THE USE OF TEPHRA TO REINTERPRET EARLY HOLOCENE GLACIAL HISTORY AT MOUNT BAKER

Electron microprobe data from Baker Set SC tephra found on Mount Baker, Washington yield new insight into the record of early Holocene glacial history on the volcano. This tephra originated from the Schrieber's Meadow cinder cone ca. 8850 14C yr BP.

Tephra Set SC has been described by many previous workers as “red scoria,” and its distribution used to constrain the age of glacier deposits on Mt. Baker. The tephra was originally mapped to be absent on a set of ridges downstream of Little Ice Age moraines on the south flank of the mountain; the absence was interpreted to indicate that these ridges were covered by ice in the early Holocene. We found that several ridges in this “scoria-free zone” are overlain by red tephra similar to Set SC tephra. We compared samples of red tephra taken from within the scoria-free zone to samples of Set SC from outside the zone. Electron microprobe analyses of glass shards show that the red tephra resembles tephra Set SC. The apparent presence of Set SC in the alleged scoria-free zone indicates that glaciers were no more extensive at 8850 14C yr BP than during the Little Ice Age. We also found Set SC and Mazama tephras within a composite lateral moraine at Easton Glacier approximately 800 m upvalley from its Little Ice Age terminal moraine, suggesting very limited ice in early Holocene time. These results are significant because they do not support the hypothesis of a significant early Holocene glacial advance in the Pacific Northwest.