IN THIS section, our concern is with the deposits of dacite pumice erupted during the closing stages of the andesitic period in Mount Mazama's history and before the extrusion of the dacite flows from the Northern Arc of Vents; in other words, before the flows of Llao Rock, Cleetwood, Redcloud Cliff, and Cloudcap.

On the south wall of the caldera, the dacite pumice in question is restricted to a single small lens, approximately a third of the distance down the wall, 1/4 mile north of Discovery Point. On account of its pink color, it is plainly recognizable even from a distance, and can be traced laterally for several hundred yards. The maximum thickness is close to 30 feet. Locally the pumice rests on andesitic lava, but generally it is underlain by coarse, bouldery till. Above it lie two thick flows of andesite separated by glacial debris. Capping the highest flow are two other layers of dacite pumice separated by till; these, however, belong to the post-andesitic activity of Mount Mazama.

Between Discovery Point and Redcloud Cliff, inter-andesitic pumice seems to be absent. The thin lenses under the moraines in Sun and Kerr notches belong to a later time, and the topmost pumice on the caldera rim is a product of the culminating explosions of the volcano. Below the dacite flows of Redcloud and Cloudcap, however, dacite pumice and welded tuff are exposed to a thickness of approximately 200 feet (plate 10). Their pale cream, buff, and pink colors contrast strikingly with the dark lavas above and below. The eye is also attracted by them on account of the fantastic forms into which they have been eroded, especially at the Cottage Rock.

The section at Cottage Rock (see figure 7) is as follows, the series beginning at the top:¹

a. The Cloudcap dacite flow.

b. Welded dacite tuff, similar to that of the Wineglass (page 60); 20 feet. This deposit is rich in angular blocks of andesite, and upward merges imperceptibly into the Cloudcap lava.

c. Pink and brown lump pumice; 3 feet.

d. Block layer consisting chiefly of lithic fragments of andesite in a matrix of dacite pumice; 4 feet.

e. Compacted, but not welded, pink and brown lump pumice crowded with fragments of andesite; 3 feet.

f. Well bedded, smoke-gray lump pumice with lithic blocks up to 2 feet across forming a fifth of the whole; 70 feet.

g. Uncompacted red, pink, and brown lump pumice with fewer lithic blocks; 50 feet.

h. Welded and streaked, red and black dacite tuff (ignimbrite), similar to the Wineglass tuff; 6 feet. This forms the conspicuous dark band at the base of Cottage Rock itself.

i. Unwelded, but compact, pink and brown lump pumice; 30 feet.

i. Strongly welded red, brown, and black tuff; 3 feet.

k. Buff and white, incoherent lump pumice with abundant lithic fragments; 10 feet.

l. Poorly exposed and doubtful glacial till; 10 feet.

m. Many flows of andesite, the four lowest of which are separated by layers of till.

Between Cottage Rock and the great V-shaped cliff of Redcloud, the deposits of dacite pumice are divided by a wedge of andesitic lava, as they are in the sections at Pumice Point, Cleetwood Cove, and Llao Rock. The presumption is, therefore, that all these pumice deposits belong to the same general period of transition in Mazama's history, before the eruption of intermediate magma finally gave way to eruption of dacite flows and basaltic scoria.

Many interesting problems are raised by the pumice and tuff deposits of the Cottage Rock section. Eruptions adequate to form 200 feet of ejecta on the caldera walls must of course have laid down a thick sheet over most of the volcano. Yet no corresponding deposits can be identified with certainty on the outer slopes. Possibly the coarse ejecta in Pumice Flat, near the base of the Union Peak volcano, are of the same age. Certainly they are not products of the final eruptions of Mount Mazama, for the winds at that time were blowing in the opposite direction.

The uncompacted and well stratified lump-pumice deposits of the Cottage Rock section presumably settled from the air in showers. On the other hand, the welded tuff which alternates with them must have been erupted in quite a different fashion. Had these ejecta been thrown high above the vents, they would have lost much of their heat and gas before reaching the ground, and therefore would not have suffered the intense compaction which now characterizes them. So firmly are the constituent particles welded together, and so finely laminated is the tuff, that only microscopic examination permits one to say with certainty that the deposits are not streaky flows of dacite obsidian. In order to produce compaction of this character, the ejecta must have remained hot and gas-rich for a considerable time. Fortunately, detailed study of similar deposits from other regions supplies an answer to the problem. These welded tuffs are products of glowing avalanches (nuées ardentes). Even so, it may be asked, how could they have retained their heat and gas long enough to cause welding when two of the layers are only 3 and 6 feet thick, respectively? The answer is, not that they were formerly much thicker, but that they were immediately buried by coarser lump pumice falling from the air. While some of the ejecta rushed down the slopes of the volcano in the form of glowing clouds, the bulk was shot high above the craters and probably fell onto the avalanches soon after they came to rest. The topmost welded tuff, though much thicker, also seems to owe its compaction to rapid burial, for it appears to have been covered immediately by a thick flow of lava.

