Paradise Lahar

Blocks as large as 8 feet in diameter are embedded in the lahar, and even larger masses are scattered on its surface. Iron oxide occurs within the deposit as thin layers and lenses, and as thin coatings on most rock fragments. The lahar is 1-5 feet thick in most outcrops, although thicknesses of as much as 15 feet are not uncommon. Owing to its general thinness, the lahar does not have a constructional topography of its own in most places, but mantles ridges, knobs, and depressions formed of bedrock and glacial drift. At Reflection Lakes, however, the surface of the lahar is hummocky, and the lakes occupy shallow basins in the deposit. In outcrops along the floor of the Nisqually River valley the lahar is 2 to 8.5 feet thick, but it veneers the valley sides to a height of more than 100 feet near the mouth of the Kautz Creek valley.

Nearly everywhere that the base of the Paradise lahar is exposed along the Nisqually River valley it rests directly on pyroclastic layer O, and there is little or no evidence of erosion along the contact. A layer of forest duff several inches thick lies on top of the lahar at several places and underlies pyroclastic layer Y and a younger lahar.

In the Paradise Park area the lahar underlies the surface south of Panorama Point and from Mazama Ridge westward to the young lateral moraines of Nisqually Glacier. According to D.R. Mullineaux (oral commun., 1969), the lahar is represented on Mazama Ridge by a deposit a few inches thick that underlies pyroclastic layer D and other layers just south of Sluiskin Falls. The maximum height of the lahar deposit in that area is about 600 feet above the floor of Paradise Valley. Farther south, a large lobe of the lahar underlies the Reflection Lakes area.

Evidently the Paradise lahar entered the Reflection Lakes area through a saddle on Mazama Ridge that is about 200 feet higher than the floor of Paradise Valley. A thin remnant of the lahar that crops out along the highway at Reflection Lakes (measured section 2) indicates that the lahar initially filled that area to a level at least 20 feet higher than the present lake level. However, the surface of the lahar was subsequently lowered as the still-fluid material drained away to the west and rejoined the mass of the lahar in the Paradise River valley. The west end of the largest lake may be dammed by laharic material that flowed off Mazama Ridge.

High remnants of the Paradise lahar both upstream and downstream from Narada Falls indicate that a very large mass of material moved down the Paradise River valley. The most conspicuous remnant is exposed in a roadcut near Ricksecker Point. The lahar there is a few feet thick and directly overlies pyroclastic layer O at a height of about 800 feet above the floor of the paradise River valley. Because of the position of the lahar on a south-facing slope overlooking the valley, there seems to be no other way to explain its presence except by the temporary filling of the valley with the lahar to a height of at least 800 feet.

Farther down the valley the Paradise lahar forms part of assemblage B. Its vertical extent indicates that the valley was at least as deep when the lahar was moving as it is today and that the lahar was temporarily several hundred feet deep at Longmire. The lahar was identified as far downvalley as a point near the mouth of Tahoma Creek, and it may be one of several lahars that crop out in the valley near National, 12 miles west of Longmire.

Stones in the lahar are principally of Mount Rainier provenance. Identification of 100 pebbles in the deposit at Reflection Lakes and at Longmire showed 75 and 80 percent, respectively, to be of Mount Rainier origin. At Reflection Lakes the deposit contains only 5 percent granodiorite in the pebble-sized fraction, but his rock type makes up 20 percent of the pebbles at Longmire. The greater portion of granodiorite at Longmire may be due to incorporation of loose gravel from the Nisqually flood plain.

Three samples of the lahars from the Paradise Park area contained 1-5 percent clay, 7-28 percent silt, and 16-44 percent sand. X-ray examination of a sample from an outcrop near the visitor center at Paradise Park indicated that montmorillonite and kaolinite are the predominant clay-sized minerals. They are accompanied by small amounts of cristobalite, feldspar, iron oxide, and quartz. Amorphous silica constituted 6.4 percent of the clay-sized fraction. It is of interest to note that a sample of Evans Creek till from the same outcrop had mostly feldspar and cristobalite in the clay-sized fraction, and only a trace amount of clay mineral was found. Some samples of the lahar are among the most poorly sorted of those examined in this study. It is especially interesting to note the poor sorting of the lahar at Ricksecker Point. The deposit there must represent a zone in the upper 10-20 feet of a lahar that was momentarily at least 800 feet deep in the Paradise River valley during flowage.

Silt and clay made up about 19 percent of a sample of the Paradise lahar that was taken from an outcrop near Longmire. X-ray diffraction analysis of the clay-sized fraction of this sample showed a kaolinite-halloysite mixed-layer mineral, and cristobalite, feldspar and a small amount of iron oxide. Amorphous silica made up 8.3 percent of the clay fraction.

