Holocene soils, geomorphic surfaces, and morphometry of two low-order drainage basins in the western Jemez Mountains, New Mexico
Tim Gere and Leslie D. McFadden
The two main tributaries of the Rio Cebolla were the focus of a geomorphologic study to illuminate how their canyons have evolved during the Quaternary. Calaveras Canyon and the Upper Rio Cebolla began developing after the cataclysmic eruption ~1.22 Ma of the Tshirege Member of the Bandelier Tuff that resulted in the formation of the Valles caldera. Spatial variations in the resistance of these volcanic units to erosion, and increased runoff potential in the Upper Rio Cebolla due to the probable capture of a high-altitude tributary basin, likely enhanced incision and widening of the axial valley in the Upper Rio Cebolla. Meanwhile, Calaveras Canyon developed a knickpoint in its long profile. Consequently, the tributaries of Calaveras Canyon possess relatively smooth, concave profiles, better adjusted to the axial channel base level than those in the Upper Rio Cebolla. Additionally, hypsometric analysis of the slopes in the Upper Rio Cebolla tributary basins indicates relatively recent steepening of the slopes in the lower parts of these basins. These differences in bedrock incision have led to differences in valley-bottom morphology. A date of ~ 4800 14C yr BP provided by charcoal excavated from ~ 2 m beneath the oldest terrace mapped in Calaveras Canyon provides a maximum age for the terraces and the tributary fans that grade to this surface. Dendrochronological methods indicate a time period of the mid-1700s to the late 1800s for the incision of a relatively fine-grained terrace surface to the boulder bar surfaces very near the level of the modern channel. Subsequent flooding events deposited a series of inset flood bar surfaces in the valley-bottom of Calaveras Canyon dated at approximately 1750 to 1870, 1941, and 1958 AD.
- Gere, Tim; McFadden, Leslie D., 2007, Holocene soils, geomorphic surfaces, and morphometry of two low-order drainage basins in the western Jemez Mountains, New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 433-440.