User:PerkinsonSC/Challis Volcanics

The Challis Volcanics is a geologic formation representing a mix of various igneous rocks covering an area of roughly 25,000 square kilometers of central and northern Idaho. The formation dates to approximately 39-52 Ma, placing it within the Eocene epoch. Covering a range between 42°N and 49°N, the Challis Volcanics represents the largest region of Eocene volcanic rocks within the United States. It was named by C.P. Ross in 1934 after the nearby town of Challis, Idaho. It contains intermediate, mafic, and felsic volcanic rocks in addition to a very limited amount of fossils. Stratigraphic relations between the Challis Volcanics and other formations are complex due to the irregular basal surface and extensive faulting throughout the Challis Volcanics' history.

Volcanic History
Volcanic activity within the Challis Volcanics is separated into two main phases. The first occurred approximately 52 Ma and involved the intrusion of shallow granitic plutons. The second phase was a period of eruptions. The oldest eruptions occurred between approximately 51 Ma and 45 Ma, and were composed of intermediate and mafic lava flows as well as silicic ash flows. These explosive early stage eruptions caused the collapse of numerous calderas, some of which formed a linear graben. The second wave of volcanism was composed of less explosive silicic eruptions, and lasted until about 39 Ma. These later eruptions were joined by intrusive rhyolitic domes, plugs, dikes, and stocks.

Geology
The Challis Volcanics is composed of mafic to intermediate to felsic volcanic rocks that are generally nonresistant to weathering and erode easily. Underlying the Miocene aged Idavada Volcanics and overlying undivided Paleozoic rocks, the Challis Volcanics contains scattered intrusive plutons composed primarily of granite. In addition to these plutons, andesitic, rhyodacitic, and dacitic lava flows are found throughout. Rhyodacite and rhyolite domes and plugs are also common. Lastly, a basal conglomerate and tuff breccia is common. The intermediate to mafic rocks are found mostly in the more northern regions and all of the rocks originate from at least seven separate calderas. The thickness of the Challis Volcanics also varies wildly, with values ranging from 400 meters to 1,500 meters thick.

Fossil Content
The Challis Volcanics has a fossiliferous unit containing examples of carbonized wood and leaf fragments in addition to some leaf imprints and coalified plant debris. Petrified wood can also be found within the basal conglomerate layer. sample citation