Geologic Overview of the Clackamas Basin

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Geologic Overview of the Clackamas Basin

• Peter Wampler
• OSU

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Cascade Mountains
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High Cascades

• Young volcanic chain
• Composite volcanoes erupting ash and pumice
• Young basalt flows and cinder cones cover large
  areas
• Glacial deposits are present near peaks

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• Basalt flows may modify drainages.
• Alters hydrology and drainage patterns.

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Western Cascades

• Produced 6 times the volcanism of the high
  cascades
• Collision during formation was head on and at a
  steeper angle
• Comprised of deeply eroded and weathered volcanic
  rocks
• Prone to mass wasting
• Columbia River Basalts occur with the same
  geographic area but are more similar to High
  Cascades in terms of erosion potential

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Tectonic Effects on Grains Size
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The Willamette Valley Province

• Valley Sediments (filling the bowl)
• Recent Alluvium (lt1,000 years old)
• Holocene (lt10,000 years old) Terraces
• Pleistocene (gt10,000 years old)
• Sand River Mudstone
• Springwater Formation
• Missoula Flood Silts

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• Clackamas River is very dynamic
• Cut down from Estacada Surface approximately 140
  ft since Missoula Floods

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Estacada Terrace