REPORT:
Postglacial Lahars From Mount Rainier Volcano, Washington

Osceola Mudflow:

The Osceola Mudflow is by far the largest mudflow of postglacial age from Mount Rainier and is one of the largest known volcanic mudflows in the world. Many of the facts concerned with the distribution, age, and origin of the mudflow have been discussed previously (Crandell and Waldron, 1956; Crandell, 1963a,b).

The Osceola was first described by Willis (1898, p.143) as "a sheet of till that covers the plateau between the Green and White Rivers and extends southwest beyond White River about the head of Fennel Creek". He correctly recognized the Mount Rainier provenance of the deposit, but thought that the Osceola had been formed by a piedmont glacier that issued into the lowland from the Cascade Range. Willis noted that knobs of Vashon till deposited during the last major glaciation are topographically higher than the surface of the adjacent Osceola deposit, and he thought that this indicated that the Osceola was somewhat older than the Vashon till.

Investigation of the Osceola (Crandell and Waldron, 1956; Crandell, 1963b, Mullineaux, 1961, 1965a, b) has shown that the mudflow overlies the Vashon Drift and that the upper part of the drift contains a soil profile. This soil was formed after the Puget glacier lobe withdrew from the southeastern part of the Puget Sound lowland, about 14,000 years ago, and before the mudflow occurred. Four radiocarbon dates obtained from wood incorporated in the mudflow range from 4,700+/-250 (W-564) to 5,040+/-150 (University of Washington radiocarbon date NO.62). When corrected for variations in atmospheric Carbon-14 and a a Carbon-14 half life of 5,730 years, these ages range from 5,550 to 5,800 years; a "true" age of about 5,700 years is here arbitrarily assumed for the Osceola.

Distribution and Volume

The mudflow is preserved at many places on the ridgetops at the head of Inter Fork valley; the highest outcrop is at the top of Steamboat Prow at an altitude of 9,700 feet. On the ridgetop between Glacier Basin and the unnamed cirque directly to the north, several feet of the mudflow veneers a lava flow. Iron oxide, apparently derived from the mudflow, has impregnated the flow rock, forming a resistant iron-rich crust at its surface. This crust was recognized by Fiske, Hopson, and Waters (1963, p.85), but they thought that it had formed beneath deeply weathered volcanic ash, whereas the crust in fact lies beneath a remnant of the mudflow.

Downstream from Inter Fork, above White River campground, the Osceola blankets a lateral moraine on the north valley wall to a height about 500 feet above the present valley floor. Pits dug on the crest of the next higher lateral moraine, which is about 1,000 feet above the valley floor, revealed a clayey deposit in the stratigraphic position of the Osceola Mudflow. The deposit is pale yellow and contains small rock fragments; it may be part of pyroclastic layer F. The mudflow evidently did not reach this height on the valley wall.

According to information provided by H.W. Anderson of the U.S. Geological Survey (written commun., December 1964), the Osceola Mudflow was penetrated at a depth of about 50 feet during drilling for a water well at the Silver Creek Ranger Station, on the White River valley floor near the north boundary of the park. The well was still in the mudflow when drilling was stopped at a depth of about 200 feet. The mudflow veneers the sides of the valley to a height of about 125 feet a few miles downstream from the Silver Creek Ranger Station. At Federation Forest State Park, about 14 miles downstream from Silver Creek, a well was reported by Mr. Anderson to have entered the Osceola Mudflow at a depth of about 200 feet beneath the valley floor. In this area the top of the mudflow forms a terrace that is about 35 feet higher than the White River.

This information from drilling on the valley floor indicates that the valley was at least 200 feet deeper when the mudflow occurred than it is today. The mudflow must have temporarily filled the valley near Silver Creek Ranger Station to a depth of more than 300 feet during movement.

The Osceola Mudflow also coats valley sides and forms terraces in the West Fork valley. The highest outcrop of the mudflow in the West Fork drainage basin, other than the one on Steamboat Prow, is at an altitude of about 6,800 feet just west of Winthrop Glacier. The mudflow crops out in streambanks along the West Fork and Winthrop Creek at many places. On the east bank of the river at the mouth of Winthrop Creek, the base of the mudflow is below river level, and its eroded top is 25 feet above the river. Successively above the mudflow area a layer of organic matter that contains logs, a coarse gravel deposit, and another lahar.

