MIOCENE AND PLIOCENE EXTENSIONAL BASINS OF NORTH-CENTRAL ARIZONA

Tertiary sedimentary basins in north-central Arizona evolved over time in response to plate tectonic processes. Prior to 18 Ma, structural basins formed adjacent to detachment faults active to the south and west of Phoenix. Before 10 Ma, limited downwarping of the crust created the Milk Creek basin, southwest of Prescott, during deposition of volcanic rocks that are the age-equivalent of the Hickey Formation. Locally deep sub-basins in the southern part of Williamson Valley and Lonesome Valley probably formed soon after the youngest volcanism in the Hickey Formation (10 Ma) in response to a changing stress regime exerted by the subducting slab. Big and Little Chino Valleys and the Verde Valley developed in response to cessation of northeast-directed subduction beneath North America and initiation of rifting in the Gulf of California at 7 Ma. Much of the geometric form of the valleys, including development of the Mogollon Rim, had formed by 4 Ma, but sedimentation within the basins continued until 1.5 Ma

The Verde Valley has a maximum sediment thickness of 780-950 m, with evaporite deposited along a narrow, central graben, fresh-water limestone throughout the basin, and conglomerate around the basin margin. The Bridgeport and Airport faults, both largely concealed but revealed by geophysical data, define the eastern and western margins of the central graben, respectively. The Verde Fault, with more than 550 m of Quaternary and Miocene displacement, forms the southwestern margin of the basin.

Big Chino Valley has a maximum sediment thickness of 750 m, with calcareous siltstone deposited in a central playa overlain by limestone-rich fanglomerate. The central playa is flanked by conglomerate in alluvial fans. Buried plugs of reversely magnetized Oligocene lati-andesite are well defined from magnetic data. Chemical and x-ray diffraction data from three wells that cross the southwestern margin of the playa indicate that the interface between playa and alluvial fan material is steep. Subsidence of the playa kept pace with deposition by alluvial fans during the early history of the basin. The Big Chino Fault forms the northeastern margin of the basin and has as much as 1100 m of Pleistocene and Miocene displacement. The tectonic framework of these basins significantly controls groundwater flow in the shallower parts of the basins.