Volcanic Stratigraphy and Physical Volcanology of Big Bend National Park

Photo of Chisos Mountains

Studies of volcanic rocks in the Chisos Mountains were part of this project.

Since the map of Maxwell and others (1967) was published, significant advances in physical volcanology have been developed allowing study of some of the most important problems of the volcanic stratigraphy and processes in BBNP using state-of-the-art techniques. Studies by the USGS during this project addressed Eocene to lower Miocene volcanism in BBNP and assisted in the interpretation that these rocks are part of an alkaline and peralkaline volcanic complex exposed over the Trans-Pecos region. Studies of the predominantly silicic systems indicated that much of what had been interpreted as intrusive sills, dikes, and laccoliths are, in fact, extrusive peralkaline densely welded ignimbrites and large-volume lava flows. Emplacement mechanisms, cooling rates and processes, mineralization, and vent source areas associated with highly viscous peralkaline rhyolites were also studied. Hydrothermally mineralized rocks abundant throughout this region were also studied to relate mineralization processes to crustal evolution. The alkalic nature of igneous activity in this region is reflected in the F- and U-rich mineralization.

Highlights & Key Findings

Stratigraphic studies carried out during this project have shown that many more ignimbrites are present in BBNP than mapped by Maxwell in the late '60's, and some of these units are quite extensive. Additionally, the unit mapped as Bee Mountain Basalt by Maxwell and others (1967) may represent two major periods of basaltic volcanism in BBNP separated by other volcanic deposits and as much as 2 m.y. This is also the case for the Wasp Spring Breccia mapped by Maxwell and others (1967); this unit represents multiple units. Results from our radiometric dates indicate that volcanism occurred in BBNP at a time not recorded in the current paleomagnetic record. A study of a skarn deposit in the Dagger Flats area was presented in USGS Circular 1327.

References

Maxwell, R.A., Lonsdale, J.T., Hazzard, R.T., and Wilson, J.A., 1967, Geology of Big Bend National Park, Brewster County, Texas, in Publication - University of Texas, Bureau of Economic Geology, p. 320.

Original link here : http://minerals.cr.usgs.gov/projects/big_bend/task2.html