Late Pliocene deposition in Culebra Lake and Pleistocene erosion of lake sediment, northeastern Pajarito Plateau, New Mexico
David P. Dethier and Andrew D. Fagenholz
Thick exposures of lacustrine sediment and interlayered tephra deposited in Culebra Lake record late Pliocene damming of the Rio Grande in White Rock Canyon and coeval dacitic and basaltic volcanism along the northeastern flank of the Jemez Mountains, New Mexico. Culebra Lake was impounded by basalt flows near White Rock at about 2.35 Ma, reached depths ≥ 80 m, covered an area of ~1000 km2 and probably extended upstream along both the Rio Grande and Rio Chama. Dacitic tephra mixed with mafic cinders and hydroclastic shards record coeval silicic and basaltic eruptions as Culebra Lake began to fill. Basal Culebra clay is as much as 12 m thick, contains pollen typical of climate cooler than the Holocene, and is composed of illite/smectite intergrade and kaolinite, both likely derived from the Colorado Plateau. Silt, sand and sandy diamict overlie the clay, thickening and coarsening to the west and north near a paleoshoreline at an elevation of ~1950 m. After the lake began to drain in late Pliocene time, aggradation predominated north of White Rock Canyon until after eruption of the lower Bandelier Tuff at 1.6 Ma. Downcutting by Rio Grande tributaries after early Pleistocene time eroded lacustrine deposits from much of the Española basin. On the Pajarito Plateau, however, slower rates of incision and denudation preserved shoreline-tooffshore facies of Culebra Lake in east-draining canyons. The Bandelier Tuff helped to preserve Culebra deposits, but failed as massive slumps where headward erosion removed lateral support from the clay layer, probably in the middle Pleistocene. Evolution from blockage to downcutting along the margin of the Pajarito Plateau records incision through the volcanic dam, but circumstantial evidence suggests a role for Pliocene climate change as well.
- Dethier, David P.; Fagenholz, Andrew D., 2007, Late Pliocene deposition in Culebra Lake and Pleistocene erosion of lake sediment, northeastern Pajarito Plateau, New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th, pp. 388-397.