New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping of the 7.5-min Guaje Mountain quadrangle is nearing completion. Approximately two-thirds of the area within the quadrangle was burned during the Cerro Grande fire of May, 2000, extending from Pajarito Mountain in the SW to Santa Clara Canyon in the north. The fire, which burned over 47,000 total acres over a period of two weeks, denuded much of the pine forest and pinon-juniper ecosystems in the quadrangle, greatly enhancing rock exposures and surface expressions of geologic features. Particularly well exposed is the intricate, valley-fill contact between the Bandelier Tuff and the Tschicoma-lava highlands that form the eastern rim of the Valles Caldera. In areas of high-intensity burn, post-fire erosion effects are clearly related to the underlying rock type. Some units, such as the highly porphyritic Tschicoma Rendija lavas, weathered extensively as a result of the fire, resulting in local streambed aggradation due to the large influx of sand to gravel-sized rock fragments. More fire-resistant units, however, have contributed far less sediment to their drainage systems, resulting in post-fire streambed incision. Numerous faults (related to the Pajarito fault zone) are more evident since the fire, particularly in south-facing canyon walls where offset of volcanic/sedimentary strata is displayed.