GEOLOGIC MAPPING OF THE VALLES CALDERA NATIONAL PRESERVE, NEW MEXICO

The Valles Caldera National Preserve (VCNP) was created by the Valles Caldera Preservation Act of 2000 to preserve and protect the historic Baca Ranch and the geologically famous Valles caldera of the Jemez Mountains volcanic field. The 89,000-acre ranch is now part of the U.S. National Forest and is governed by the Valles Caldera Trust. In 2002, new geologic mapping commenced within the VCNP and at present this mapping is being published as a series of 7.5-minute quadrangles by the New Mexico Bureau of Geology & Mineral Resources (Socorro, NM). Two of the four quadrangles that cover significant portions of the caldera (Redondo Peak and Bland) are now complete and two others will be finished by 2006 (Valle Toledo and Valle San Antonio). Eventually, the geology of the Valles caldera will be published as a special color map at 1:24,000.

The 22-km-diameter Valles Caldera is the world's type resurgent caldera and it is the source of the famous Bandelier Tuff. The southwestern resurgent dome is also host to a classic 200 to 300°C, liquid-dominated hydrothermal system circulating at depths of 600 to 2000 m. The new mapping has contributed to the following major research findings: 1. Uplift and faulting of the Valles resurgent dome have exposed large, rootless megabreccia blocks composed of precaldera rocks immersed in densely welded, intracaldera Bandelier Tuff. The largest blocks are 0.2 to 2.0 km long, consisting of Pennsylvanian through Quaternary rocks. 2. Maximum resurgence of at least 1000 m occurred immediately after caldera formation. High-precision Ar/Ar dates indicate that this structural resurgence took place within 54 ka of caldera formation. Resurgence averages 3.7 cm/yr and probably not less than 1.9 cm/yr. 3. Many lakes have existed in the caldera at various times. The earliest formed immediately after the caldera formed and has left tilted lacustrine and hydromagmatic deposits draped around the lower to middle flanks of the resurgent dome. Another lake may have filled much of the caldera at around 800 ka. Younger lakes occurred in the north and east valleys of the caldera at about 550 to 520 ka. A final lake occurred in Valle Grande at roughly 50 to 60 ka.