Geology of The Grand Teton Area - Precambrian Deposition, Metamorphism, and Intrusion

Precambrian Deposition, Metamorphism, and Intrusion

Perhaps 3,000 million years ago in Precambrian time, sand, limey ooze, silt and clay were deposited in a marine trough (accurate dating is not possible, due to subsequent partial recrystallization of the resulting rock). Interbeded between these layers were volcanic deposits, probably from an island arc. These sediments were later lithified into sandstones, limestones, and various shales. These rocks were 5 to 10 miles (8 to 16 km) below the surface when orogenies (mountain-building episodes) around 2,800 to 2,700 million years ago intensely folded and metamorphosed them, creating alternating light and dark banded gneiss and schist. Today these rocks dominate the Teton Range with good examples easily viewable in Death Canyon and other canyons in the Teton Range. The green to black serpentine created was used by Native Americans to make bowls.

Sometime around 2,500 million years ago, blobs of magma intruded into the older rock, forming plutons of granitic rock. Extensive exposures of this rock are found in the central part of the range. About 1,300 to 1,400 million years ago in Late Precambrian, 5 to 200 foot (1.5 to 60 m) thick black diabase dikes intruded as well, forming the prominent vertical dikes seen today on the faces of Mount Moran and Middle Teton (the dike on Mount Moran is 150 feet (46 m)). Some of the large dikes can be seen from the Jenny Lake and String Lake areas.

More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocks. The Precambrian rocks were uplifted during this gap in the geologic record known as an unconformity; exposed to erosion they were gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly subsided and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years.