Geologic History of the Mojave

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Geologic History of the Mojave
Paul Kepley
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Scope and Scale of Geologic History

• Geologic time nearly incomprehensibly long
• Not a complete account
• Pieced together from present evidence

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Precambrian

• No known features date earlier than the
  Precambrian Age
• Oldest known rocks date between 1.6 Bya and 1.8
  Bya

Schist, and Gneiss with Granitic intrusion.
(These metamorphic rocks are some of the oldest
rocks in the region, and in the state in general.)
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Precambrian

• 1Bya, shallow seas entered region
• -Caused sedimentary deposits on continental
  shelf
• -Event recorded as the Great Unconformity

The Great Unconformity seen in the Mojave
records the same event shown in this image of the
Grand Canyon.

• Seas retreated, leaving behind sandstone,
  limestone, dolomite and shale.
• Many rocks subsequently metamorphosed

Precambrian Dolomite (Death Valley)
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Paleozoic

• Sea levels changed, advancing and retreating over
  Mojave
• Fossil remains include stromatolites and
  invertebrates (such as trilobites).
• Rocks in Death Valley preserve carbonate reefs,
  including corals, brachiopods, etc.
• Many of the rocks formed in this age were
  destroyed by later processes.

Cambrian Trilobite (Olenellus gilberti ) -
Waucoba Spring
Algae Nodules Girvanella, Echo Canyon
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Mesozoic

• Triassic -gt Cretaceous
• Volcanic Arc developed all along Western North
  America (due to subduction).
• (Cordillera Range)
• Arc developed into Sierra Nevada Batholith
• Produced plutonic and volcanic features across
  Greater Mojave Region
• Granitic rocks of this age abundant across Mojave
  and Southern California.

Stegosaurus? Probably not here But granite is!
Granite! Lots of granite!
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Paleocene ? Oligocene

• Subduction continued to occur
• Volcanic activity stopped
• Moved further east into Rockies

Uplift occurs in the manner depicted above.

• Region underwent uplift
• Resulted in erosion

Rocks tilted due to uplift (from Hewlett program
photos)
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Miocene

• Subduction zone along west North American Plate
  slowly switches to transform faulting
• Resulting from tectonic uplift, volcanism, and
  crustal extension, the Basin and range province
  forms
• Forms interior draining basin

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Pleistocene

• Lakes begin forming due to climate cycles
• Jet stream cut off, leads to ice age,
• Sea level rises and falls with cycles of
  glaciation
• -streams incise into valley during low sea
  level
• -valley floods during high sea level leading
  to deposition and filling
• Leads to modern river systems
• More volcanism occurs

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Hole in the Wall