ABSTRACT: Sedimentary Record of Climatically Induced Lake-Level Fluctuations, Pyramid Lake, Nevada

SMOOT, JOSEPH P., U.S. Geological Survey, Denver, CO

Pyramid Lake is a large, perennial, terminal lake occupying a fault-bounded basin fed by the Truckee River. Repeated changes in the depth and area of the lake during the past million years are attributed to fluctuations in regional rainfall and evaporation. The stratigraphic record of these climatically induced changes was examined in a 60 m long core (PLC87) from the north shore of the lake and in 40 m high outcrops of lacustrine and deltaic deposits along the incised Truckee River canyon on the south side of the lake.

In PLC87, a systematic succession of sedimentary features at a 10 m scale reflects changes from subaerial deposits to deepest water deposits (>100 m): (1) brecciated silts and clays, (2) oscillatory-rippled coarse-to-fine sand, (3) thin beds of bioturbated silty clay, and lenses of oscillatory-rippled sand, (4) bioturbated silty clay with graded, skeletal silt erosional lags, (5) bioturbated silty clay with random burrows, (6) bioturbated silty clay and clay with horizontal burrows and remnant laminae, (7) finely laminated graded silt and clay (with dropstones), or laminae of biologic debris and clay (varves?). Decimeter-scale alternations of fabrics within the sequences indicate smaller depth fluctuations.

Transgressive sequences in the Truckee River canyon are similar to those in PLC87 except that thin-bedded turbidites are common and total sediment thicknesses are nearly doubled. Regressive sequences are dominated by deltaic deposits, rich in reworked sediment. Small-scale fluctuations in depth are indicated by stacking of condensed sequences of proximal and distal delta deposits.

The sedimentary record of lake depth changes appears to be much clearer at a finer scale than published records based on biological or chemical criteria.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)