LATE CENOZOIC RECORD OF CLIMATIC AND TECTONIC CHANGES AT THE JUNCTION OF THE SOUTHEASTERN COLORADO PLATEAU, SOUTHERN ROCKY MOUNTAINS, AND CENTRAL RIO GRANDE RIFT

Neogene strata of the northwestern Albuquerque Basin, Rio Grande rift (RGr), lie east of the junction of the southern Rocky Mountains (SRM) and southeastern Colorado Plateau (SECP) and provide a long-term record of erosion and sedimentation that reflects changing tectonic, climatic, and geomorphic conditions. Widespread deposition on the SECP ceased after formation of the Oligocene San Juan, Latir, and Mogollon-Datil volcanic fields. The presence of Pedernal chert on the Sierra Nacimiento crest indicates partial or complete burial of this SRM range prior to ~26 Ma; present relief is largely due to erosion following deposition of this distinctive chert. Lava flows in the Acoma-Zuni section of the SECP preserve parts of the paleolandscape since 8 Ma. By 3.73 Ma low-relief surfaces contained channels eroded into Upper Cretaceous strata ~500 m above the present Rio Puerco and SECP.

SECP and SRM denudation patterns yield nonunique clues as to first-order causes such as relief, climate, or basin geometry. RGr subsidence generated tectonic relief, influenced drainage-basin development, and transport and storage of sediments into the rift. Rapid sedimentation and tectonic subsidence formed closed basins until late Miocene time. During the past 5 m.y., fault-slip rates slowed to <0.2 mm/yr, active rift-margin faults stepped basinward, and local horsts rose from previously subsiding areas. Pliocene sediment accumulation slowed to <70 m/m.y. Concurrent climatically driven changes in sediment delivery, storage, and onset of extrabasinal axial-river transport modified rift sedimentation patterns. Basinal gravels derived from the SECP and SRM were restricted to the margin of the Albuquerque Basin until Pliocene time, when the laterally extensive Ceja Mbr (Arroyo Ojito Fm) prograded across much of the basin until ~2-2.5 Ma. This progradation roughly coincides with global cooling and increased sediment production, and possible Pliocene onset of Rocky Mountain glaciation. Sedimentation of the axial Rio Grande continued towards the eastern basin margin until ~800 ka, when widespread aggradation ceased with the entrenchment of the Rio Grande Valley and other nonglaciated drainages in southwestern North America, supporting regional climate change interpretations for early Pleistocene incision.