Geological Survey Professional Paper 729—B Volcanic Stratigraphy of the Quaternary Rhyolite Plateau in Yellowstone National Park

The Huckleberry Ridge Tuff and the rhyolitic and basaltic lavas associated temporally and spatially with it are discussed here as the first volcanic cycle of the rhyolite plateau. The record of this cycle is quite incomplete because much of the source area of the Huckleberry Ridge and associated rocks was disrupted by later deformation and is now covered by younger rocks. Little else than marginal parts of the first-cycle volcanic fields can be observed within Yellowstone National Park. However, parts of a large caldera related to eruption of the Huckleberry Ridge Tuff are preserved in the southern Island Park area and between Grassy Lake Reservoir and the Red Mountains (figs. 1, 2).

JUNCTION BUTTE BASALT

The basalts of the Yellowstone rhyolite plateau, although not nearly as voluminous as the rhyolites, occur widely around the plateau margins. None of them, however, crops out within the source areas of the major ash-flow tuffs. Thus it is somewhat difficult to assign individual basaltic units confidently to the different volcanic cycles in a genetic sense. One can reasonably say that basaltic volcanism was intermittently active around the margins of the plateau before, during, and after each of the major rhyolitic cycles. Nevertheless, it is possible to show that certain basaltic units are time and stratigraphic relatives of the major rhyolitic units, and it is in this sense that the Junction Butte Basalt is discussed with the first volcanic cycle.

The name Junction Butte Basalt is given here to basalts in northern Yellowstone National Park that conformably underlie the Huckleberry Ridge Tuff. The name comes from the prominent butte between the Yellowstone and Lamar Rivers, but the type locality is designated as the west side of the Grand Canyon of the Yellowstone opposite the mouth of Deep Creek (fig. 2). Outcrops of Junction Butte Basalt occur in the valley of the Yellowstone River and on adjacent uplands from near Deep Creek to Blacktail Deer Creek and Mount Everts. The basalts unconformably overlie the Absaroka Volcanic Super-group and Paleozoic and Precambrian rocks. The top of the Junction Butte commonly is an erosionally stripped surface, but locally the Huckleberry Ridge Tuff, the Mount Jackson Rhyolite, or the Lava Creek Tuff overlies it.

At its type locality the Junction Butte lies in a paleovalley eroded in Eocene volcanic rocks of the Absaroka Supergroup and is overlain unconformably by the Wapiti Lake flow of the Mount Jackson Rhyolite and by the Lava Creek Tuff. The basalt section at the type locality is about 90 m thick and consists of three or four thick flows. A few kilometers downstream, below the mouth of Tower Creek, only a single flow of the Junction Butte Basalt is exposed. This flow, the Overhanging Cliff flow, is about 30 m thick where it forms the spectacular roadside cliff just north of Tower Creek. In this cliff and in other good exposures along the canyon rims downstream, the flow is characterized by well-developed two-tiered columnar jointing. The lower colonnade is commonly about 2-1/2—3 m thick, and small blocks from the upper zone, or entablature, of irregularly intersecting prismatic joints generally form conspicuous black talus cones beneath cliff exposures. The basalt is black and aphanitic in hand specimen and contains only sparse plagioclase phenocrysts. In the valley of Tower Creek the Overhanging Cliff flow and a lithologically similar overlying flow lie topographically below the Huckleberry Ridge Tuff. Although the contact is not well exposed, parallelism of the respective outcrop bands along 2 km of a narrow canyon suggests that the basalts are concordant beneath the ash-flow tuff.

A few meters of gravel and associated sand and silt commonly lies at the base of the Junction Butte Basalt. Inasmuch as earlier workers had difficulty in determining age relations between certain outcrops of the Overhanging Cliff flow and the Yellowstone Tuff, we carefully searched the underlying gravels for clasts from ash-flow tuffs of the Yellowstone Group. We have failed to find any, although a few cobbles of welded tuff from the underlying Eocene Absaroka Volcanic Supergroup do occur. Howard (1937, p. 36-80) similarly noted the presence of less than 1 percent rhyolites in the same gravels. These gravels contrast with the younger sediments of The Narrows which contain moderately abundant (commonly 10-15 percent) welded tuff cobbles of the Yellowstone Group—although, for reasons not clear to us, Brown (1961, p. 1182-1183) stated that he could find no rhyolite clasts in either of the two gravels we distinguish in the Tower Creek area. These observations support the conclusion that the Overhanging Cliff flow is older than the Huckleberry Ridge Tuff.

Basalt which we regard as Junction Butte crops out conformably beneath the Huckleberry Ridge and above Cretaceous shales and sandstones on the southeast side of Mount Everts, on the steep slopes extending about 3 km west from just northwest of Blacktail Pond. The basalt ranges in thickness from a few meters to about 100 m. Two flows are present in the sequence, and the outcrops closest to Blacktail Pond form a buried mound of scoria and cinders, probably representing the source vent for the flows.

The Junction Butte Basalt has not been dated isotopically, but it probably is only slightly older than the 2-m.y.-old Huckleberry Ridge. Flows in the Tower Creek—Grand Canyon area all have stable reverse remanent magnetic polarities and, therefore, probably are less than about 2.4 m.y. old (Cox, 1969, fig. 4). The two flows on Mount Everts are normally polarized and may be older or younger, but preservation of a cinder cone conformably beneath the Huckleberry Ridge suggests that the basalt is barely older than the tuff.