The distribution of the Paradise lahar in the area between Panorama Point and Ricksecker Point suggests that the lahar originated in one or more huge avalanches of moist rock debris that swept down the south flank of the volcano. The clay mineralogy of the deposit indicates that it was derived in part from hydrothermally altered rocks, perhaps at the summit of Mount Rainier. Although I once thought that the deposit at Paradise Park was of the same age as the Osceola Mudflow (Crandell, 1963a, p.B139), I now know that the Paradise lahar is somewhat older because it is stratigraphically below pyroclastic layer D, which is older than the Osceola (Mullineaux, oral commun., 1969). It seems likely that counterparts of the slides that resulted in the Greenwater lahar in the White River valley caused the Paradise lahar and were triggered in the same way.

Although the distribution of the lahar in the Paradise Park area indicates that the lahar originated in an avalanche down the south side of Mount Rainier, the area of transition from an avalanche to a lahar is not known. At Paradise Park, where the transition may have occurred, grain-size distribution in samples of the deposit indicate a rather wide variability in texture and a range in sorting coefficients of 7.75 - 13.6. median diameters range from 0.2 to 13 mm in these samples. Samples obtained from two locations further downvalley, where there is little doubt that movement was that of a lahar, have sorting coefficients of 15.75 and 16.25 and median diameters of 1.4 and 4.3 mm, respectively. One might expect that the deposit would be most poorly sorted where it was moving as a rapid avalanche and that it would show somewhat better sorting where it was moving as a lahar. However, the opposite seems to be true, if the samples described here are representative.

The original volume of the Paradise lahar is difficult to determine because the deposit is highly variable in thickness, and nowhere does it form a broad thick fill in the Nisqually River valley. Beyond the community of National, 6 miles west of the park, Evans Creek Drift directly underlies surfaces on both sides of the river; it the lahar reached this far downvalley, it must have been confined to a narrow channel, and its top could not have been appreciably higher than the present flood plain. The depth of the channel of the Nisqually River at that time is not known.

The volume of the lahar was originally estimated to be 400-500 million cubic yards on the basis of the heights to which it reached on valley sides, the assumption that the Paradise and Nisqually River valleys were simultaneously filled with the lahar upstream from Ricksecker Point, and the probability that the lahar was draining from one area while its crest was passing points further downvalley (Crandell, 1963a). Subsequent geologic mapping has shown that the total area covered by the lahar probably is no more than about 13 square miles, and its original average thickness in this area after it came to rest and lost much of its water content probably was between 5 and 10 feet. The estimated volume is either 67 million or 134 million cubic yards, according to whether the smaller or larger average thickness value is used in the computation.

The great depth of the Paradise lahar at some places, notably in the lower Paradise River valley, seems entirely out of proportion to such a relatively small volume, even if it is assumed that the lahar probably contained an additional volume of water of as much as one-third of the estimated volume of the deposit. The anamolous relations involved can be illustrated by considering the capacity of the lower Paradise River valley. Near Ricksecker Point, the valley ahs a cross-sectional area of about 130,000 square yards up to a height of about 800 feet above the valley floor. The 1-mile segment of the valley immediately east of Ricksecker Point has an estimated volume of at least 230 million cubic yards up to a similar height. Thus, it is apparent that even the total estimated volume of the lahar could not entirely fill this short valley segment at one time. The problem becomes even more acute when it is remembered that a considerable part of the estimated volume of the lahar remained in the area above the lower Paradise River valley and in the Nisqually River valley above Ricksecker Point. Moreover, to explain the great depth of the lahar in the Paradise River valley by any conventional means, it is necessary not only to have this valley segment full of mud, but simultaneously to have mud of comparable thickness in the Nisqually River valley at the mouth of the Paradise River.

On the basis of the assumption that the estimated volume of the lahar is not grossly in error, the most probable explanation of the contradictory relations just described is that the lahar moved across Paradise Park and down the Paradise River valley in a single massive transient wave with a height of as much as 800 feet. Such a wave must have been generated by a large avalanche of wet, clayey, rock debris from an area high on the south flank of the volcano, or perhaps at its summit. Because of momentum the avalanche-generated wave swept across Paradise Park, rather than moving directly down the Nisqually River valley. The valley swings from a southward course to a southwestward course just west of Panorama Point and Alta Vista, surged across Paradise Park, and slopped up onto Mazama Ridge just south of Sluiskin Falls. The bulk of the material, however, was deflected by Mazama Ridge to the southwest and down the Paradise River valley.

The transient wave probably lost much of its height when it debouched from the Paradise River valley at Ricksecker Point. Its momentum must have carried it high on the flank of Rampart Ridge along the west side of the Nisqually River valley. At Longmire, a little more than 1 mile downvalley, the lahar was at least 80 feet and probably several hundred feet deep. As the lahar wave progressed farther down the Nisqually River valley, it gradually lost height as the valley widened and as more and more volume was lost by material being left behind as a coating on the lower valley walls and floor.