BURIED LANDSCAPES: RECONSTRUCTING THE PALEO-TOPOGRAPHY OF THE CERRO TOLEDO INTERVAL, JEMEZ MOUNTAINS VOLCANIC FIELD, NEW MEXICO

The near instantaneous emplacement of a major ignimbrite preserves the pre-existing landscape; subsequent exposure by erosion can be used to reconstruct the topography of this buried terrain. The Pajarito Plateau, a 16-25 km wide, 48 km long feature that flanks the east side of the Jemez Mountains volcanic field in north-central New Mexico is a world class example of this “sandwich effect” in which a prior landscape is essentially “sealed” between two major ash flows (Otowi and Tshirege Members of the Bandelier Tuff at 1.6 and 1.2 Ma), with windows into the past subsequently re-exposed by a series of narrow, deeply incised, parallel canyons. Structure contour maps derived from recent field observations of exposed contacts, combined with previously mapped geologic surface data and deep well drill-hole data, provide a glimpse of the topography that developed on the surface of the Otowi Member during the ~ 380,000 years (the Cerro Toledo interval) prior to eruption of the second (Tshirege) tuff member. The non welded Otowi was easily eroded, resulting in a landscape characterized by rolling hills with gentle gradients. Episodic eruptions of plinian ash throughout the Cerro Toledo interval resulted in pulses of sediment that periodically overwhelmed developing drainage systems. Regional base level was controlled by the ancestral Rio Grande, whose location shifted in response not only to the silicic volcanism from the Jemez Mountains to the west, but also to mafic flows from the Cerros del Rio volcanic field (~3.0-1.1 Ma) to the east. The basaltic flows from the east created a broad resistant tableland which provided local knickpoints for streams draining the Otowi headlands, allowing broad washes to form adjacent to the master stream. In addition, occasional landslides, earthquakes, and undercutting of the ancestral Rio canyon formed ephemeral impoundments which may have temporarily raised base level along particular reaches influencing rates of tributary drainage network development and affecting where large, episodic outflows of sediment would have been deposited.