RHYOLITE OF BROAD CREEK

The rhyolite of Broad Creek underlies the Huckleberry Ridge Tuff and crops out only in the area east of the Grand Canyon of the Yellowstone on the east side of Broad Creek between about 1/2 and 2-1/2 km south of the Wapiti Lake Trail (fig. 2). A single flow about 60 m thick forms the unit as exposed. The flow forms the base of the exposure at this locality. The rhyolite contains about 30 percent quartz, sanidine, and plagioclase phenocrysts, and it has the steeply dipping flow layering typical of the upper portions of rhyolitic lava flows.

This early rhyolite of the first cycle has not been dated isotopically but appears to be conformable beneath the overlying welded tuff. We presume that the flow represents some of the earliest rhyolitic activity of the Yellowstone rhyolite plateau but that it is not much older than the 2-m.y.-old Huckleberry Ridge.

LEWIS CANYON RHYOLITE

The Lewis Canyon Rhyolite is named here for the excellent exposures in Lewis Canyon between Lewis Lake and the Snake River. The formation consists of several bulbous, nonstratiform rhyolitic lava flows, but we have recognized no more than one in any single section. The north side of Glade Creek, between the Snake River and the east end of Grassy Lake Reservoir, is designated the type area (fig. 1). In this area, the Lewis Canyon Rhyolite overlies the Huckleberry Ridge Tuff and Cretaceous sedimentary rocks and is overlain by the Lava Creek Tuff. The contacts in this area are not well exposed, but the relations are clear from mapping. The rhyolite is well exposed on the north side of Glade Creek along the meadows about 3 km southeast of Grassy Lake Reservoir. The flow is more than 200 m thick in the type area.

Exposures of the formation are excellent in Lewis Canyon, but the base is not exposed there. As in the Glade Creek area, the Lewis Canyon is overlain by the Lava Creek Tuff. The base of the Lewis Canyon Rhyolite is again poorly exposed above Paleozoic and Mesozoic sedimentary rocks about 7 km east of Lewis Canyon, at the east end of nearly continuous exposures along the north side of the Snake River between the Lewis River and Red Creek.

The rhyolite has 30-40 percent conspicuous phenocrysts, including abundant much-embayed sodic oligoclase, generally about 5 millimeters long, and less abundant, slightly smaller phenocrysts of quartz, sanidine, clinopyroxene, and opaque oxides. Despite the abundant plagioclase and relatively sparse sanidine, chemical analyses show the rocks to be rhyolites.

Boyd (1961, p. 392) regarded the Lewis Canyon Rhyolite as part of his Jackson flows, but we separate the Lewis Canyon from the Mount Jackson Rhyolite (as we call the remainder of his Jackson flows) on the basis of their different lithology, particularly the phenocryst assemblages, and on what appears to us to be a significantly different relation to the Huckleberry Ridge and Lava Creek Tuffs, despite the occurrence of both rhyolites between the two. The Mount Jackson Rhyolite is conformable beneath the Lava Creek and was erupted from vents around what was to become the third-cycle, Yellowstone caldera. By contrast, the Lewis Canyon Rhyolite is overlain on an erosional unconformity by the Lava Creek and appears to be closer in age to the first-cycle Huckleberry Ridge Tuff, the Lewis Canyon filling a segment of the first-cycle caldera. The Lewis Canyon is petrographically similar to early postcaldera rhyolite lavas (the Upper Basin Member of the Plateau Rhyolite) similarly situated in the Yellowstone caldera. This suggests parallel roles in evolution of the first and third volcanic cycles for these similar rhyolites.

The age of the Lewis Canyon Rhyolite has not yet been determined isotopically.

SEDIMENTS AND BASALTS OF THE NARROWS

Gravels and basalts near The Narrows of the Grand Canyon of the Yellowstone, below Tower Creek (fig. 2), were included by Pierce, Christiansen, and Richmond (1970) in the Osprey Formation. K. L. Pierce and we have found in subsequent work that the sediments and basalts in the vicinity of The Narrows formerly included in the Osprey postdate the Huckleberry Ridge Tuff but predate the Lava Creek Tuff. The correlation with the Osprey, therefore, was in error. We will use informal terminology for this unit until the pertinent geologic relations can be discussed more fully in a later paper. For the present we note only that four basaltic lava flows, most of them very thick and with conspicuous two-tiered columnar jointing, are interlayered with the sediments of The Narrows. The four flows all contain plagioclase phenocrysts, and the upper flow is particularly coarse grained.

The sediments and basalts of The Narrows lie in a paleovalley cut through the Huckleberry Ridge Tuff and the underlying Junction Butte Basalt, and the gravels contain abundant clasts of the Huckleberry Ridge. The Lava Creek Tuff has not been found directly overlying these sediments and basalt but is judged to be younger on the basis of the reverse paleomagnetic polarities of all four basalt flows and on the basis of a K-Ar age of 1.6 m.y. for sanidine from an ash bed interlayered in the unit.