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The Castle Rock area includes approximately 65 km2 occupying the area of intersection of Grant, Harney and Malheur Counties of eastern Oregon. The products of this study are a geologic map of the area and stratigraphic and lithologic descriptions of the rock units within the area. The "unnamed igneous complex" is the oldest rock unit exposed within the area. This complex consists of more than 670 m of flow-on-flow basaltic lava flows deposited in Late Miocene time. Faulting and intrusion of rhyolitic dikes resulted in the Castle Rock ridge being uplifted along numerous north-northwest-trending faults. Eruption of the Dinner Creek Ash-Flow Tuff from these rhyolitic dikes introduced numerous local volcanic structures and ultimately caused subsidence of the area around Black Butte. The resultant tuff sheet is more than 300 m thick locally. Silicic lava flows and breccias, pumice lapilli-tuffs and basaltic lava comprising a section more than 700 m thick, were deposited over the Dinner Creek Tuff in very Late Miocene time. These rocks, the Strawberry Volcanics, were derived from sources primarily outside the Castle Rock area. Following folding, continued faulting and extensive erosion, the tuffaceous, lacustrine and fluviatile sediments iv of the Juntura and Drewsey Formations and associated volcanic rocks were deposited through Early and Middle Pliocene time, respectively. A thin rhyolitic welded ash-flow tuff was deposited during deposition of the Drewsey Formation. During Pleistocene to Recent time faulting and folding have continued on a small scale and erosion of the lacustrine sediments has left stream deposits on pediment surfaces 470 m above the present stream level.

The Geology of the Castle Rock Area, Grant, Harney and Malheur Counties, Oregon