PALEOTOPOGRAPHY ON THE OTOWI MEMBER OF THE BANDELIER TUFF: IMPLICATIONS FOR LANDSCAPE EVOLUTION OF THE JEMEZ MOUNTAINS, NEW MEXICO

The contact between the 1.6 Ma Otowi and 1.2 Ma Tshirege members of the Bandelier Tuff is well exposed in Seven Springs 7.5’ quadrangle in the western Jemez Mountains. Elevations of the contact between the units extracted from a digital elevation model of the area are used to construct a paleotopographic surface on top of the Otowi member. The Tshirege member filled a relatively rugged topography that developed on the Otowi member during the ~400,000 years between eruptions of the Valles caldera; local relief is generally on the order of 20 to 30 m, although along the Rio Cebolla, relief is ~100 m. The base of the Tshirege member generally dips toward the southwest, away from a drainage divide north of the map area and away from the Valles caldera rim. Elevations of the base of the Tshirege member range from 2680 to 2740 m in the north central part of the area and along the topographic rim of the caldera to about 2500 m near Fenton Lake.

Some of the paleocanyons cut into the Otowi member in the western Jemez Mountains follow the general trend of modern canyons, but are offset to the west; however, many of the paleocanyons trend more to the southwest compared to the trend of modern drainages. In the northern and western Jemez Mountains, little sediment is preserved between the two tuff members, but fluvial gravels are found in the southwestern and southern part of the range. In places, the fluvial gravel is composed of material recycled out of the underlying Otowi member. Elsewhere, the fluvial gravel includes rounded clasts of locally derived Tertiary Paliza Canyon andesite and basalt and Permian sandstone. Epiclastic sediment and tephra deposits of the Cerro Toledo interval are present between the tuffs on the Pajarito Plateau in the eastern Jemez Mountains. The paleotopography on the Otowi in the Seven Springs area, combined with these observations from elsewhere in the Jemez Mountains, are used to provide a snapshot of the landscape around the Valles caldera 1.2 million years ago.