GRAVEL-CAPPED RIDGES RECORD PLUVIAL AND EROSIONAL HISTORY OF MUD LAKE PLAYA, RALSTON VALLEY, NYE COUNTY, NEVADA

Distinctive linear ridges from 3 to 6 m wide, from 0.3 to 3 m high, and up to 2.5 km long surround the Mud Lake playa, located on the Nevada Test and Training Range, which has limited access. Ridge segments align with one another to form individual ridge systems many kilometers long that surround the playa. As many as eight ridge systems separated by linear depressions parallel one another, with each successive ridge crest increasing in elevation away from the playa. Each ridge crest is consistent in elevation around the circumference of Mud Lake. The ridges are capped by 12 to 25 cm thick gravel deposits composed of moderately well-sorted, subangular to subrounded pebbles 1 to 5 cm in diameter. These gravel deposits are in contrast to the poorly sorted, angular gravel in the alluvium around Mud Lake. Similar deposits have been interpreted as barrier and off-shore bars. Here the gravel deposits resemble ancient beach gravel lag deposits from Lake Lahontan and modern beach deposits from Walker Lake. Sorting, clast size, and clast rounding of the Mud Lake beach deposits are related to the slope of the shoreline. Gently dipping slopes (3.3 m drop per km) on the north side resulted in thinner deposits of smaller, less rounded pebbles than the steeper slopes (15.4 m drop per km) on the south side. Steeper beach slopes result in greater inherent wave energy for sorting and rounding. Walker Lake provides a modern analog, where the steep beaches are characterized by coarse gravel and gentle beaches are characterized by sand and fine gravel. The beach gravel lag-deposits of Mud Lake acted as caprock protecting the underlying alluvium from erosion as deflation and runoff removed unconsolidated material from the surrounding desert. Successively lower ridges record the position of progressively younger pluvial beaches. The distribution of these ridges records the recessional history of pluvial Mud Lake (331 km2 at its highstand), which experienced periods of prolonged stability, with minor adjustment, at 1,609 m, 1,603 m, 1,597m, and 1,591 m above sea level. These periods of relative stability, and formation of lag-deposits, were separated by periods of accelerated retreat and decreased size of Mud Lake.