PATTERNS OF CONGLOMERATE DEPOSITION ASSOCIATED WITH THE UNROOFING OF THE SOLITARIO DOME, PRESIDIO AND BREWSTER COUNTY, TEXAS

The Solitario dome is one of the US southwest’s most striking geologic features, a circular formation so large it can be viewed from space. At almost 16 km across, the dome is the uplift over one of the largest laccoliths in the world. It is located within the Trans-Pecos Magmatic Province in Presidio and Brewster counties, Texas and comprises the northeast area of Big Bend Ranch State Park. Late Eocene to early Oligocene uplift of the Solitario dome generated coarse sediments comprising the Solitario Conglomerate (Tsg), an approximately 63 m thick mixed-clast formation outcropping to the west and the north of the dome. Tsg represents an alluvial fan network that is interpreted as the depositional product of major and minor pulses of uplift related to the igneous activity under the Solitario between 36 and 30 Ma (Henry and Davis, 1996). Near Fresno Canyon immediately west of the dome, the Solitario Conglomerate outcrops in Arroyo Mexicano where it conformably overlies the Chisos Formation. The base of the conglomerate is composed chiefly of silicic igneous lithic fragments; vertically, the silicic igneous clasts decrease in occurrence as more Cretaceous limestone clasts appear. At the top of the formation, clasts of Paleozoic cherts are present. This depositional sequence is consistent with the unroofing sequence of rocks uplifted by the Solitario dome. This study analyzes clast compositions through the vertical section, noting first appearances of clasts and recording changes in clast sizes; thus, reconstructing the uplift history of the Solitario dome. Tsg occupies an interval of 36 to 30 Ma (Henry and Davis, 1996). An ash layer was recently found within the conglomerate that could further constrain the timing of unroofing.