At a point about 2.5 miles upstream from the mouth of West Fork valley the Osceola veneers the northwest valley wall to a height of about 320 feet above the valley floor; and at the mouth of the valley the mudflow extends below river level. If the pre-Osceola valley of West Fork was as deep as that of the White River, the Osceola may have temporarily reached a depth of at least 500 feet in the lower part of the West Fork valley.

Forty miles downvalley from Mount Rainier, just inside the Cascade mountain front, the White River passes through a bedrock gorge that is now blocked by Mud Mountain Dam. The gorge lies at the south edge of the White River valley, which is about 3 miles wide in this vicinity. A long, narrow, flat-topped ridge (Mud Mountain) several hundred feet high, which extends southward across the broad White River valley, is made up largely of unconsolidated deposits that range in age from early or middle Pleistocene to late Pleistocene. The Osceola Mudflow was temporarily at least 450 feet deep in this part of the valley and cascaded in a sheet nearly a hundred feet deep down the west slope of Mud Mountain to the floor of a glacial melt-water channel. The mudflow poured from the Mud Mountain area into the Puget Sound lowland along several different routes.

West of the mountain front the Osceola spread widely on a plain of Vashon till and melt-water deposits. The mudflow is only a few feet thick on some topographic scarps and till drumlins; elsewhere it is as much as 75 feet thick. A high degree of fluidity permitted the mudflow to flow long distances down narrow topographic depressions on the drift plain. As a result the Osceola finally came to rest in the lowland as a lobe as much as 8 miles wide and 20 miles long with very uneven or digitate margins.

The mudflow profoundly changed the lower course of the White River. Before the mudflow, the river turned southward where it debouched from the mountain front and followed the valley of South Prairie Creek to join the Puyallup River near Orting. When the mudflow rushed out onto the drift plain, one lobe of it extended down the old White River valley, but the greatest share flowed northwesterly across the plain, inundating all but the highest relief features. As the mudflow became more dilute and approached the consistency of a muddy stream, it became the White River; this river began to cut a new valley along the axis of the broad lobe, trending northwestward toward Auburn (Crandell, 1963b).

The mudflow crops out in banks of the Puyallup River and has been penetrated in wells drilled into the flood plain south of Sumner (Crandell, 1963b). The Osceola also has been identified at a depth of about 265 feet below sea level, beneath the floor of the Puyallup-Duwamish valley, 4 miles northwest of Auburn (Luzier, 1969, p.14). At the time of the mudflow the Puyallup-Duwamish valley was an arm of Puget Sound between Orting and Renton. This arm was filled first by the mudflow and later by deposits of the White River (Mullineaux, 1961, p.185; Crandell, 1963b, p.A68).

At least 30 blocks of reddish-brown breccia derived from Mount Rainier are scattered on the surface of the mudflow. The largest measures 30 by 40 feet and stands 20 feet above the mudflow surface; it is located about 4 miles west of Enumclaw. Nearly all these blocks are situated along topographic breaks in the mudflow surface that coincide with scarps in the buried drift plain. More than half the blocks in the Buckley-Enumclaw area lie along a northwest-facing scarp 1 mile west of Enumclaw that represents a side of a buried melt-water channel; the opposite side was formed by the glacier itself (Crandell, 1963b, p.A66, pl.2). Blocks are probably concentrated along this scarp because the mudflow thinned considerably while flowing over it, and the large blocks became grounded on the underlying surface much as icebergs become grounded in shallow water. The presence of lower ground immediately northwest of the scarp permitted the fine matrix of the mudflow to drain away and leave blocks standing above the surrounding surface. The concentration along this scarp, as well as a lack of blocks to the southeast, suggests that this was the first place in the lowland where the mudflow thinned sufficiently for the blocks to become grounded.

The volume of the Osceola Mudflow can be estimated on the basis of the known area of distribution of the flow, if several assumptions are made concerning its total inferred area and thickness: The mudflow originally covered a total land area of at least 100 square miles, in addition to a submerged area in the Puyallup River valley where the mudflow extended into a former arm of Puget Sound. The mudflow probably extended as far south in the Puyallup River valley as the confluence of the Puyallup and Carbon Rivers, as far west as the town of Puyallup, and northward in the Green River valley to the outskirts of Kent and covered a submerged area of at least 27 square miles. The mudflow has subsequently been removed from a considerable part of its former total area by stream erosion or has been buried by younger alluvium. The mudflow ranges in thickness from a few feet to at least 200 feet. It is a reasonable assumption that the Osceola had an original average thickness of 20 feet over the land areas it covered; if it did, the mudflow would have had a volume of about 2 billion cubic yards. If the Osceola had had a comparable average thickness in the submerged areas, there would have been an additional volume of about 660 million cubic yards, making a probable total volume of a little more than half a cubic mile.

