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

The Sierra Quemada is a ring-dike complex in the south-central part of Big Bend 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 exposure of resistant intrusive rocks. It is well known that most ring-dike structures like this are related to caldera activity. Structurally, the complex is a dome with units outside the ring-like feature that are stratigraphically continuous and exhibit a gentle (20-30^o) but consistent quaquaversal dip. Inside the ring dike the strata are jumbled and show an irregular pattern of strikes and dips with dip angles that vary from essentially horizontal to greater than 60 degrees. Also, inside the ring structure the units that can be identified are distinctive and not like those outside, are highly altered, and are not correlated easily. Several faults that we relate to collapse and/or resurgence occur only inside the ring.

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. Similarly, pumice-clast and lithic-fragment sizes in Tmet decrease regularly away from the ring structure. Geophysically and geochemically the rocks associated with the ring structure are distinct from those outside. Significantly, petrographic and geochemical analyses of lithic fragments within the “explosion breccia” indicate that they are similar to rock units older than Tmet. None of the lithics are similar to units that are younger than Tmet. All of these features are compatible with, and indicate the presence of, a typical resurgent caldera that fits in a relative chronological sense with the age of Tmet. Implications include that mapping can be simplified by using genetically related units such as compound cooling units and that maps based mainly on radiometric ages can be misleading