EVIDENCE OF CALDERA ACTIVITY IN THE SIERRA QUEMADA, BIG BEND NATIONAL PARK, TEXAS

The Sierra Quemada is a ring-dike complex that has been interpreted by us and others as a caldera. The ring structure is apparent from topographic and geologic maps as well as air photos and satellite imagery and is caused by resistant intrusive rocks. Most ring-dike structures like this are related to caldera activity. Structurally, the complex is a dome with quaquaversal dip away from the ring dike but with an irregular pattern of strikes and dips inside the structure. Dip angles inside the ring vary from essentially horizontal to greater than 60 degrees. Several faults that we relate to collapse and/or resurgence occur only inside the ring. The rock units outside the structure generally are stratigraphically continuous and can be traced readily. Inside the ring structure the units generally are different than those outside, are highly altered, and are not correlated easily.

Probably the best evidence of caldera activity is the presence of a thick (> 130m/400 ft) tuff or “explosion breccia” that we interpret as an intra-caldera equivalent of the Mule Ear Spring Tuff Member (Tmet) of the Chisos Formation. We have speculated that the tuff was erupted from this structure. No rocks similar to Tmet, or any younger volcanic units from the park, are found within the ring structure. Outside of the Sierra Quemada, Tmet thins more or less regularly to the south and west. In addition, pumice clast and lithic fragment sizes in Tmet decrease regularly away from the ring structure. Geophysically and geochemically the rocks inside the structure are distinct from those outside. All of these features are compatible with, and indicate the presence of, a typical resurgent caldera. Implications include that mapping can be simplified by using genetically related units such as compound cooling units and other caldera-related features.