THICK MIO-PLIOCENE NON-MARINE SALT DEPOSITS IN THE SOUTHWESTERN USA: IMPLICATIONS FOR DEVELOPMENT OF THE COLORADO RIVER DRAINAGE SYSTEM

Thick non-marine Mio-Pliocene halite deposits reside in many half grabens in the Basin and Range of the SW USA. These deposits have major implications for the paleogeographic evolution of the region and also have significant value as industrial minerals and for solution cavern storage. Halite accumulated in continental playas in enclosed basins and is commonly associated with other evaporites, such as gypsum and anhydrite. These evaporites record a period of internal drainage coinciding with and following major Miocene extension while preceding the through-going Colorado River (CR) drainage.

Thick halite deposits are documented in the Luke, Detrital, Hualapai, Picacho, Bristol Lake, and Overton Arm basins. Thickness of halite is as much as 1-2.5 km in the Hualapai (Red Lake salt) and Luke basins. Thinner, but substantial, halite deposits (>350 m) have been drilled in the Bristol, Picacho, and Detrital basins. Thick evaporite deposits are generally associated with large catchments proximal to major rivers, such as the CR and Gila River, and which drain broad elevated terranes (e.g. Colorado Plateau (CP)). Other basins proximal to the CR and its major tributaries have high potential for containing thick salt, including the Mesquite basin in southern Nevada, Big Sandy and Dutch Flat basins in NW Arizona, Higley-Chandler, Paradise, and Rainbow basins in central Arizona, and Blythe-McCoy, Yuma, and Mohawk basins in SW Arizona, all of which have major gravity and aeromagnetic minimums compatible with thick halite.

Basins in lower reaches of the CR system were integrated relatively rapidly through sequential, down-stream spill-overs (House et al., 2008). In contrast, lakes persisted as long as ~5 m.y. in more upper reaches (e.g. Grand Wash Trough). Our new tephrochronology of interbedded tephras constrains the uppermost salt in the Hualapai basin to 5.6 Ma and salt in the Picacho basin to 8.6 Ma. The 5.6 Ma age immediately precedes development of the CR in the Lake Mead region. This correspondence strongly suggests that the thick evaporites are related to incipient development of the CR system. We envision a precursor to the CR and its tributaries delivering large amounts of water to the Lake Mead and other regions, which ponded in axial basins and spilled into satellite basins forming broad playas and salt pans.