A CHRONOSEQUENCE ON POST-GLACIAL FLUVIAL TERRACES IN THE SOUTH FORK AND UPPER RIO GRANDE RIVERS

A lack of recent tectonic activity indicates that the terraces along the upper Rio Grande River are primarily the result of changes in climate after the Last Glacial Maximum. As such, these terraces are important in understanding millennial scale changes in climate that are more similar to modern climate change. We mapped terraces along the Rio Grande and the South Fork of the Rio Grande near South Fork, Colorado with the goal of understanding their distribution. After mapping, we dug 16 soil pits across the three dominant terrace units and described soils using the methodology laid out by Pete Birkeland in his foundational textbook. We calibrated the resulting chronosequence using a combination of radiocarbon and OSL ages. Soil morphology varies significantly between the various terrace units indicating that soils form relatively quickly in the area despite a climate that features cold winters and arid summers. The uppermost terrace is interpreted as outwash associated with glacial retreat. The two intermediate terraces likely represent brief pauses during post-glacial incision. The youngest terrace is Late Holocene in age and differs from the other terraces in that it is sand-dominated and lacks significant cobbles. This sedimentology suggests that the youngest terrace formed via overbank processes and that the Rio Grande has generally retained its channel geometry over the past ~2 ka. This surface is also an important cultural location as Native American artifacts are common on the surface.