A MODIFIED HISTORY OF LAKES MANIX AND MOJAVE, CALIFORNIA

New field work adds to conclusions of prior studies of the history of Lakes Manix and Mojave. These prior studies showed that Lake Manix, the terminal basin of the Mojave River until the late Pleistocene, reached its last highstand at an altitude of 543 m at ~18 ka. Overtopping a sill, it discharged eastward to fill Lake Mojave to ~289 m. During and after this time, Afton Canyon was incised between the two basins and Lake Mojave intermittently discharged northward toward Death Valley.

New studies have found several wavecut scarps and barriers in the Manix basin that suggest a highstand before 18 ka at ~556 m altitude. Features in the Soda Lake and Silver Lake basins suggest that Lake Mojave may once have attained an altitude of 295 m or higher. Minimum-limiting 14C ages of ~52 ka and 35 ka on groundwater-discharge tufa at the north end of Silver Lake suggest elevated water tables that may reflect the presence of an older Lake Mojave.

Evidence from two alluvial remnants (538 m) and slip-off river terraces (~536 – 521 m) found downstream of the Lake Manix sill on the north rim of Afton Canyon suggest Lake Manix overflowed prior to the cutting of the modern Afton Canyon. The alluvial remnants are ~5 m thick and consist of locally derived fan deposits interbedded with fluvial sand and fine gravel. The surface soil and five paleosols that overlie the fluvial deposit have profile development index (PDI) values ranging from 0.01 to 0.14, collectively indicating an older age than soils on lower fluvial terraces <18 ka. Although potential influence from the nearby Manix Fault is presently unknown, these observations suggest a connection between Lakes Manix and Mojave prior to the late Pleistocene.

Nine river terraces are inset within Afton Canyon, below the base of Lake Manix deposits. Soil development in three of these fluvial deposits at 463, 451, and 439 m is uniformly weak with PDI values of ~0.03. The presence of multiple terraces suggests incremental incision since the ~18-ka highstand of Lake Manix, though this incision occurred quickly enough that soils can not distinguish terraces of different ages.