THE OSCEOLA MUDFLOW FROM MOUNT-RAINIER - SEDIMENTOLOGY AND HAZARD IMPLICATIONS OF A HUGE CLAY-RICH DEBRIS FLOW

The 3.8 km(3) Osceola Mudflow began as a water-saturated avalanche dur ing phreatomagmatic eruptions at the summit of Mount Rainier about 560 0 years ago, It filled valleys of the White River system north and nor theast of Mount Rainier to depths of more than 100 m, flowed northward and westward more than 120 km, covered more than 200 km(2) of the Pug et Sound lowland, and extended into Puget Sound, The lahar had a veloc ity of approximate to 19 m/s and peak discharge of approximate to 2.5 x 10(6) m(3)/s, 40 to 50 km downstream, and was hydraulically dammed b ehind a constriction, It was coeval with the Paradise lahar, which flo wed down the south side of Mount Rainier, and was probably related to it genetically. Osceola Mudflow deposits comprise three facies, The ax ial facies forms normally graded deposits 1.5 to 25 m thick in lowland s and valley bottoms and thinner ungraded deposits in lowlands; the va lley-side facies forms ungraded deposits 0.3 to 2 m thick that drape v alley slopes; and the hummocky facies, interpreted before as a separat e (Greenwater) lahar, forms 2-10-m-thick deposits dotted with numerous hummocks up to 20 m high and 60 m in plan, Deposits show progressive downstream improvement in sorting, increase in sand and gravel, and de crease in clay, These downstream progressions are caused by incorporat ion (bulking) of better sorted gravel and sand, Normally graded axial deposits show similar trends from top to bottom because of bulking, Th e coarse grained basal deposits in valley bottoms are similar to depos its near inundation limits, Normal grading in deposits is best explain ed by incremental aggradation of a flow wave, coarser grained at its f ront than at its tail. The Osceola Mudflow transformed completely from debris avalanche to clay rich (cohesive) lahar within 2 km of its sou rce because of the presence within the preavalanche mass of large volu mes of pore water and abundant weak hydrothermally altered rock. A sur vey of cohesive lahars suggests that the amount of hydrothermally alte red rock in the preavalanche mass determines whether a debris avalanch e will transform into a cohesive debris flow or remain a largely unsat urated debris avalanche, The distinction among cohesive lahar, noncohe sive lahar, and debris avalanche is important in hazard assessment bec ause cohesive lahars spread much more widely than noncohesive lahars t hat travel similar distances, and travel farther and spread more widel y than debris avalanches of similar volume, The Osceola Mudflow is doc umented here as an example of a cohesive debris flow of huge size that can be used as a model for hazard analysis of similar flows.