Bruneau-Jarbidge caldera

The Bruneau-Jarbidge caldera (sometimes called a supervolcano) is located in present-day southwest Idaho. The volcano erupted during the Miocene, between ten and twelve million years ago, spreading a thick blanket of ash in the Bruneau-Jarbidge event and forming a caldera. Animals were suffocated and burned in pyroclastic flows where they stood, or where they collected at a water hole, such as at Ashfall Fossil Beds, located far downwind in northeastern Nebraska.

The existence of the event was discovered in 1971 by Prof. Mike Voorhees, a paleontologist at the University of Nebraska State Museum, at a lagerstätte near Royal, Nebraska, the Ashfall Fossil Beds; there, two hundred fossilized rhinoceros remained at a single site, together with the prehistoric skeletons of camels and lizards, horses and turtles, a death assemblage preserved in two meters of volcanic ash.

By its uniquely characteristic chemical "fingerprint" and the distinctive size and shape of its crystals and glass shards, the volcano stood out among dozens of prominent ashfall horizons laid down in the Cretaceous and Tertiary of central North America. The event responsible for this fall of volcanic ash was identified at Bruneau-Jarbidge, 1600 kilometers west in Idaho. Prevailing westerlies deposited distal ashfall over a vast area of the Great Plains.

References

* [http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml BBC: "Supervolcanoes"] Program transcript, 3 February 2000
* [http://www.geo.mtu.edu/~raman/papers/Nebraska.pdf W. I. Rose, C. M. Riley, and S. Dartevelle, "Sizes and Shapes of 10-Ma Distal Fall Pyroclasts in the Ogallala Group, Nebraska"] (pdf file) Includes bibliography.
*Izett, G. A. 1981. "Volcanic ash beds: recorders of Upper Cenozoic silicic pyroclastic volcanism in the western United States." "Journ. Geophysical Res." 86:10, 200–10, 222.