ORIGIN OF THE WALKER LAKE CINDER LAYER AND ITS IMPLICATIONS FOR DATING THE SADDLE MOUNTAIN ERUPTION, SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

Of the dozens of Quaternary cinder cones in the San Francisco volcanic field, only a few have been istopically dated. The ages are based on K-Ar analyses, except in the case of Saddle Mountain. Its age is purportedly constrained by 14C analysis of Walker Lake sediments, which underlie cinders assumed to have been erupted by Saddle Mountain. The goal of this study is to determine the origin and depositional history of the cinder layer in Walker Lake.

Walker Lake sits in the unbreached crater of a small, basaltic cinder cone 5 km south of Saddle Mountain and has been accumulating sediments through Quaternary time. The sediments are carried into the crater by wind, slope processes, and, possibly, nearby eruptions. In the 1980s, workers interpreted a 1-m-thick cinder deposit as distal tephra fallout from the eruption of nearby Saddle Mountain. Organic material in lake mud beneath the cinder layer yielded an age of 15,440 ± 250 14C yr BP, which workers accepted as the time of the Saddle Mountain eruption.

The gross cinder-layer geometry was determined by hand augering at 11 sites within the Walker Lake basin. Three small test pits exposed its internal stratigraphy. The cinder layer is underlain by at least 5 m of gray, orange, and brown lake mud containing pure-cinder laminae. Cinders comprising the main cinder layer are angular and matrix-free. The layer thickens from 40 cm at the basin margin to 230 cm toward the center of the basin, suggesting focusing by down-slope movement. Bedding, grading, and sorting are consistent with deposition by water-mediated slope processes. Specifically, the cinders compose distinctly reversed-graded beds of nearly equal thickness. Brown lake mud containing cinder laminae overlies the main cinder layer at the basin center but grades radially outward into stony silt. The frequency and young age of the cinder laminae in this upper mud unit indicate a hillslope rather than eruptive source.

The 14C age of lake mud underlying the cinder layer is a maximum age for deposition by slope processes; it does not record primary deposition by fallout. The cinders may be reworked Saddle Mountain tephra, but the length of time separating fallout and reworking is not known. We suggest that the cinders are washed-in local constituents of the Walker Lake cone, which has basaltic composition similar to Saddle Mountain.