GEOLOGIC MAPPING IN BANDELIER NATIONAL MONUMENT, NEW MEXICO

Bandelier National Monument (BNM) consists of nearly 34,000 acres that were set aside on February 11, 1916 to preserve prehistoric, scientific and scenic values. BNM occupies the southern portion of the Pajarito Plateau whose canyons and mesas contain remnants of Ancestral Puebloan communities that flourished during the 12th to mid-16th centuries. In 2000, new geologic mapping commenced within and around the BNM 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 quadrangles that cover significant portions of BNM (Frijoles and Bland) are now complete and their geology is the subject of this abstract. Eventually, it is anticipated that the geology of BNM will be published as a special map at 1:24,000.

BNM is the type locality of the famous Bandelier Tuff, a voluminous series of ash flow tuff sheets (ignimbrites) that erupted from nearby Valles and Toledo calderas at 1.25 Ma and 1.61 Ma, respectively. Rhyolitic fall deposits of El Cajete Pumice (50 to 60 ka), an eruption from within the Valles caldera, overlie the Bandelier Tuff within most of the monument. The new mapping provides some insights into Puebloan culture. For example, there is a relation between the location of smaller Puebloan ruins and El Cajete deposits. Thus, it appears that ancestral Puebloan inhabitants grew crops on the pumice because of its sponge-like ability to store water. On the west side of BNM, the mapping has revealed that a large debris avalanche from Rabbit Mountain forms a tongue between outflow sheets 2 and 3 of the Upper Bandelier Tuff. Possibly, explosions from Valles caldera caused collapse of the Rabbit Mountain dome between two ignimbrite eruptions. This deposit contains huge quantities of easily extractable obsidian blocks and was a mining site for ancestral Puebloans. The western part of the BNM is cut by the north-trending Pajarito fault system, the local active boundary fault of the Rio Grande rift. The normal, down-to-the-east Pajarito fault exhibits about 200 m of vertical displacement on the Upper Bandelier Tuff in the monument. Studies outside the monument indicate the fault system has had three Holocene surface rupturing earthquakes.