LATEST PLEISTOCENE AND HOLOCENE LAKE LEVEL FLUCTUATIONS, PYRAMID SUBBASIN OF LAKE LAHONTAN, NEVADA, USA

The drop in lake levels after the most recent Lake Lahontan highstand (approx. 13,000 C-14 ybp) left smaller isolated lakes in linked or individual subbasins. Histories of these latest Pleistocene and Holocene post-Sehoo highstand lakes are poorly understood despite their importance for active tectonic, archaeological, and paleoclimatic studies.

We find evidence for at least two post-13,000 C-14 ybp lake transgressions in the Pyramid subbasin. Constructional shoreline features associated with each lake stand can be differentiated on the basis of geomorphic character and AMS C-14 dates of charcoal and fossil molluscs incorporated into alluvial fan and lacustrine deposits.

The highest, oldest suite of shorelines occurs between approx. 1208 m and 1230 m elevation. The shoreline features are subdued and often discontinuous in character and unconformably abut older recessional shorelines. We placed a trench across a constructional lacustrine beach berm at 1212 m. An intact nacreous pelecypod shell was obtained from lacustrine sands and gravels exposed in the trench. AMS C-14 dating of this shell yielded an age of 10,890 +/- 35 C-14 ybp (12,819–13,126 cal. ybp, 2 sigma). This older lake stand may be related to the Younger Dryas climatic event, or northward deflection of the Truckee River following regional isostatic rebound due to dessication of Lake Lahontan (Adams et al., 1999), or both.

A lower, younger suite of shorelines occurs between approx. 1185 m and 1208 m elevation. The shoreline features are very sharp, continuous, and crosscut a Truckee River meander surface postdating the older suite of shorelines. A trench in the lowest constructional lacustrine bar of this series at approx. 1185 m exposes lacustrine sand and gravel deposited on subaerial debris flow deposits. Detrital charcoal incorporated in the deposit limits debris flow emplacement to after 4,235 +/- 40 C-14 ybp (4630-4860 cal. ybp), indicating that lake level was lower than ~1185 m at this time. Abundant gastropod shells (Pyrgulopsis nevadensis) in the base of the lacustrine deposits yield an age of 3595 +/- 35 C-14 ybp (3,830-3980 cal. ybp). The 1185-1208 m shorelines thus appear to record a previously unrecognized late Holocene lake stand.