Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

MUDFLOW AT PARADISE PARK

A very large landslide of clayey altered rock avalanched down the south side of Mount Rainier between 5,800 and 6,600 years ago. Upon reaching the lower slopes of the volcano, the avalanche became a mudflow that spread over the Paradise area. Here the mudflow deposited a plastic yellowish-orange mixture of unsorted clay, sand, and rock fragments a few inches to more than 15 feet thick. Blocks in the deposit are as large as 6 feet in diameter. The presence of thin remnants on top of Mazama Ridge indicates that the mudflow must have been 600 feet deep in the adjacent Paradise River valley. Farther downvalley, at Ricksecker Point, the mudflow was temporarily at least 800 feet deep. The mudflow lies on top of pumice layer O (fig. 12).

PUMICE LAYER O from Mount Mazama (Crater Lake) forms a thin yellow band at the base of the yellowish-orange mudflow that blankets Paradise Park. This outcrop is at Ricksecker Point, and its height above the floor of the adjacent Paradise River valley indicates that the mudflow was temporarily at least 800 feet deep here. (Fig. 12)

OSCEOLA MUDFLOW

The Osceola Mudflow is the largest mudflow that has originated at Mount Rainier and is one of the largest volcanic mudflows in the world. It originally covered an area of more than 100 square miles in the Cascade Range and the Puget Sound lowland, and its volume probably was more than half a cubic mile. Radiocarbon determinations of logs in the mudflow indicate an age of about 5,700 years.

The mudflow is an unsorted mixture of plastic clay, sand, and rock debris that includes boulders as large as 15 feet in diameter (fig. 13) It is purplish gray where fresh and weathers to yellowish orange. Remnants of the mudflow high on the sides of the White River and West Fork valleys show that both valleys were temporarily filled with streams of mud more than 500 feet deep. After the crest of the mudflow passed a given point, most of the material drained away downvalley, leaving veneers a few feet to a few tens of feet thick on the valley walls. Much of the flowing mud finally came to rest in the Puget Sound lowland, 40—70 miles away.

The presence of the Osceola Mudflow on ridgetops above Glacier Basin and at the top of Steamboat Prow indicates that it originated even higher on the volcano. This distribution and a large clay content suggest that the mudflow was caused by huge slides of altered rock material from a former summit of the volcano. These slides probably were triggered by volcanic explosions, and they produced the large crater-like depression at the top of the volcano that contains the present summit lava cone.

Roadcuts expose the mudflow at the White River Ranger Station (fig. 4, locality 16), and one of the best outcrops is in the valley of Inter Fork about 1.5 miles west of White River campground (fig. 13; fig. 4, locality 17)

OSCEOLA MUDFLOW is exposed on the south bank of Inter Fork. Slope wash from the bouldery yellow mudflow has nearly hidden the underlying gray Evans Creek till, which can be seen at the lower left. (Fig. 13)

INTERBEDDED MUDFLOWS AND ALLUVIUM

Mudflows are interbedded with alluvium in terraces or benches along the sides of nearly every major valley in the park. Individual mudflows are from about l to 15 feet thick and typically are unsorted mixtures of sand, clay, and stones of many sizes. Individual mudflows lack bedding, although a succession of several mudflows, or an alternation of mudflows and alluvium, may give an outcrop a layered appearance. Most mudflows are purplish gray where unweathered, and yellowish orange or yellowish brown where weathered. Many of these mudflows resulted from floods caused by eruptions. The volcano has repeatedly erupted hot rock debris and lava flows which melted snow and ice. The resulting floods that cascaded down the flanks of the cone became mudflows by scouring loose alluvium and moraines on the valley floors. Other mudflows, however, probably originated during periods of unusually heavy rainfall, when parts of recent moraines became saturated and slid and flowed downvalley.

Alluvium that is interlayered with the mudflows was formed mainly by streams and rivers like those of today. It generally consists of sand and gravel in which some boulders are as large as 6 feet in maximum diameter, and it ranges in thickness from a few inches to at least 50 feet. Alluvium can be distinguished from mudflows mainly by the presence of bedding and by lenses and layers of well-sorted material.

Pumice layers that locally lie between deposits of alluvium and mudflows are commonly interbedded with rooted stumps, logs, and other organic matter formed on forest floors that have since been buried. As many as four successive buried forest layers can be seen in the steep banks of Kautz Creek near the Wonderland Trail (fig. 4, locality 18), and the north bank of Tahoma Creek shows a similar sequence about a mile upstream from Tahoma Creek campground (fig. 4, locality 19). Some pumice layers, whose age is known, permit age bracketing of interbedded deposits, and the presence or absence of other layers on top of the deposits permits subdivision of the mudflows and alluvium into map units of four ages. (See explanation for the geologic map.) Unit A includes deposits older than pumice layer O; unit B contains the deposits younger than layer O and older than layer Y; unit C represents the deposits younger than layer Y but older than layer W; and unit D includes the deposits younger than layer W. The surface deposit represented on the map commonly is underlain by mudflows and alluvium of older units.