Geology of The Death Valley Area - Early Sedimentation - Pahrump Group

Pahrump Group

The Pahrump Group of formations were deposited from 1200 to 800 mya in the Amargosa aulacogen. This was after uplift-associated erosion removed whatever rocks covered the Proteozoic Complex. Pahrump is composed of, from oldest to youngest:

• Crystal Springs Formation,
• Beck Spring Dolomite,
• Kingston Peak Formation.

Outcrops of this group can be seen in a highly metamorphosed belt that extends from the Panamint Mountains to the eastern part of the Kingston Range, including an area near the Ashford Mill site.

Uplift eventually exposed the crystalline complex to erosion. Arkose conglomerate and mudstone of the lower Crystal Spring Formation were created from muddy debris derived from stream erosion of these uplands. A warm shallow sea spread over the area as the Amargosa aulacogen slowly subsided; thick sequences of lime-rich ooze with abundant colonies of algae called stromatolites were then laid down. Dolomite and limestone resulted, forming the middle part of the Crystal Spring Formation. The upper part was formed after silt and sand destroyed the algal mat, forming siltstone and sandstone. Laterally extensive diabase sills of molten rock later intruded above and below the carbonate rock layers; commercial grade talc was formed from thermal decay of carbonate rock at its contact with the lowest sill, which covers hundreds of square miles (many hundreds of km²). Today the formation is 3,000 feet (910 m) thick.

The Death Valley region once again rose above sea level, resulting in erosion. The Amargosa aulacogen then slowly sank beneath the seas; a sequence of carbonate banks that were topped by algal mats of stromatolites were laid on top of its eroded surface. Eventually these sediments and fossils became the Beck Spring Formation, which is 1,000 feet (300 m) thick.

Another round of uplift exposed the Beck Spring rocks and the underlying Crystal Spring to erosion; subsequent faster subsidence of the Amargosa aulacogen broke these formations into islands in later Proterozoic time. The resulting large sequence of thick conglomerate beds of pebbles and boulders in a sandy and muddy matrix that blanketed basins between higher areas is known as the Kingston Peak Formation. This formation is prominent near Wildrose, Harrisburg Flats, and Butte Valley and is 7,000 feet (2,100 m) thick.

Part of the Kingston Peak resembles glacial till by being poorly sorted and other parts have large boulder-sized dropstones resting in a fine-grained matrix of sandstone and siltstone. Similar deposits are found over North America during the same period, some 700 to 800 mya. Geologists therefore hypothesize that the world at that time was affected by a very severe glaciation, perhaps the most severe in geologic history (see Snowball Earth). The youngest rocks in the Pahrump Group are from basaltic lava flows.