COMPARISON OF TERRACE LEVELS, PALEO CLIMATE INCISION RATES, AND ARCHEOLOGICAL SITES NEAR MONTROSE, COLORADO

Despite extensive terrace mapping by the Colorado Geologic Survey on the eastern flank of the Uncompahgre Plateau near Montrose, Colorado, there are no published studies describing the geomorphic evolution of localized fluvial terraces. The study area includes the Dry Creek and Roubideau Canyon drainages that head at the crest of the Uncompahgre Plateau, as well as drainages that run east from these canyons and terminate in the Uncompahgre farming valley. The northern extent of the study area is Escalante Canyon to Spring Creek Canyon on the south.

This landscape is known to host some of the oldest archaeological sites on the margins of the Colorado Plateau, though no sites older than 8000 BP have been identified in the study area. Our study questions will give us a better understanding of landscape evolution since the late Quaternary: 1) what, if any, are the differences in terrace histories spatially and between smaller and larger arroyos; 2) where are archaeological sites, particularly Late Pleistocene and Early Holocene sites, most likely to occur; 3) how did the climate vary during the period of terrace formation?

This collaborative effort between a USGS chronology lab, an archaeological consultancy, and soil geomorphologists will identify, sample, and date terraces and eroding sites within the selected research areas. Three initial luminescence dates on two levels of terraces and palynology samples were taken from drainages east of the canyons and near their termination in the valley. These samples indicate that fluvial terraces formed during colder, wetter climatic conditions (20 ka and 12 ka). Charcoal lenses (dated from 12 ka BP to 9.5 ka BP) within alluvium sampled from cut banks and from bucket augers on terraces are being investigated to determine if they are from natural or cultural activity. Fine-grained massive deposits tend to overlie the coarse matrix-supported fluvial deposits. The massive deposits do not exhibit soils or soil like structures or internal stratigraphy (cross bedding, graded bedding, etc.). Investigation of these massive units is on-going. Currently, we are determining the age(s) of these deposits and their origin (colluvial, paludal, slackwater, or eolian dust deposits).