Marysville Buttes is an extinct complex volcanic plug surrounded by steeply folded and faulted Cretaceous and younger sediments. It occupies a circular outcrop area of approximately 20 square miles, and forms a bold topographic feature rising above the flat low Sacramento Valley.

The Marysville Buttes intrusive developed during post-Eocene time with the piercement and doming of 7,500 feet of Upper Cretaceous and middle and upper Eocene beds by a rhyolite and andesite plug approximately 4 miles in diameter. It is believed later andesite formed the central core and progressively deformed the sedimentary beds. During late Pliocene to Pleistocene, showers of pyroclastics, crudely interbedded with mudflows, were deposited on the denuded dome. This formed a cone, which has subsequently been deeply eroded.

Beneath the valley floor near Colusa, a similar large plug which has been indicated by seismic and well records lies concealed beneath approximately 5,000 feet of Cretaceous and younger sediments.

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Possible other deep-seated intrusives in the area domed overlying sediments, in some cases resulting in gas accumulations.

The Buttes gas field produces from lenticular Cretaceous sands in a trap which may have been further improved by the Buttes intrusion. Whereas the surface expression of the plug and distorted bordering sediments occupies an area of 20 square miles, the area beneath the surface which is influenced by this volcanic disturbance is at least 120 square miles.

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