Grand Teton National Park - Geology

Geology

Grand Teton National Park has some of the most ancient rocks found in any U.S. National Park. The oldest rocks dated so far are 2,680 ± 12 million years old, though even older rocks are believed to exist in the park. Formed during the Archean Eon (4 to 2.5 billion years ago), these metamorphic rocks include gneiss, schist and amphibolites. Metamorphic rocks are the most common types found in the northern and southern sections of the Teton Range. 2,545 million years ago, the metamorphic rocks were intruded by igneous granitic rocks, which are now visible in the central Tetons including Grand Teton and the nearby peaks. The light colored granites of the central Teton Range contrast with the darker metamorphic gneiss found on the flanks of Mount Moran to the north. Magma intrusions of diabase rocks 765 million years ago left dikes that can be seen on the east face of Mount Moran and Middle Teton. Granite and pegmatite intrusions also worked their way into fissures in the older gneiss. Precambrian rocks in Jackson Hole are buried deep under comparatively recent Tertiary volcanic and sedimentary deposits, as well as Pleistocene glacial deposits.

By the close of the Precambrian, the region was intermittently submerged under shallow seas, and for 500 million years various types of sedimentary rocks were formed. During the Paleozoic (542 to 251 million years ago) sandstone, shale, limestone and dolomite were deposited. Though most of these sedimentary rocks have since eroded away from the central Teton Range, they are still evident on the northern, southern and western flanks of the range. One notable exception is the sandstone Flathead Formation which continues to cap Mount Moran. Sedimentary layering of rocks in Alaska Basin, which is on the western border of Grand Teton National Park, chronicles a 120 million year period of sedimentary deposition. Fossils found in the sedimentary rocks in the park include algae, brachiopods and trilobites. Sedimentary deposition continued during the Mesozoic (250–65 million years ago) and the coal seams found in the sedimentary rock strata indicate the region was densely forested during that era. Numerous coal seams of 5 to 10 ft (1.5 to 3.0 m) in thickness are interspersed with siltstone, claystone and other sedimentary rocks. During the late Cretaceous, a volcanic arc west of the region deposited fine grained ash that later formed into bentonite, an important mineral resource.

From the end of the Mesozoic to present, the region went through a series of uplifts and erosional sequences. A mountain-building episode known as the Laramide orogeny started to uplift western North America 65 million years ago and eventually formed the ancestral Rocky Mountains. This cycle of uplift and erosion left behind one of the most complete non-marine Cenozoic rock sequences found in North America. Conglomerate rocks composed of quartzite and interspersed with mudstone and sandstones were deposited during erosion from a now vanished mountain range that existed to the northwest of the current Teton Range. These deposits also have trace quantities of gold and mercury. During the Eocene and Oligocene, volcanic eruptions from the ancestral Absaroka Range buried the region under various volcanic deposits. Sedimentary basins developed in the region due to drop faulting, creating an ancestral Jackson Hole and by the Pliocene (10 million years ago), an ancestral Jackson Lake known as Teewinot Lake. During the Quaternary, landslides, erosion and glacial activity deposited soils and rock debris throughout the Snake River valley of Jackson Hole and left behind terminal moraines which impound the current lakes. The most recent example of rapid alteration to the landscape occurred in 1925 just east of the park, when the Gros Ventre landslide was triggered by spring melt from a heavy snowpack as well as heavy rain.