Abstract: Late Quaternary Sedimentary Environments of the Walker Lake Half Graben, Nevada

Terence C. Blair, John G. McPherson

The Walker Lake basin, located in the Basin and Range Province of west-central Nevada, is a modern half graben bounded on its tectonically active west side by the Wassuk Range, and on the passive east side by the Gillis Range. The sedimentary environments of the basin have a classical half-graben asymmetrical distribution, with the axial Walker River and the perennial Walker Lake occurring at, or near the active fault margin, locally separated from the actual margin by alluvial fans or fan-deltas with short radii. In contrast, alluvial fans and fan-deltas with relatively long radii occupy the passive side of the basin.

The alluvial fans, as well as the subaerial component of the fan-deltas, consist principally of clast-rich debris-flow lobes and levees interstratified with washed debris-flow intervals. The debris-flow deposits typically are indurated where derived from footwall zones composed of volcanic rocks, and are loose and easily erodible where derived from quartz monzonite. The subaqueous component of the fan-deltas is dominated by poorly sorted, sandy cobble to pebble gravel deposited below wave base in steep foreset beds that tangentially merge with the lake bottom along typically sandy toe sets. Bottomset prodeltaic sand, silt, and mud in nearly horizontal beds occur at the distal margins of the fan-deltas, becoming less sandy with distance from the delta proper.

The fan-delta deposits are strongly modified in the vicinity of the Walker Lake shoreline due to the effect of high levels of wave energy generated by winds of variable intensity and direction. The waves achieve extensive sorting in the shallow upper shoreface zone, winnowing sand and clay from the fan deposits to produce rounded pebble or cobble gravel beaches. The strongly oblique angle of impingement of the waves on the shoreline typically generates longshore currents. These currents actively erode the fans as well as deposit recumbent spits consisting of beach gravels and back-barrier ponds into which washover gravel, sand, mud, and algal mats accumulate. Wave erosion of the fans is most extensive along fault sectors where hangingwall subsidence is greatest, and is least at fault ector terminations, where subsidence is less. Areally extensive accumulations of fan delta and recumbent spit sequences occur in these later sites, whereas deep lake deposition dominates the former.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994