VERY HIGH SHORELINE DEPOSITS INDICATE THE PRESENCE OF AN EARLY LAKE LAHONTAN ALONG THE NORTHERN SIERRA NEVADA

Serendipitous field observations found incredibly high beach gravels in and along valleys of the northern Sierra Nevada that are far above the pluvial middle to late Pleistocene high stands of Lake Lahontan. Well-rounded pebble to boulder gravels and tufa are consistently found at elevations up to 1645 m, more than 300 m above the highest Lahontan Eetza and Sehoo shorelines. They are deeply dissected, blanketing ridge crests and mountain slopes and are commonly reworked into alluvial fan deposits. More than 30 m of bedded, unconsolidated beach deposits are exposed in Eagle Canyon Pass on the Hungry Ridge range crest. At least two distinct deposits outcrop up to an elevation of 1630 m, indicating that this lake level was reached at least twice. Some of the highest beach gravels found to date occur at elevations of 1645-1660 m. It is not clear what role, if any, tectonics has played. The areal extent of such lakes would have been vast, extending throughout the basins in and along the northern Sierra Nevada, including the Reno and Carson City basins, Verdi basin, Sierra Valley, and Mohawk Valley. The greatest depth of the lakes would have been more than 500 m. Morrison (1991) identified multiple early Lahontan lakes but did not believe any exceeded 1332-1335 m elevation. But Reheis et al. (2002) found high Lake Lahontan deposits at 1400 m in the Walker basin and proposed an age of 650 ka based on tephra data. Similar-age tephra beds (the ~600 ka Rockland and Dibekulewe beds) occur in lower elevation fine-grained deposits in the Reno and Carson City basins, but the stratigraphic relationship between these likely lacustrine deposits and the high beach gravels is unknown. The ages and total extent of these high beach deposits are therefore presently uncertain.