GEOLOGIC HISTORY AND URANIUM POTENTIAL OF THE BIG JOHN CALDERA, SOUTHERN TUSHAR MOUNTAINS, UTAH.

The Big John caldera is an obscure subsidence structure on the western flank of the Tushar Mountains, within the Marysvale volcanic field of west-central Utah. The caldera subsided about 23 m. y. ago in response to ash-flow eruptions that deposited the Delano Peak Tuff Member of the Bullion Canyon Volcanics. During caldera development and subsequent filling and erosion, several geologic environments were formed that were favorable for the concentration of uranium; these environments form the focus of this report describing the major geologic features and main mining areas of the Marysvale volcanic field.