Texture and Minerology

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Origin

The water component of the mudflow may have been as much as 800 million cubic yards during movement if it is assumed that the material required a water content of 33 percent in order to flow. The rock debris probably was derived from an active hydrothermal area on the volcano; if so, the clayey material could have had a high moisture content from condensation of steam. Moreover, if the avalanches contained steam as well as hot rock debris, snow could have been melted to provide added moisture as the masses moved down the flank of the volcano.

An adequate source of material was not known when the Osceola was first recognized as a mudflow. Crandell and Waldron (1956, p.360) suggested that the mudflow was initiated by a volcanic explosion that expelled masses of hydrothermally altered rock from the northeast flank of the volcano, but I (Crandell, 1963a) later suggested that the mudflow originated at the summit of the cone. The source area I postulated also accounted for a "missing summit" of the volcano that was first noted by Russell (1898). The present summit of Mount Rainier is at the top of a young lava cone constructed within a large depression about 1-and-1/4 miles across; high points on the rim of this depression are at Point Success, Gibraltar Rock, and along the ridge between Liberty Cap and Russell Cliff. These high points indicate that the summit of the volcano was removed above an altitude of about 14,000 feet. Both Russell (1898) and Matthes (1914) suggested that the top of the volcano had been blown off by a mightly volcanic explosion, but Coombs (1936) proposed that the former summit was removed piecemeal during a series of eruptions. Fiske, Hopson, and Waters (1963) concluded that the summit was lowered by subsidence into magma, by outward slumping and flowage of solfatarized rock in the central part of the volcano, or by headward glacial erosion into the weak core rock. I (Crandell, 1963a) proposed that the summit of the volcano slid off and formed very large rockslides and avalanches that descended the northeast side of the cone to form the Osceola Mudflow.

The discovery by D.R. Mullineaux (oral commun., 1969) of newly erupted material in an airlaid deposit formed at the same time as the Osceola Mudflow suggests that the rockslides and avalanches were caused by volcanism. The airlaid deposit is layer F, and its correlation with the mudflow is based on stratigraphic relations, on its resemblance to the matrix of the mudflow, and on the similar radiocarbon ages of the two deposits. Layer F does no occur on top of the Osceola Mudflow and thus cannot be younger, and organic material beneath layer F at Cowlitz Park has an age of 5,800 years (sample W-2053).

According to Mullineaux, layer F is made up of three units in the Yakima Park-Berkeley Park area. The basal unit, 1-3 inches thick, consists mostly of clay and fragments of dense rock, about half of which have been strongly altered. The clay is mostly montmorillonite. The middle unit, 1/4 to 1 inch thick, consists mostly of glass-encrusted mineral euhedra and pumice, and the top unit is 1-3 inches of clay that contains abundant altered and unaltered dense rock fragments as well as mineral euhedra and pumice. Mullineaux believes layer F to be primarily of pyroclastic origin. He concludes that the pumiceous ash and lapilli in layer F demonstrate and eruption of magma and visualizes that the following sequence of events occurred. A phreatic explosion threw dense rock fragments and clay onto areas northeast of the volcano; this explosion was closely followed by an explosive eruption of magma that formed the thin layer of pumiceous ash and lapilli. Still later, pumiceous material was erupted along with dense rock fragments and clay. The initial explosion probably caused the slides of altered rock that formed the Osceola Mudflow, but perhaps less likely, the rockslides could have been triggered by earthquakes accompanying the eruption.

There is little evidence concerning the behavior in detail of the Osceola Mudflow, and one can only speculate about its characteristics during flowage. The mudflow probably moved downvalley in a series of large surges, each initiated by an avalanche of moist, clayey rock debris. The mudflow's tremendous volume resulted in hydraulic damming and ponding at valley constrictions in the Cascade Range and caused inundation of a broad area of the lowland. In the absence of any topographic or stratigraphic evidence to the contrary, it is assumed that the Osceola's entire fluid mass came to rest before any part of it dried enough to solidify. The time involved in the formation, movement, and emplacement of the mudflow may have been several days, or as little as a few hours.