Plate 26. Panorama of the caldera wall: Cleetwood Cove

No inter-andesitic dacite pumice occurs on the caldera walls between Redcloud Cliff and Cleetwood Cove. Beneath the Cleetwood dacite, however, pumice is again well developed. In the conspicuous white slide on the west wall of Cleetwood Cove (plate 26), the measurable thickness approximates 150 feet, and if the deposits continue beneath the talus to the edge of the lake, their total thickness must approximate 250 feet. The topmost 60 feet, immediately beneath the Cleetwood lava, consists of well bedded pumice charged with blocks of andesite up to 3 feet in diameter. By contrast, few of the pumice lumps measure more than 3 inches across, and the majority are not even 1/2 inch in diameter. Beneath this block-rich layer lies a thick accumulation of equally well stratified but finer pumice in which the content of andesite fragments is much less. A thin wedge of andesitic lava partly separates the two types of pumice, and for a depth of 6 feet the older pumice has been reddened thereby. Three other tongues of andesite are interbedded with the pumice near the base of the slide. The record of events here is therefore as follows: first, alternating eruptions of andesitic lava and fine dacite pumice; then increasingly violent explosions of pumice during which great quantities of andesitic debris were torn from the walls of the vents; and finally, the extrusion of the Cleetwood dacite flow. Had the pumice been blown from the vent which erupted the Cleetwood lava, the dips of the bedded pumice would radiate from the center of Cleetwood Cove. Actually, they conform with the dips of the interbedded andesites and suggest a common source near the summit of Mount Mazama.

On the opposite wall of Cleetwood Cove, interandesitic pumice is poorly exposed, outcropping just below the Cleetwood dacite to a thickness of only 35 feet (plate 26). Judging by the abundance of pumice in the talus below, however, there may be a thick series of similar ejecta reaching to the edge of the lake.

The next important exposures of inter-andesitic pumice may be seen in the upper part of Pumice Point, where they form a bold, white face (plate 11, figure 1). The lower half of the point consists of andesitic flows and breccias. Resting on the glaciated crust of the topmost lava is a thin and irregular lens of bouldery till and glacial sand. Above this is 47 feet of coarse lump pumice mixed with large blocks of andesite. After these ejecta had been laid down, they were partly buried by a thick flow of andesite. Both the lava and the pumice were then overridden by ice and a second layer of glacial till was deposited. The ice subsequently retreated, and a dark soil bearing plentiful remains of decayed vegetation accumulated on the moraines. Renewed explosions of dacite pumice then took place, probably from vents at or near the summit of Mount Mazama. These continued until between 130 and 170 feet of ejecta buried the old soil (figure 8). There must have been many explosions of varying intensity, for the deposits are well bedded and differ markedly in coarseness. In some layers the pumice bombs rarely exceed a few inches in diameter; in others they measure a yard across and have pink crusts produced by oxidation of escaping gases. In the finer layers, the fragments of foreign andesite are also small; in the coarser layers, some blocks reach a diameter of 2 feet and they may make up more than a tenth of the whole. Perhaps the rate of deposition and the temperature of the ejecta also varied from time to time, for whereas most of the pumice is white or pale buff and only loosely compacted, certain bands are pink and sufficiently compacted to form small cliffs, as if the ejecta had been hotter, richer in gas, and more rapidly erupted.

Inter-andesitic dacite pumice is again exposed in the upper part of the walls overlooking Steel Bay, where it alternates with lenses of till and flows of andesite (plate 28). Because most of it is concealed by talus and much is difficult of access, the relations have not been determined with the desirable accuracy.

The alternation of andesitic lava with dacite pumice is nowhere more graphically shown than on the caldera walls beneath the east "wing" of Llao Rock (plate 28). Three or four slaggy, red-crusted wedges of andesite may be seen here within a mass of stratified pumice carrying abundant and large blocks of andesite. The maximum thickness of the pumice approximates 300 feet. Above this deposit, and separating it from the dacite flow of Llao Rock, is another stratum of pumice, between 20 and 50 feet thick.

Obviously, a long interval of time elapsed between the eruptions of the two deposits of pumice, for a deep glacial valley was carved across the inter-andesitic pumice before the younger pumice was laid down on the valley side. Probably the older pumice was blown from the central, summit vents of Mazama; the younger pumice, on the other hand, may represent the first products of the vent from which the lava of Llao Rock was extruded.

On the west wall of the glacial valley, the succession is strikingly different from that just described. Instead of a thick series of bedded pumice deposits with lenticular flows of andesite, there are many flows of andesite, a few short and thin lenses of pumice, and at least two layers of bouldery till, the topmost of which forms the floor over which the Llao dacite spread. Both of the observed tills contain much pumiceous debris. Possibly this reworked pumice and the thin lenses of pure pumice associated with it correspond to the thick sheet of pumice on the opposite side of the glacial valley. There is, however, no pumice corresponding to that molded on the east side of the valley. Why the successions on the two sides of the valley are so different is not apparent.

This concludes the account of the inter-andesitic dacite pumice on the caldera wall. Following this period of activity, a semicircular arc of vents opened on the north flank of Mount Mazama, possibly in response to a northward enlargement of the underlying magma chamber.

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