FOLDED FORMAT: 1 color plate (approx. 36" by 60") plus 20 p. text
Compiler: Fred W. McDowell
This map and text cover an area of eastern Chihuahua state adjacent to the Rio Grande and the Big Bend of Texas. The area contains an 1100-m-thick volcanic section very similar in lithology and age (by Ar-Ar dating) to that exposed in both Big Bend National and State Parks. This includes, from older to younger, a heterogeneous sequence very much like to the Chisos Formation, a thick locally derived rhyolitic flow complex comparable to the Tule Mountain trachyandesite, distal thin ignimbrites similar in age to the Mitchell Mesa ash-flow tuff (the largest unit in the Trans-Pecos volcanic field), and a caldera source for both the 31 Ma San Carlos tuff and the 28 Ma Santana tuff. The caldera is an unusual trap-door type with a hinge zone on the southwest and two separate collapse and eruption margins around the north and east. Its outer diameter is approximately 25 km, which is unusually large for the tuffs that erupted from it, suggestive of a shallow collapse. Inflation or tumescence prior to the eruptions modified a preexisting Laramide fold by bowing it outward toward the north and east; a 31.5 Ma granitoid was intruded into the fold axis, resulting in the formation of skarn deposits in the surrounding limestones of the fold.
Published: 10/19/2010
Product Category: Maps and Charts