THE ANTIQUITY OF MOUNT ST. HELENS AND AGE OF THE HAYDEN CREEK DRIFT

Tephra layers generated by explosive eruptions at Mount St. Helens (MSH) form invaluable late Pleistocene and Holocene markers beds throughout western North America. The oldest tephras, Set C, consist of distinctive quartz-biotite-cummingtonite-hornblende-phyric dacites. Set C predates the Fraser glaciation and commonly overlies till correlated with the Hayden Creek Drift (HCD) of the Mount Rainier region, deposited during the penultimate glaciation in the Washington Cascade Range. The ages of Set C and the HCD are poorly known. Based on weathering characteristics and ¹⁴C dates, Crandell (1987; USGS PP 1444) inferred an age of 50-35 ka for Set C and about 60 ka (=Marine Isotope Stage (MIS) 4) for the underlying till near the volcano.

We have found many places in the Lewis River valley downstream from MSH where the HCD contains dacite clasts mineralogically equivalent to Set C. Locally, Set-C-like tephra underlies till, and at several localities fluvial deposits composed almost exclusively of similar dacites are intercalated with drift. These sediments are found as high as 400 m above the valley floor, too high to be remnants of a valley-wide fill. We interpret them as syneruptive volcanic debris that accumulated in glacier-margin lakes. Generally, little or no till is found at elevations above the ice-contact beds, indicating deposition occurred near the Hayden Creek glacial maximum.

Plagioclase in tephra beneath till about 33 km SW of MSH has an ⁴⁰Ar/³⁹Ar plateau age of 247±12 ka. Further downstream, plagioclase separated from a pumice-lapilli bed in ice-contact beds has an ⁴⁰Ar/³⁹Ar plateau age of 270±20 ka. The source vent for these deposits may be Goat Mountain, a glaciated dacitic plug-dome about 8 km W of MSH, for which we obtained a K-Ar whole-rock age of 296±7 ka.

These results indicate that silicic volcanism in the vicinity of MSH began much earlier than previously recognized; some beds assigned to Set C may be as old as 300 ka. Near the volcano, deposits from the oldest eruptions have apparently been buried or removed by glacial erosion; they are preserved only in more distal settings. The data also indicate that the glacial advance that deposited the Hayden Creek Drift in the southern Washington Cascade Range reached a maximum about 275-250 ka and thus correlates with MIS 8 and with the Double Bluff Drift in Puget Sound.