Geodiversity Atlas—Greater Yellowstone Network Index

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Geology and Stratigraphy of the Greater Yellowstone Network Parks

The Greater Yellowstone Inventory and Monitoring Network (GRYN) consists of four national park units in northwestern Wyoming, southern Montana, and eastern Idaho. Bighorn Canyon National Recreation Area, Grand Teton and Yellowstone National Parks, and the John D. Rockefeller, Jr. Memorial Parkway are the core of the 7.3-million-hectare (18-million-acre) Greater Yellowstone Ecosystem. These four units include a broad range of climatic zones, habitat types, elevation profiles, and encompass great biological and geological diversity. Geographically the network contains the headwaters of the Bighorn, Shoshone, Snake, and Yellowstone Rivers. These watersheds are characterized by landscapes influenced by a diversity of geologic processes and support distinct plant and animal communities including a diversity of large mammals. Taken together, the unique geologic, hydrologic, and biological characteristics of this region are world-renowned.

map of yellowstone region
Map of Greater Yellowstone Network parks including: Bighorn Canyon National Recreation Area (BICA), Grand Teton National Park (GRTE), John D. Rockefeller, Jr. Memorial Parkway (JODR), and Yellowstone National Park (YELL).

NPS image.

A Brief Geologic History—Greater Yellowstone Network

A few examples of events and Network resources in each geologic time period are highlighted below, from youngest to oldest.

The geological record for the Cenozoic in the GRYN units becomes more discontinuous with the uplift of more mountainous terrain associated with the Laramide Orogeny. The Cenozoic is not represented by any formal stratigraphic units within BICA. However, Eocene (see Paleogene) volcanics are represented by widespread deposits in GRTE, JODR, and YELL associated with the nearby Absaroka volcanic field active between approximately 55 and 44 Ma. A thick sequence of extrusive flows, lahars, ash, and volcaniclastic strata are part of the Absaroka Volcanic Supergroup which includes a complex sequence of units including the Sepulcher and Lamar River formations present in the northern portion of YELL (Smedes and Prostka 1972).

Oligocene (see Paleogene) rocks are not exposed at the surface in any GRYN unit, but the Miocene is well-represented at GRTE, primarily by the Colter and Teewinot formations. The Colter Formation is composed of volcanic debris, indicating the presence of a volcanic source near the north end of Jackson Lake between about 20 and 13 Ma. The succeeding Teewinot Formation was deposited in a large lake. A local volcanic center, the Jackson Hole volcanic field, was active around 8 Ma, but much more extensive volcanism arrived near the end of the Miocene in the form of the Yellowstone hotspot.

The first evidence of the Yellowstone hotspot in the immediate vicinity of the GRYN begins near the end of the Miocene (see Neogene), with the development of the Heise volcanic field in eastern Idaho between 7 and 4 Ma. Evidence of these eruptions is either absent or buried at YELL, but small remnants of the 5.51 Ma Conant Creek Tuff can be found in GRTE and JODR. By about 2 Ma, the hotspot emerged beneath the Yellowstone Plateau. Three major eruptions resulted in the creation of three distinct calderas: the Huckleberry Ridge Caldera (Huckleberry Ridge Tuff) at about 2.05 Ma; the Henry’s Fork Caldera (Mesa Falls Tuff) at about 1.3 Ma; and the Yellowstone Caldera (Lava Creek Tuff) at about 0.64 Ma (640,000 years ago).

Quaternary glaciation, beginning approximately 2.6 Ma, has significantly shaped the landscape of the GRYN parks, especially GRTE, JODR, and YELL. The Jackson Lake basin displays dramatic examples of glacial features including glacial outwash deposits and terraces. Small remnant glaciers continue to be present at high elevations in GRTE. Today, the iconic geologic features and landscapes of the GRYN parks attract visitors, photographers, artists and scientists from around the world.
The beginning of the Mesozoic in northwestern Wyoming is marked by Lower Triassic rocks, exhibiting evidence of multiple transgression-regression cycles. The Upper Triassic rocks include a more terrestrial sequence known as the Chugwater Formation. In the BICA area, the Permian and Triassic rocks below the Chugwater Formation are a unit locally referred to as the Goose Egg Formation.

The Early Jurassic is poorly known in the parks of the GRYN, being represented only by a small exposure of the Nugget Sandstone in GRTE, the remnant of a large dune system. Marine deposition returned during the Middle and early Late Jurassic, leaving some of the most fossiliferous rocks of the network parks in the form of the Gypsum Spring and Sundance formations of southern BICA, GRTE, JODR, and southern YELL, and the roughly equivalent Ellis Group of northern BICA and northern YELL. A shallow sea known as the Sundance Sea arrived from the northwest and advanced and retreated several times. During periods of marine regression, dinosaurs and other vertebrates left tracks in the coastal regions and in shallow water, as seen at BICA. With the final retreat of the Sundance Sea in the early Late Jurassic, a sustained period of terrestrial deposition began, lasting about 50 million years into the Cretaceous. The latest Jurassic portion is represented by the Morrison Formation.

