INTERPRETATION OF AEROMAGNETIC ANOMALIES OVER THE PINE CANYON CALDERA, BIG BEND NATIONAL PARK, TEXAS

The Big Bend National Park region of west Texas has experienced extensive Tertiary volcanism. The region’s volcanic stratigraphy and magmatic structures, however, are poorly understood. To advance understanding of the igneous geology, a high-resolution aeromagnetic survey was flown over the region in 2002. Some of the most interesting measured anomalies correlate with the Pine Canyon caldera, one of a dozen Tertiary calderas in Trans-Pecos Texas and source of rhyolites and ash flow tuffs of the South Rim Formation. The two oldest members of the South Rim Formation remain today within the caldera boundary: The Pine Canyon Rhyolite is the major caldera-forming unit, and the Boot Rock member (unofficially named), a series of ash flow tuffs, caps topographic ridges in the caldera. Highly rugged topography and strong magnetic remanence make aeromagnetic interpretation difficult, but many conclusions can still be drawn. Comparisons between anomaly boundaries, pseudogravity gradients, Euler deconvolution solutions, topography, and geologic contacts show that magnetic sources laterally within the caldera boundary are not buried and that the topography is strongly magnetized. Magnetic lows over high ridges, 2.5-D models, quantitative comparison of measured anomalies with synthetic terrain effects, and field polarity and susceptibility measurements indicate that the Pine Canyon Rhyolite is strongly reversely magnetized, and also that the Boot Rock member is probably weakly normally magnetized. Immediately to the south and southeast of the caldera boundary are prominent positive and negative anomalies, respectively, that lack surface expression; these are caused by buried sources, probably intrusions. A pseudogravity high that straddles the southern portion of the caldera boundary indicates the presence of a magnetic source that is more deeply seated than the magnetic anomalies caused by rocks cropping out at the surface. 2.5-D modeling results show enhanced ideas of caldera structure and geometry. Also, new radiometric dates for selected units inside and immediately outside the caldera provide constraints on the temporal development of the Pine Canyon caldera.