Sonoma orogeny

The Sonoma orogeny was a period of mountain building in western North America. The exact age and structure of the Sonoma orogeny is controversial. The orogeny is generally thought to have occurred during the Permian / Triassic transition, around 250 million years ago, following the Late Devonian Antler orogeny. The Sonoma orogeny was one of a sequence of accretionary events along the Cordilleran margin, possibly caused by the closure of the basin between the island arc of Sonomia and the North American continent. Evidence of this event has been reported throughout western North America, but most distinctly in northwest Nevada.

The orogeny was named by Silberling and Roberts, who identified it with the Havallah Formation, originally thought to date to the Pennsylvanian and Permian ages. The age range of the Havallah has since been revised to include rocks of Late Devonian and Mississippian age and its name was changed from Havallah Formation to Havallah sequence. Some geologists dispute whether convergent plate tectonics produced the Sonoma orogeny.

The Havallah sequence is the stratigraphic unit universally associated with the Sonoma orogeny, but units of the same age range and roughly the same lithic composition, along the western and northern margins of the Havallah, are also relevant. These units include the Inskip Formation in the East Range and a series of formations in the Hot Springs Range. These units consist of basalt, felsite, bedded chert, limestone, and detrital rocks ranging from conglomerate to argillite that accumulated in a trough west of the Antler orogenic belt.

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