New Mexico Geological Society Annual Spring Meeting — Abstracts

During the past few million years, tectonism and Rio Grande aggradation and erosion have controlled the dynamics of landscape evolution in the southeastern Albuquerque basin of the Rio Grande rift. Landscape elevations, courses of the Rio Grande, and paths of large tributaries are controlled by tectonic uplift and subsidence, and sediment erosion and aggradation. Sediment transport and storage along the Rio Grande and tributaries are related to episodic fluctuations in climate-determined discharge. The basin-scale interplay of tectonics and sediment storage is reflected in the geometry of alluvial fans, the maximum level of aggradation, and the eastern extent of Rio Grande deposits.

Quaternary tectonism is suggested by paths of tributaries, and the broad removal and contrasting burial of equivalent Pliocene deposits. The south-trending Rio Puerco hooks eastward near Bernardo. Southeastern tributaries trend northwest toward Bernardo, joining the Rio Grande in upstream directions. High-level (Pliocene?) alluvial deposits west of the southern Los Pinos Mountains are poorly preserved south of the Palo Duro-Cibola divide. Equivalent-aged Pliocene rift sediments are buried or at lowest exposures along the Rio Grande valley, indicating a steeper slope between uplifts and basins. These observations suggest the southern Albuquerque basin subsided while the rift flanks uplifted during Quaternary time despite the paucity of observed fault scarps.

Elevations of maximum aggradation of Rio Grande well-rounded mixed-gravel deposits as high as 1,554 m (114 m above the river) are exposed in Abo Arroyo to Turututu (Black Butte) to the headwaters of Pascual Arroyo (east of Contreras). Rio Grande deposits continue south along the west flank of Joyita Hills. These deposits contain clasts of 1.4-Ma Rabbit Mountain obsidian and rare clasts of Bandelier Tuff (Tshirege Mbr). The deposits extend approximately 10 km east and southeast from the modern Rio Grande channel, 3 km farther east and 34 m higher than previously mapped.

The previously interpreted top of basin fill at 81 m above the Rio Grande floodplain is interpreted now as an inset terrace tread. Exposures in Abo Arroyo show two bluff-line unconformities of inset terraces with the two western middle Pleistocene straths closely spaced (instead of one) approximately 39-43 m above the present Rio Grande. The oldest bluff line is between cemented Rio Grande deposits and a package of uncemented strath of Rio Grande channel and floodplain deposits overlain by Abo Arroyo fan deposits. The cemented Rio Grande deposits contain the Blancan camel Hemiauchenia gracilis and horse Equus cumminsii. The second bluff line and package of similar facies is approximately 15 m west, and the basal strath gravel is 1.5-2 m below the strath to the east. Abo Arroyo also exposes a higher, more gravelly terrace (?) above the cemented unit and a lower, extensive terrace near Veguita with a tread 29 m above the floodplain similar to terraces mapped farther south.