Crustal Thinning and Rifting
A new rift opened that started to break apart the supercontinent Rodinia, which North America was then a part of. 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 now resides.
The first formation to be deposited in this setting was the Noonday Dolomite, which was formed from an algal mat-covered carbonate bank. Today it is up to 1,000 feet (300 m) thick and is a pale yellowish-gray cliff-former. The area subsided as the continental crust thinned and the new ocean widened; the carbonate bank soon became covered by thin beds of silt and layers of lime-rich 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.
Read more about this topic: Geology Of The Death Valley Area
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