The Early Cretaceous is represented by the Cloverly Formation in southern BICA, GRTE, JODR, and southern YELL, and the Kootenai Formation in northern BICA and YELL. These formations were predominantly deposited by fluvial and lacustrine processes in a variety of channel, floodplain, and lake settings. A complex and controversial interval of rocks above the main bodies of the Cloverly and Kootenai formations represents the approach of the final shallow continental sea to submerge central North America. This sea, known as the Western Interior Seaway or Cretaceous Interior Seaway, was a major geographic feature of North America for much of the Cretaceous, at its greatest extent bisecting the landmass into eastern and western sections and connecting the Arctic Ocean to the Gulf of Mexico. Seaway deposition often resulted in thick, dark marine shales with occasional pulses of coarser sediment and thin beds of bentonite, the remains of volcanic ash layers. In the GRYN parks, near the western margin of the seaway, marine deposition existed for nearly 30 million years, between approximately 100 and 70 Ma (million years ago), although in GRTE, JODR, and YELL marine deposition was mostly over by about 85 Ma (Tweet et al. 2013).

Near the end of the Cretaceous, as the seaway was retreating, western North America was affected by a major mountain-building event known as the Laramide Orogeny. The Laramide Orogeny is thought to have resulted from a marine plate subducting at a shallow angle under western North America and interacting with the underside of the North American plate, causing large blocks of crust to rise. This process lasted into the Eocene (see Paleogene), resulting in the uplift of the present-day Rocky Mountains.
The beginning of the Paleozoic in the paleogeographic area represented today in part by the parks of the GRYN is marked by the transgression of the shallow Sauk Sea during the early Cambrian. The arrival of this ancient sea is represented by the widespread coastal unit known as the Flathead Sandstone. This sandstone unit is overlain by the Gros Ventre Formation and Gallatin Limestone, representing marginal marine and deeper marine deposition along the fluctuating shoreline. A diverse assemblage of Cambrian marine invertebrate fauna has been documented from these units exposed in each of the network parks. The marine depositional environments spanned into the late Cambrian when there was a transition to a period of erosion. Middle and upper Cambrian rocks are exposed in BICA, GRTE, and YELL.

The Ordovician, Silurian, and Devonian are not well represented in the GRYN parks, with two episodes of deposition found in BICA, GRTE, and YELL. The older rocks belong to the Bighorn Dolomite, another widespread shallow marine formation deposited during a period when the network parks were near the equator. Another extended episode of erosion and non-deposition prevented the preservation of any Silurian rocks. The rock record resumes in the Late Devonian with the Darby Formation and equivalent Jefferson and Three Forks formations, representing another interval of shallow marine deposition.

The Late Devonian formations within the GRYN are the beginning of over 300 million years of geologic history with few substantial gaps. The most complete sedimentary sequence occurs within GRTE, spanning from the Upper Devonian into the Eocene (see Paleogene), and it is likely that the only major gaps are in the upper Permian and probably the Middle Triassic. One of the most widespread units mapped in the GRYN parks is the Madison Group (formerly Madison Limestone), deposited during the Early and Middle Mississippian as part of a marine transgression across much of the northwestern United States. A brief depositional hiatus after the deposition of the Madison sediments resulted in an episode of cave and void formation. Much later this expanded into larger caves during the late Cenozoic, as seen at BICA.

The Amsden Formation is a primarily marine Mississippian–Pennsylvanian formation exhibiting time-transgressive deposition across Wyoming from west to east and is exposed in BICA, GRTE, and YELL. Overlying the Amsden Formation is the coastal terrestrial deposition of the Tensleep Sandstone of BICA, GRTE, and southern YELL, and the Quadrant Sandstone of northern YELL. Permian strata are represented by transgression-regression cycles consisting of interfingering of coastal and nearshore sedimentary deposits. There are three predominant formations during the Permian: a carbonate unit named the Park City Formation deposited in mudflat and shallow shelf settings; the Phosphoria Formation composed of phosphatic shales, carbonaceous rocks, and cherts, deposited in deeper marine basins; and the Shedhorn Sandstone, composed of eolian and beach sandstone.
The parks of the GRYN collectively preserve an extensive geologic history spanning from the Archean to the present (see geologic time scale). This history includes some of the oldest exposed rocks in the NPS, including Precambrian intrusive igneous and high-grade metamorphic rocks exposed on the flank of East Pryor Mountain in BICA, within the Teton Range in GRTE, and in the Beartooth and Gallatin Mountains in YELL (Tweet et al. 2013).

Type Sections—Greater Yellowstone Network

black and white report cover for type section inventory

The geologic history above is excerpted from a report titled, "National Park Service geologic type section inventory: Greater Yellowstone Inventory & Monitoring Network". Type sections are essential reference locations for the geoscientists who study geologic history and paleontology. A summary of the type sections in each park can be found at the links below.

The full Network report is available in digital format from https://irma.nps.gov/DataStore/Reference/Profile/2280034

Please cite this publication as:

Henderson, T., V. L. Santucci, T. Connors, and J. S. Tweet. 2020. National Park Service geologic type section inventory: Greater Yellowstone Inventory & Monitoring Network. Natural Resource Report NPS/GRYN/NRR—2020/2198. National Park Service, Fort Collins, Colorado.

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