PLUVIAL SHORELINES AND THE DETERMINATION OF EROSION RATES IN THE GREAT BASIN USING GPS AND C-14 DATA FROM MUD LAKE PLAYA, NEVADA

Distinctive linear ridges from 3 to 6 m wide, 0.3 to 3 m high, and up to 2.5 km long surround the Mud Lake playa located in the Ralston Valley, Nevada. Ridge segments align with one another to form individual ridge systems that are circumferential to the playa. Each ridge system exists at a unique elevation around the playa and is separated from adjacent ridge systems by linear depressions parallel to the ridges. These ridges are capped by beach gravel lag deposits formed by pluvial lakes during the Late Pleistocene and Early Holocene. The sequential ridges record the shrinking and disappearance of pluvial Mud Lake. Following the disappearance of the pluvial lake the beach gravel lag deposits acted as a protective caprock as erosion and ablation removed detritus from the adjacent alluvium. Beach gravel-capped ridges at 1594, 1592, and 1589 m above sea level contain fossil stromatolites that yield C-14 ages from 23,560 +/-170 to 29,820 +/- 300 years. These beaches were abandoned after 23,560 yrs. B.P. and local pluvial lakes and wetlands completely disappeared after 10,570 +/- 70 yrs B.P., based on C-14 ages from nearby seep deposits. Real Time Kinematic GPS surveys measured the local relief across and between these ridges and these data were used to measure the depth of erosion since the beaches were abandoned. Erosion and ablation removed as much as 112 cm of detritus from one location between the upper and middle ridges. The presence of well developed desert pavement indicates that ablation alone removed as much as 20 cm of fine detritus from a similar location along the strike of this valley. A maximum of 65 cm of ablation was recorded between the middle and lower ridges. This yields an ablation rate of up to approximately 2.8 mm/century (0.028 mm/yr) at this location within the Great Basin.