Crustal thinning and rifting
Late Precambrian Noonday Formation scoured in Mosaic Canyon by episodic flow. (USGS photo)
At the same time the Earth was apparently in a severe glaciation (see above), a rift started to open and a sea flooded the subsiding region. The rifting zone was part of a system of zones responsible for breaking apart the supercontinent Rodinia and creating the Pacific Ocean. One of the three arms of the local rifting zone, the Amargosa Rift, failed to split the continent. A shoreline similar to the present Atlantic Ocean margin of the United States (with coastal lowlands and a wide, shallow shelf but no volcanoes) lay to the east near where Las Vegas, Nevada, now resides.
The first formation to be deposited was the Noonday Dolomite. It was formed from an algal mat-covered carbonate bank. Today it is up to 1000 feet (300 m) thick and is a pale yellowish-gray cliff-former. The area subsided as the continental crust thinned and the Pacific widened; the carbonate bank soon became covered by thin beds of silt and layers of limy ooze. These sediments in time hardened to become the siltstone and limestone of the Ibex Formation. A good outcrop of both the Noonday and overlying Ibex formations can be seen just east of the Ashford Mill Site.
An angular unconformity truncates progressively older (lower) parts of the underlying Pahrump Group starting in the southern part of the area and moving north. At its northernmost extent, the unconformity in fact removed all of the Pahrump, and the Noonday rests directly on the Proterozoic Complex. An ancient period of erosion removed that part of the Pahrump due to its being higher (and thus more exposed) than the rest of the formation.
Late Precambrian Noonday Formation scoured in Mosaic Canyon by episodic flow. (USGS photo)
At the same time the Earth was apparently in a severe glaciation (see above), a rift started to open and a sea flooded the subsiding region. The rifting zone was part of a system of zones responsible for breaking apart the supercontinent Rodinia and creating the Pacific Ocean. One of the three arms of the local rifting zone, the Amargosa Rift, failed to split the continent. A shoreline similar to the present Atlantic Ocean margin of the United States (with coastal lowlands and a wide, shallow shelf but no volcanoes) lay to the east near where Las Vegas, Nevada, now resides.
The first formation to be deposited was the Noonday Dolomite. It was formed from an algal mat-covered carbonate bank. Today it is up to 1000 feet (300 m) thick and is a pale yellowish-gray cliff-former. The area subsided as the continental crust thinned and the Pacific widened; the carbonate bank soon became covered by thin beds of silt and layers of limy ooze. These sediments in time hardened to become the siltstone and limestone of the Ibex Formation. A good outcrop of both the Noonday and overlying Ibex formations can be seen just east of the Ashford Mill Site.
An angular unconformity truncates progressively older (lower) parts of the underlying Pahrump Group starting in the southern part of the area and moving north. At its northernmost extent, the unconformity in fact removed all of the Pahrump, and the Noonday rests directly on the Proterozoic Complex. An ancient period of erosion removed that part of the Pahrump due to its being higher (and thus more exposed) than the rest of the formation.