PLIOCENE INCISION OF THE RIO GRANDE IN NORTHERN NEW MEXICO

Stratigraphic and geomorphic features of the Rio Grande rift between Espanola and Socorro indicate the river system began to cut into basin-fill in late Pliocene time, not middle Pleistocene as previously held (e.g., Connell et al., 2000). Between 2.8-2.2 Ma, extensive basalt flows buried a low-relief landscape across the rift. The earliest magmas at Cerros del Rio, Santa Ana Mesa, and Isleta in the basin centers produced hydromagmatic eruptions (Kelley and Kudo, 1978), indicating little incision had taken place at this time (high water table). No thick or extensive fluvial deposits accumulated in this part of the Rio Grande rift during and immediately following eruptions despite continued subsidence (except near very active Pajarito and La Bajada faults; e.g., Sawyer et al., 2001). The widespread hiatus in deposition indicates the drainage system had begun to erode into the upper Santa Fe Group, perhaps as early as 2.5 Ma. The oldest significant post-basalt fluvial deposits are coarse sand and gravel beds restricted to a narrow tract along the eastern side of the modern Rio Grande (e.g., Maldonado et al., 1999). They are younger than 1.6 Ma (contain clasts of older Bandelier Tuff) and locally as young as 1.2 Ma (contain tephra of the younger Bandelier eruption) and overlie tilted and eroded upper Santa Fe. We interpret these deposits as the oldest, highest inset river terrace fill in the rift, underlying the Sunport geomorphic surface of central Albuquerque about 300 feet above grade. Twenty miles south at Los Lunas, basalt erupted at about 1.1 Ma over upper Santa Fe deposits (capped by a thick calcic soil) and flowed 400 feet down toward the Rio Grande where it buried a topographic bench slightly higher than the Sunport surface. The Los Lunas relationships indicate that the Rio Grande had deeply incised the Santa Fe Group by 1.1 Ma and that Santa Fe deposition by the ancestral Rio Grande-Rio Puerco fluvial system had ceased long before.