RHYOLITES AND ASSOCIATED DEPOSITS OF THE VALLES-TOLEDO CALDERA COMPLEX

The centerpiece of the Jemez Mountains volcanic field of north-central New Mexico is the Valles-Toledo caldera complex which formed with two voluminous eruptions of Bandelier Tuff, about 400 to 475 km³ each, at 1.25 Ma (Valles caldera) and 1.61 Ma (Toledo caldera). Additionally renowned is the Valles caldera’s resurgent dome, which preserves a record of over 1000 m of structural upheaval of the caldera floor within 54,000 years of caldera formation. All of the numerous rhyolite lavas and domes, small and large volume pyroclastic flows, fallout tephras, and a variety of volcaniclastic deposits of the caldera complex constitute the Quaternary Tewa Group. Extensive new geologic mapping and new ⁴⁰Ar/³⁹Ar ages provide fresh insights into the relationships of Tewa Group units. We propose eleven completely new units of member rank, with descriptions of lithology, contact relations, distribution, and type areas. These new members are parts of the Bandelier Tuff, Cerro Toledo, and Valles Rhyolite formations, and serve to depict the magmatic and geomorphic evolution of the area before, during, and following each of two major caldera-forming episodes. With results from mapping efforts in the Jemez Mountains revealing the broad implications and interrelations of some established units, we redefine the Cerro Toledo Formation and demote three formal members (El Cajete, Battleship Rock, and Banco Bonito) to bed or flow rank. Because it has been repeatedly shown in published works that one formation (Cerro Rubio Quartz Latite) was originally defined based on erroneous relations, we propose its formal abandonment. Additionally, we propose formal abandonment of one established member (Valle Grande Member of the Valles Rhyolite) because of lack of utility and widespread disuse. Our proposed revisions embody the practices of geologic mappers, and serve to better clarify relations of volcanic and volcaniclastic rocks through the evolution of the Valles-Toledo caldera complex. The new formal stratigraphy that we propose will provide a flexible, but robust framework for on-going and future research in the Valles-Toledo caldera complex. (http://geoinfo.nmt.edu/publications/periodicals/nmg/32/n1/Gardner.pdf)