Graded Lahars in the Type Sections of the Ellensburg Formation, South-Central Washington

Abstract

The volcaniclastic Miocene-Pliocene Ellensburg Formation west of Yakima contains up to about 15 lahars. These are massive, sheet-like deposits with an average thickness of 2-5 m (6-15 feet). The lahars are characteristically graded, having a thin fine-grained (medium sand) basal layer which abruptly grades into a massive, coarse, poorly sorted central portion that may contain blocks up to 3 m (9 feet) in diameter. This part grades into a bedded or cross-bedded top zone (fine to medium-grained sand, partly pebbly) that may be transitional to horizontally bedded tuffs. Clay- and siltsized fractions form less than 10 percent of all three zones. Heavy minerals, chiefly hornblende, hypersthene and magnetite are abundant in the 125-200mu fraction and may constitute as much as 20 percent of the weight of this fraction. Most lahars are predominantly composed of fragments of a pink to light violet hypersthene-hornblende dacite. The lahars probably originated on the flanks of explosive volcanoes in an ancient Cascade chain at least 60 km (40 miles) to the west. They seem to have moved partly as inertia type flows which would explain the inverse grading: pebbles and blocks moved upward during transport, leaving a layer of medium sand at the base.