Geology of The Grand Teton Area - Tertiary Uplift and Deposition

Tertiary Uplift and Deposition

The tectonic setting of western North America changed drastically as the Farallon Plate under the Pacific Ocean to the west was shallowly subducted below North American Plate. Called the Laramide orogeny, the compressive forces generated from this collision erased the Cretaceous Seaway, fused the Sierran Arc to the rest of North America and created the Rocky Mountains. This mountain-building event started in the Mesozoic 80 million years ago and lasted well into the first half of the Cenozoic era 30 million years ago.

Some 60 million years ago, these forces uplifted the low-lying coastal plain in the Teton region and created the north-south-trending thrust faults of the nearby Wyoming Overthrust Belt. Uplift intensified and climaxed a few million years later early in the Eocene epoch when large thrust and reverse faults created small mountain ranges separated by subsiding sedimentary basins. One of the reverse faults, the north-south trending 10 mile (16 km) long Buck Mountain Fault, elevated what is today the central part of the Teton Range.

By about 34 million years ago, these forces had uplifted a broad part of western Wyoming into a continuous high plateau. This region includes areas now occupied by the Teton Range, Gros Venture Range, Wind River Mountains and other mountain ranges to the south and east of the Tetons. A separate area of uplift called the Targhee Uplift formed north of park borders around this time.

Subsequent erosion of the Targhee Uplift was driven by steepened stream gradients. Gravel, quartzite cobbles, and sand from this erosion eventually became the 5,000 foot (1,500 m) thick Harebell Formation seen today as various conglomerates and sandstones in the northern and northeastern parts of the park. In the Paleocene epoch large amounts of clastic sediment derived from uplifted areas covered the Harebell Formation to become the Pinyon Conglomerate. The lower members of this formation consist of coal beds and claystone with conglomerate made of quarzite from the Targhee uplift above.

The subducting Farallon Plate was eventually completely consumed below the North American Plate, bringing an end to the Laramide orogeny. Hot and semi-plastic rock deep below western North America responded to the lack of compression beginning 30 million years ago by slowly rising; gradually pushing the overlying rock sideways both east and west. Blocks of the brittle upper crust responded by breaking along roughly parallel north-to-south trending normal faults that each have a subsiding basin on one side and a mountain range on the other. This stretching may have begun to tear apart the previously-mentioned high plateau in western Wyoming around this time, but evidence from ancient sediments indicates that the Teton Fault system developed much later (see below). An eastward-moving intensification of this process began 17 million years ago, creating the Basin and Range geologic province in Nevada and western Utah. Stretching of the crust in this region eventually exceeded 200 miles (320 km), doubling the distance between Reno, Nevada and Salt Lake City, Utah.

Waning of the Laramide orogeny coincided with volcanic eruptions from two parallel volcanic chains separated by a long valley in the Yellowstone-Absaroka area to the north. Huge volumes of volcanic material such as tuff and ash accumulated to great depth in the Grand Teton area, forming the Absaroka Volcanic Supergroup. Additional eruptions east of Jackson Hole deposited their own debris in the Oligocene and Miocene epochs.

Sediment collected in various lakes in the area from around 17 to 15 million years ago, becoming the Miocene-aged Colter Formation. Beginning around 13 million years ago (also in the Miocene), a 40-mile (64-km) long steeply east dipping normal fault system called the Teton Fault started to vertically move two adjacent blocks. One block, the Jackson Hole basin, moved down while the other block, containing the westward-tilting eastern part of the Teton Range, moved up; thus creating the youngest mountain range in the Rocky Mountains. Most of the downward movement occurred right next to the fault, resuling in a 15° tilt of the Colter Formation. No sediment was deposited on top of the tilted Colter Formation for up to three million years, resulting in an angular unconformity as the tilted Colter partially eroded away.

Around 10 million years ago, Jackson Hole's first large freshwater lake was impounded by east-west fault movement in what is today the southern end of the park. Geologists call this fault-scarp dammed body of shallow water Lake Teewinot and it persisted for around 5 million years. The resulting Teewinot Formation of lakebed sediments sits directly on the Colter and consists of limestones and claystones mixed with volcanic material and fossilized clams and snails. All told, sediments in the Tertiary period attained an aggregate thickness of around 6 miles (10 km), forming the most complete non-marine Tertiary geologic column in the United States. Most of these units within the park are, however, buried under younger deposits.

Eventually all the Mesozoic rock from the Teton Range was stripped away and the same formations in Jackson Hole were deeply buried. A prominent outcrop of the pink-colored Flathead Sandstone exits 6,000 feet (1,830 m) above the valley floor on the summit of Mount Moran. Drilling in Jackson Hole found the same formation 24,000 feet (7,300 m) below the valley's surface, indicating that the two blocks have been displaced 30,000 feet (9,100 m) from each other. Thus an average of one foot of movement occurred every 300 years (1 cm per year on average).