DISRUPTION OF A MID-MIOCENE ECOSYSTEM ASSOCIATED WITH INCIPIENT “YELLOWSTONE HOTSPOT” VOLCANISM IN NORTHWEST NEVADA

The “Yellowstone Hotspot” appears to have left a trail of calderas generally decreasing in age toward the northeast, culminating in the Yellowstone caldera. The High Rock caldera complex (NE-SW: Virgin Valley, Badger Mountain, High Rock, and Hog Ranch), located in northwestern Nevada at the northern edge of the Great Basin, is among the oldest in the sequence. Their associated volcanics have yielded radiometric age dates of ~14.5-16.5 ma.

The focus of our research is the High Rock (Soldier Meadow) caldera. Paleontological and stratigraphic evidence reveal that in the interval between caldera collapse and the uplift of a resurgent dome, the High Rock caldera teemed with wildlife resembling today's diversity at Ngorongoro Crater in Tanzania. Fossils indicate a temperate climate with three-toed horses, camels, rhinos, paleo-antelope, paleo-deer, oreodonts and horse/sloth-like animals called chalicotheres. Shoreline sediments have yielded the bones of birds and beaver that plied the shores of caldera lakes. Lacustrine deposits reveal insects, fish, clams and snails, and a horned gopher tunneled the shore. Bones from bear-dogs, wolves and badgers evince a healthy ecosystem. Shales have yielded foliage of oak, maple, birch, redwood and cedar while petrified trees add poplar and chinquapin.

After initial caldera collapse, smaller explosive eruptions influenced the ecology, sometimes entrapping forests and wildlife in layers of volcanic ash. Some forests were entombed upright while others appear to have been toppled Mt. Saint Helens style. Some volcanic blasts splintered trees into fragments, becoming nature's petrified “chipboard” when the debris settled to lake bottoms or amassed in deltaic distributaries. Lahars swept up and preserved a wealth of animal bones. At least one ballistic-charged volcanic blast embedded pumice fragments up to 2 cm in diameter into the trunks of subsequently petrified trees.