Geological Survey Bulletin 1444 Geology and Thermal History of Mammoth Hot Springs

The oldest exposed rocks in the immediate vicinity of Mammoth Hot Springs are Mesozoic (Jurassic and Cretaceous) marine and nonmarine sedimentary rocks (Ruppel, 1972). (See geologic time scale in table 1.) Mount Everts (fig. 3) contains an excellent exposure of part of the Cretaceous sedimentary section, and the hill opposite the entrance to the Terrace Loop Road (pl. 1) is composed of Jurassic rocks. Sediments and fossils in these rocks reveal a history of widely fluctuating sea level that produced environments ranging from shallow oceans to swamps and river flood plains (Fraser and others, 1969).

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Mount Everts and Main Terrace (fig. 3). The Cretaceous sedimentary rocks of Mount Everts are capped in the right half of the figure by Pleistocene volcanic rocks erupted from the Yellowstone caldera. Main Terrace (with Jupiter Springs on the left, the blue pool of Main Spring in the center, and Canary Springs, marked by vapor plumes, on the right) is the light-colored travertine terrace in the middle of the figure. Foreground shows part of tree-studded Prospect Terrace.

Beginning in Late Cretaceous time, the area was subjected to intense compression that caused the existing rocks to be folded, faulted, and uplifted to form mountains (Keefer, 1971).³ This disturbance in the Earth's crust, known as the Laramide orogeny, lasted until early Eocene time. The remainder of the Eocene Epoch was distinguished by voluminous outpourings of lava called the Absaroka Volcanic Supergroup. Near Mammoth Hot Springs, the Absaroka crops out on Sepulcher Mountain (fig. 2). Bunsen Peak, also shown in figure 2, is a small intrusive rock body related to this period of volcanism.

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³Much of the geologic history of the Mammoth area given in this section is extracted from Keefer's paper, which provides an excellent account of the geologic history of Yellowstone National Park.

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The Oligocene and Miocene Epochs were a comparatively quiet time during which the landscape underwent very little change. However, the region was once again subjected to intense mountain-building forces in the Pliocene Epoch.

Large volumes of volcanic rocks flooded the Yellowstone region a second time during the Pleistocene Epoch. The yellowish cliffs of Golden Gate (fig. 2) and the top of Mount Everts (fig. 3) represent a very small part of the extrusive rocks erupted from the Yellowstone caldera (see index map of fig. 2) during this volcanic episode.

Three periods of glaciation, called pre-Bull Lake (180,000 to 300,000 years ago), Bull Lake (125,000 to 170,000 years ago), and Pinedale (10,000 to 50,000 years ago), also affected the Yellowstone region during the Pleistocene Epoch (Keefer, 1971; Pierce and others, 1976). The glacial deposits that form Capitol Hill and those shown elsewhere on plate 1 are of Pinedale age (Pierce, 1973).

The youngest geologic features found in the Mammoth area are landslides and travertine terraces deposited from hot-spring waters. The hot-spring activity, beginning in Late Pleistocene time and extending to the present, forms the subject matter for this report.