The Hualapai basin, northwestern Arizona, contains one of the thickest known, nonmarine halite deposits in a continental rift. The basin is a large half-graben in the hanging wall of a listric normal fault along the eastern margin of the Basin and Range province. Seismic reflection and drill-hole data indicate that the little-deformed halite is 2.5 km thick in the central part of the basin, approaches ∼200 km3 in volume, and has a three-dimensional lenticularwedge geometry. An age of 9-13 Ma is suggested for the salt because it lies in the upper, more gently tilted part of a growth-fault sequence, and extension in the region is bracketed between 16 and 9 Ma. The texture and bromine content of the halite, dominance of halite, and S and O isotopic values of intercalated and capping anhydrite indicate that halite deposition took place in an intracontinental playa that accommodated regional groundwater discharge. Several events conspired to produce this unusually thick salt deposit, including regional aridity, a broad catchment basin with a closed drainage network, ample supplies of Na+ and Cl-, and rapidly developing accommodation space. In addition, oxygen isotopic values indicate a lacustrine origin for a thick (∼300 m) upper Miocene-lower Pliocene limestone in the Grand Wash trough directly north of the Hualapai basin. Thus, Miocene-Pliocene evaporite deposits in northwestern Arizona and southern Nevada do not represent the northern extent of the ancestral Gulf of California. A lack of upper Miocene-Pliocene marine deposits and little recent faulting in the region imply that most of the differential relief between the southwestern Colorado Plateau and adjacent Basin and Range developed 16-9 Ma during major extension. The abundance of Cenozoic nonmarine evaporites in the Basin and Range implies that a nonmarine origin should not be overlooked for large belts of synrift evaporites on passive continental margins. Primary, lenticular-wedge geometries characterize synrift salt deposits and may facilitate the development of large salt diapirs.
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