New Mexico Geological Society Annual Spring Meeting — Abstracts

Valles Caldera has contained multiple lakes since resurgence at >1.1 Ma. The three youngest lakes recognized from geologic mapping formed subsequent to the three youngest episodes of volcanism. The youngest was formed at ca. 50-60 ka by burial of the East Fork Jemez River canyon by thick deposits of El Cajete pumice. This lake was 10 km long and 5 km wide, covering much of the Valle Grande; beach ridges, spits, and wave-cut shorelines mark its extent. The El Cajete-dammed lake probably drained rapidly once an outlet was established, sending an outburst flood down San Diego Canyon. An older, larger lake also occupied much of the Valle Grande, formed by damming of the East Fork by ca. 520 ka South Mountain rhyolite flows. This lake was longer lived, and well logs indicate at least 85 m of clayey diatom-rich sediment may be associated with this lake. An additional lake occupied most of the northern moat of the caldera, extending about 19 km eastward from a ca. 580 ka San Antonio Mountain rhyolite dam into Valles San Antonio, San Luis, Santa Rosa, and Toledo. The part of the lake in Valle Toledo completely filled with diatom-rich sediment and is conformably overlain by fluvial deposits. Downstream, however, fluvial terraces unconformably overlie lacustrine sediments and indicate draining before the lake filled with sediment, caused by incision of the outlet. Surface and subsurface data indicate the presence of additional post-resurgence lakes in the caldera, although their ages and characteristics are less well constrained.