Abstract

The Sierra Quemada caldera is a major volcanic center that erupted a large volume of material about 34 million years ago in and around Big Bend National Park. The caldera activity produced a large amount of various igneous materials and culminated in the eruption of the Mule Ear Spring Tuff Member of the Chisos Formation. The tuff is thicker in and near the caldera and thins rapidly away from it. Numerous small intrusive bodies are related to two stages of caldera development: radial dikes formed before the main eruption; complex concentric plutons formed with ring-fracture and resurgent magmatism.

Structurally, the Sierra Quemada area is a high in which the surrounding rocks dip away from and are truncated by the caldera margin. The caldera is cut by the Burro Mesa fault group which forms a northwest-southeast trending complex graben system. The main fault hinges southeast of the caldera and has a displacement that increases from 200-300 ft (66-90 m) in that part of the caldera to over 3,000 ft (915 m) northwest of the caldera near Blue Creek Canyon.

Geophysical studies include more than 3,000 ground magnetic values covering about 75 mi (125 km) of traverse. Characteristics derived from these data include: 1) elevated values over the caldera compared to surrounding areas, 2) very high magnetic relief at ring-fracture and resurgent intrusions, 3) subdued relief of the southwest interior of the caldera relative to the northeast part, and 4) higher intensities outside the caldera to the southeast.