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

Most previous workers considered the rhyolites and basalts of the Yellowstone plateau to be of Pliocene age. Richmond and Hamilton (1960) and Hamilton (1960) first called attention to relations which suggested a late Quaternary age for some of the youngest flows. Stratigraphic relations that we have worked out, together with extensive and continuing potassium-argon dating by J. D. Obradovich, show that virtually all of the plateau volcanism was younger than about 2 m.y. (million years) and that some was as young as about 70,000 years. We consider all of this time span to be within the Quaternary, and this seems to accord with the present thinking of most stratigraphers.

The volcanic rocks of the Quaternary rhyolite plateau are virtually all either rhyolites or basalts, and the rhyolites are greatly predominant. The rare rocks of intermediate composition are mixed lavas with rhyolitic and basaltic components. Areally, the plateau consists of two parts, as demonstrated by Boyd (1961)—an outer ring that consists largely of rhyolitic welded tuff but also lesser amounts of rhyolitic and basaltic lavas, and an inner area covered mainly by rhyolitic lava flows, some of them very large (fig. 2). The inner lavas partly filled an enormous elliptical caldera, 70 by 45 km (kilometers) across, and spilled over its margins in places, especially on its southwest side.

Although rimmed by mountains on its other sides, the Yellowstone rhyolite plateau along its southwestern margin overlooks the eastern Snake River Plain, a topographically lower area also covered largely by Quaternary volcanic rocks, principally basalts but including subordinate rhyolites. The transitional Island Park region lies between the plateau and the plain and was shown by Hamilton (1965) to be a large caldera. Our study extends to include the Island Park area, and although we do not discuss it here at length, its history is intimately related to that of the Yellowstone rhyolite plateau and is partly contemporaneous.

We have found that the ash-flow tuffs which largely constitute the outer part of the Yellowstone rhyolite plateau and its outliers comprise three major ash-flow sheets, each principally a single cooling unit separated from the others by major unconformities. These three ash-flow sheets form a basis for dividing the entire volcanic section into three parts, each of which represents a volcanic cycle in the evolution of its respective source area. The plan of the paper, therefore, is to describe first the three major ash-flow sheets and then to discuss the volcanic stratigraphy of the rhyolite plateau in terms of these cycles. Table 1 summarizes the stratigraphic nomenclature outlined here. Figure 3 shows some of the stratigraphic relations between the units outlined in table 1. Because the second cycle was completely confined to the Island Park region, it will not be discussed at length.

TABLE 1. — Summary of stratigraphic nomenclature

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