PLEISTOCENE PLUVIAL LAKE DEPOSITS AT THE POVERTY HILLS, INYO COUNTY, CA: EVIDENCE OF GREATER EFFECTIVE PRECIPITATION DURING MARINE ISOTOPE STAGES 22-19 AND 5A/4 IN THE SOUTHWESTERN GREAT BASIN

The Poverty Hills (PH) are a 300-m-high upland encompassing 12 km2 in the Owens Valley about 70 km south of Bishop, California, and 60 km north of pluvial Owens Lake. Bound on the east side by the Owens Valley fault zone, the rocks exposed in the PH are highly brecciated Paleozoic metasedimentary rock, Cretaceous granodiorite, and Quaternary basalt. Two lacustrine sequences are described in buttress unconformity with the PH: (1) In the east lacustrine sandstone to freshwater diatomite beds dip up to 40 degrees. (2) In the north gently dipping (~5 degrees) lacustrine beds are exposed in a Highway 395 roadcut. The PH have been described as both a pressure ridge and landslide deposit.

We provide 40Ar/39Ar dating and tephrochronology from tephra beds within the two suspected Pleistocene lacustrine deposits. Four tephra beds were identified Interbedded with the diatomite-bearing sediments in the eastern PH. Two of these have glass shard compositions similar to the 0.8-1.2 Ma Upper Glass Mountain tephra beds. An 40Ar/39Ar date of 0.966 ± 0.007 Ma was determined on one of these beds. Closely overlying an Upper Glass Mountain tephra bed is a previously undescribed tephra informally named herein the tuff of Poverty Hills. The source of the tuff of Poverty Hills is unknown; the major and minor element composition is similar to the Pliocene tuffs of Curry canyon (~3.58 Ma), however, the trace element composition differs. The fourth tephra bed near the top of the sedimentary sequence is the 0.772 Ma Bishop tephra bed. In the northern PH, intercalated with the gently folded lacustrine beds is a tephra bed whose glass shard composition matches the 80-60 ka older Mono Craters ash beds.

At 60 km north and 65 m above the highstand of pluvial Owens Lake, the PH freshwater lake deposits represent a separate lake formed by impoundment of the Owens River behind the Poverty Hills and basalt lava flows to the east during a period of greater effective precipitation. The 0.966-0.772 Ma lake deposits correspond with Marine Isotope Stages (MIS) 22 to 19 when a perennial lake occupied Owens and Searles valleys and the Sherwin glaciation mantled the adjacent Sierra Nevada range. The 80-60 ka lake beds correspond with MIS 5a/4 and pluvial Owens Lake, Deep Springs Lake (CA) and Walker Lake (NV). Neither deposit is equivalent to the ~2.6-2.3 Ma Waucoba Lake Beds 15 km to the north.