CORING FOR ASH: A POSSIBLE NEW TEPHRA FROM MOUNT BAKER VOLCANO

A small alpine lake northeast of Mount Baker volcano preserves what initial analysis suggest may be either a previously reported (Hyde and Crandell, 1978) but rarely preserved tephra from the volcano correlative to the SP tephra, 10870 ± 80 ¹⁴C yr. B.P., or an unidentified tephra. Alternatively, the deposit may be eroded sediment from a Table Mountain columnar andesite outcrop present in the cliff above and to the east of the lake.

In the fall of 2007, we collected three short sediment cores from a small, shallow, unnamed lake near the Heather Meadows Visitors Center. The lake is roughly circular, approximately 50 m in diameter with a maximum depth of 3.6 m. We cored the lake using a modified Livingstone piston corer (50.8mm diameter barrel); all cores showed a similar upper stratigraphic sequence: organic mud (gyttja) interbedded with two known tephras (Mount Baker Set BA/OP, ~5740 ¹⁴C yr B.P.; and Mazama O, ~6850 ¹⁴C yr B.P.). Two of the cores also contain a dark, silty-to-sandy glass-bearing basal unit up to 12 cm thick that physically resembles other Mount Baker tephras. Initial Scanning Electron Microscope/Energy Dispersive Spectroscopy analysis of the glass shards in the basal unit suggests that the composition of the grains is within the compositional range of the SP tephra (average SiO₂ value of 66.85%) and the BA tephra (average SiO₂ value of 68.42%). A compositional analysis of glass from the Table Mountain andesite should help determine if it is a possible source for the basal unit. Of note, no unit correlating to this dark sand was found in Highwood Lake, which is only 1.4 km down-valley of our lake. Additional coring in several other alpine lakes in this area during winter, 2008, should help to constrain the distribution and composition of this unit.