STRATIGRAPHIC RELATIONS IN THE CHISOS GROUP IN THE HAYES RIDGE AREA OF BIG BEND NATIONAL PARK, TEXAS, USA

We have conducted mapping in the Hayes Ridge area of Big Bend National Park as part of an effort to create a new geologic map of the park. This area includes outcrops of Chisos Group intruded by a ring dike complex that forms the arcuate topographic feature referred to as Hayes Ridge. This ring dike complex is associated with the South Rim Formation and the Pine Canyon Caldera. In the Hayes Ridge area, the upper section of the Chisos Group is truncated by the eruptive products of the Pine Canyon Caldera, while the base is not exposed. Here, the Chisos Group includes a silt- and clay-stone that grades upward into a cross bedded sandstone interbedded with claystone toward the top. This assemblage is intruded by a sequence of narrow dikes, approximately one meter in width, followed by the emplacement of larger three meter wide dikes. The first rhyolitic dike sequence has approximately fifteen percent sanidine phenocrysts, two percent glassy quartz, a grayish-blue groundmass, and an approximate trend of ninety degrees. A second phase of dike emplacement is slightly more porphyritic, composed of approximately twenty percent sanidine phenocrysts, minor quartz phenocrysts, a finer groundmass, and an approximate trend of sixty-five degrees. This second phase of dike emplacement is sub parallel to the main Hayes ridge dike structure, and is petrographically similar.

Chemistry of the Hayes Ridge structure is similar to that of the welded tuffs at Emory Peak and Crown Mountain, leading us to believe there is a genetic relationship. We feel that the change in dike orientation is related to a change in the stress field related to both the development of the Hayes Ridge ring dike complex, and the final eruption of the Pine Canyon caldera.