GEOMORPHIC AND INCISION HISTORY OF THE NORTHERN RIO GRANDE RIVER GORGE NEAR QUESTA, NEW MEXICO

We investigate relationships between river incision, aggradation, and widening in a ~5 km stretch of the northern Rio Grande River Gorge near the confluence with the Red River in New Mexico using detailed geomorphic mapping and cosmogenic ³He surface exposure dating. This wide and deep stretch of the river exhibits a unique set of cohesive, stacked toreva blocks, incoherent landslides, rockfalls, and slumps developed within ~3-5 Ma Servilleta basalts and intercalated Pliocene Santa Fe Group gravels. Toreva blocks of paired upper and lower Servilleta basalt are best developed along the eastern side of the river—they exhibit coherently-dipping, patinated tops that can be reconstructed to the gorge walls, but with toes that are truncated or buried by later deposits. Located below these landslide features is a flight of fill and fill-cut terraces spanning 5 levels at elevations of 49, 39, 29, 22, and 7 m above the modern river grade. Terraces at 29 m can be correlated across and along the river axis, whereas others are more locally developed. All terraces exhibit well-defined boulder levees and risers, and rounded and sculpted basalt clasts.

A preliminary ³He surface exposure age of ~103 ka was obtained from the uppermost terrace tread (Qt5); additional dating is in progress. If initiation of gorge incision was coeval with capture of the San Luis Valley of southern Colorado at ~440 ka, average incision rates prior to the formation of Qt5 were 618 m/my. This incision was likely coincident with toreva block formation and substantial gorge widening, as the toreva blocks predate terrace development. Average incision from ~103 ka to present day appears to have been faster, with minimum rates of 475 m/my. Gorge narrowing is observed during this time period, with only minor widening accommodated by slumping and incoherent landsliding that post-date terrace treads. This period of incision was punctuated by aggradational events that may correlate with late Pleistocene MIS climate cycles. These rapid incision rates mirror the phenomena seen in other western US river systems since the Pliocene.