and less abundant Cretaceous(?) granitic plutons. Tertiary (?) volcanic rocks overlie, and locally are in fault contact with, the older crystalline rocks. Most of the western half of the quadrangle is covered by alluvium, sand dunes, and Holocene basalt. The area is cut by northwest-striking faults, one of which exhibits evidence of right-lateral displacement.