Geology of The Grand Teton Area - Mesozoic Deposition

Mesozoic Deposition

Mesozoic deposition changed from primarily marine to a mix of marine, transitional, and continental that varied over time as crustal conditions altered the region. By the close of this era, 10,000 to 15,000 feet (3,000 to 4,500 m) of sediment accumulated in 15 recognized formations. The most extensive non-marine formations were deposited in the Cretaceous period when the eastern part of the Cretaceous Seaway (a warm shallow sea that periodically divided North America in that period) covered the region. Their sediment came from rock eroded from a mountain chain east of the seaway interbeded with ash from volcanos west of the seaway in the Sierran Arc (a long volcanic island chain like the modern Andes Mountains but in island form). This ash eventually became bentonite, a clay which expands in water and thus causes landslides in the park.

Regional uplift in latest Cretaceous time caused the seaway to retreat and transformed the Grand Teton area into a low-lying coastal plain that was frequented by dinosaurs (a fossilized Triceratops was found east of the park near Togwotee Pass). Coalbeds were eventually created from the swamps and bogs left behind after the last stand of the seaway retreated. Coal outcrops can be found near abandoned mines in and outside of the eastern margin of the park. Outcrops of older Mesozoic-aged formations can be found north, east, and south of the park.