POST-RESURGENCE LAKES OF THE VALLES CALDERA, NEW MEXICO

Abstract

Valles caldera has contained multiple lakes since resurgence at ≥1.2 Ma. Many details of the history of these lakes are yet to be defined, but a partial framework has been developed from recent mapping and other studies. Three large lakes, each >20 km2 in area, formed as a result of damming of drainages during the three youngest episodes of volcanism, and earlier volcanism also likely blocked drainages and impounded lakes. Much of Valle Grande was inundated at ca. 50-60 ka following burial of the East Fork Jemez River canyon by thick El Cajete pumice deposits. This lake was ~10 km long and up to 5 km wide; beach ridges, spits, and wave-cut shorelines mark its extent. The El Cajete lake was probably short-lived, draining rapidly once an outlet was established. An older, somewhat larger lake, ~11 km long, also occupied much of Valle Grande, formed by dam- ming of the East Fork by ca. 521-552 ka South Mountain rhyolite flows. Drilling logs indicate at least 90 m of clayey diatom- rich sediment are associated with this lake, and recent studies suggest it persisted for at least 170 ky. A third lake occupied much of the northern moat of the caldera, extending ~20 km eastward from a ca. 557 ka San Antonio Mountain rhyolite dam into Valle San Antonio, Valle San Luis, Valle Santa Rosa, and Valle Toledo. The part of the lake in Valle Toledo filled with diatom- rich sediment that is conformably overlain by fluvial deposits, and an inferred delta front is preserved downstream in eastern Valle San Antonio. Farther west, fluvial terraces unconformably overlie lacustrine sediment and indicate draining before the lake filled with sediment, caused by incision of the outlet. Preliminary OSL analyses, supported by a 14 C date, suggest that upper Valle Toledo lake beds have an age of ca. 40-50 ka, indicating either a very long-lived lake or a younger damming event, although a younger dam has not been identified. Surface and subsurface data indicate the presence of additional post-resurgence lakes in the caldera, although their age and characteristics are less well constrained.