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Absaroka Volcanic Province

Bradley Smith

Regional Field Geology of the Northern Rockies

Yellowstone Lake (Left edge) and part of Yellowstone National Park which is partially made up of the Absaroka volcanic province.

Thank you!

References

Thorofare Group

  • Named after its predominant feature, Thorofare creek, the Thorofare Group sits at the top of the Absaroka Volcanic province (stratagraphically speaking)
  • Is roughly 6000 feet thick in most areas
  • Consists of light-colored volcaniclastic Two Ocean Formation, the Tepee Trail Formation, and the well-bedded light-gray Wiggins Formation.
  • They make up the southern portion of the Absaroka Volcanic Province and about 1/3 of Yellowstone National Park.

View of northwest side of Mt. Washburn which lies in the northern part (Washburn Group) of the Absaroka Volcanic Province.

  • Smedes, Harry W., and Harold J. Prostka.ABSAROKA VOLCANIC SUPERGROUP (n.d.): n. pag. USGS. USGS. Web.
  • Johnson, Leslie C. Absaroka Volcanic Province. Michigan: Leslie C. Johnson, 2015. Print.
  • Spark, Renee R. et al. "Watch GSA TV." GSA2014 Home Page. GSA, n.d. Web. 30 July 2015.

Washburn Group

  • Named for the Washburn Range which makes up the northern most portion of the Absaroka Range is upwards of 3000 ft thick in some regions yet thinner in others.
  • It consists, in large part, of volcaniclastic strata which is composed of hornblende and pyroxene andesite
  • The group makes up the lower 1,500 feet of the northwester portion of Yellowstone National park and forms spectacular cliffs that overlook the Lamar River
  • Most of the rock was erupted from several contemporaneous active centers
  • Made up of the Sepulcher Formation, the Lamar River Formation, and the Cathedral Cliffs Formation.

Types of Volcanoes Found Here

Sunlight Group

  • Mostly weathered down andesitic and basaltic stratovolcanoes
  • Also the coalesced deposits of epiclastically reworked material (rock made on the earth's surface by collecting fragments of preexisting rock)

  • Named for a sequence of dark colored pyroxene andesite lava flows, volcaniclastic rocks and potassic basalts.
  • The Sunlight group sits atop the Washburn group throughout the northern part of the volcanic field
  • On the Sunlight's southern edge it lies on top of prevolcanic rocks
  • Made up of the Crascent Hill Basin, the Wapiti Formation, and the Trout Peak Trachyandesite

Basalt Columns (Gallatin Range)

Right photo: the parallel lines are known as "arrest lines". Arrest lines are created when the basalt column is forming, as the rock cracks down the joint several centimeters at a time and where the crack stops is where the line is made.

Back to the Eocene(53 MYA)

Lahar Breccia

  • Beginning of volcanic activity in the north-western region of the current United States. Volcanic activity did not stop until roughly 38 MYA
  • Spans 23,000 square kilometers
  • Make up a large chunk (roughly 1/3) of Yellowstone national park

Geologic Composition of the Area

  • Most of the rocks in the province are intermediate to mafic in composition
  • Interestingly the northern portion of the province contains many potassium poor rocks such as large basalt columns, lahar formed breccia, and andesite
  • Whereas the southern section is very rich in potassium holding rocks like the quaternary rhyolites closer to Yellowstone
  • The most common minerals that were produced in the area are hornblende, biotite, and pyroxene
  • The province is divided into 3 groups
  • Washburn
  • Gallatin Range
  • Sunlight
  • Beartooth Range
  • Thorofare
  • Southern Section and parts of Yellowstone

more lahar breccia

Lahar Breccia (Silken Skein Falls)

Basalt columns

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