New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jemez Mountains volcanic field in northern New Mexico, erupted catastrophically at 1.12 Ma, depositing the upper Bandelier Tuff (UBT) and forming the 20 km wide Valles caldera (VC). San Diego Canyon (SDC), containing the Jemez River (JR), provides the only outlet from the VC through a breach in the southwestern caldera rim.

24 km downstream from the breach, have mapped and surveyed terrace deposits and buried axial gravels. Terrace treads, terrace remnants, and buried gravels are at elevations of roughly 194, 187, 176, 120, 79, 51, 31,20, 14, 10,7 and 5 m above the modem JR channel. A river gravel buried beneath the UBT, 176 m above grade, provides a well-dated end-point from which to calculate incision rates as well as data on the depth of a paleo-SDC. The average incision rate over the last 1.12 Ma is at least 0.26 m/ka assuming the overlying thickness of UBT to have been 117 m (conservative).

Many researchers have speculated on the timing of the first breaching of the VC with estimates ranging from roughly 1 Ma to 0.48 Ma. Thirteen post-UBT rhyolite domes, flows, and tuffs within the caldera have been dated by previous workers. Among the youngest of these is the Banco Bonito flow (BB), dated at about 130-205 ka (Self et al., 1991,1. Geo. Res., 96: 4107). The first appearance of BB clasts on the 20-m terrace
provides a maximum age for it and a minimum age for the higher terraces. The highest appearance of caldera-derived volcanic clasts is within the gravels of a terrace remnant 79m above grade. The age of this deposit provides a minimum age for the first breaching of the VC. Rhyolite gravels have been found within a terrace remnant 120m above grade, but, at present, their source is unknown. Petrographic matching of caldera derived rhyolite clasts to dated rhyolites and amino-acid data from fossil gastropods preserved within terrace deposits are being attempted in order to better constrain the terrace ages and the timing of breaching events.