TIMING OF LAKE PHASES IN THE AFTON ARM OF LAKE MANIX, MOJAVE DESERT: RELATIONS TO PALEOCLIMATE AND DRAINAGE-BASIN CHANGES

The Afton arm is the eastern subbasin of pluvial Lake Manix in the Mojave Desert of south-central California. Interbedded lake and fan deposits in the Afton arm were extensively exposed by dissection after Lake Manix drained east to form Lake Mojave and cut Afton Canyon. Previous studies recognized a major lake phase during OIS 6(?) and several high-lake episodes during OIS 4(?), 3, and 2. Based on detailed mapping, new ¹⁴C dating, and preliminary U-series ages on lake tufa, we interpret the Afton subbasin record as 3 major lake phases. The first entry of lake water occurred due to failure of a dam about 20 km upstream of Afton Canyon; deposits of this oldest lake phase extend as high as about 530 masl. U-series ages from tufa possibly associated with this lake phase range from about 210 to 150 ka and suggest deposition during OIS 7(?) and OIS 6. An intermediate lake phase is represented by thick green mud and nearshore sand and gravel deposits that can be traced at least as high as 550 masl; tufa from the base of the mud yielded several ages between 138 and 71 ka and minimum-limiting ¹⁴C ages on shells above the tufa are 49.8 to 39.9 ka. Deposits of this phase are capped by fan gravel with a weak soil; the gravel thins toward the basin axis and represents a significant hiatus and lowering or desiccation of the lake. The youngest lake phase is represented mostly by beach sand and gravel that extend to as high as 543 masl in spits and barriers constructed on top of fan gravel. Subtle unconformities and very weak soils, together with ¹⁴C and U-series ages, show that this phase consists of three subunits recording highstands at about 34.7 to 31.9 (possibly as old as OIS 4), 29.6 to 25.4, and 23.1 to 20.5 ¹⁴C ka.

This record suggests that Lake Manix achieved high levels during several episodes corresponding to OIS 6, 5, 4(?), 3, and 2, in contrast to pluvial lakes farther north in the Great Basin whose highstands correspond mainly with glacial stages. These data suggest that high levels of winter precipitation necessary to sustain Lake Manix may be influenced by ENSO-like conditions and southward positions of the Aleutian Low, with or without southward depression of the polar jet.