LATE QUATERNARY ALLUVIAL GEOCHRONOLOGY AND GEOMORPHOLOGY OF LOWER COMB WASH, SAN JUAN COUNTY, UTAH

Understanding Quaternary erosional and depositional cycles in arid drainages is critical to preventing the heavy loss of property like that which occurred at the end of the 19th century in the American southwest. The goals of this study were to reconstruct the late Quaternary geologic history of lower Comb Wash in southeastern Utah and examine the cyclicity of erosional/depositional cycles in relation to climatic variability.

Comb Wash an ephemeral tributary of the San Juan River in semi arid southeastern Utah on the Colorado Plateau. Five fluvial terraces were mapped in the lower 27 km: T1-T3, BCT, MCT, and Ta, oldest to youngest, respectively. Deposition of T1 is constrained by radiocarbon analysis and began before 13,000 cal yr B.P., ending by 11,000 cal yr B.P. T2 is inset 5 meters below T1. A single radiocarbon date indicates that deposition of T2 began by 11,000 cal yr B.P. and ended by 5000 B.P. based on overlying archaeological material. T3 is the dominant terrace throughout the upper portion of the study area and was deposited between 4500 and 1200 cal yr B.P. based on radiocarbon and ceramic analysis of in situ organic and archaeological material. The big cottonwood terrace (BCT) is distinguished from T3 by a lower topographic position and the presence of large, century old cottonwood (Populus fremontii). BCT sediment range from A.D. 800 to 1880 based on radiocarbon analysis and dendrochronology, respectively. The medium cottonwood terrace (MCT) was deposited between A.D. 1940 and 1980 based on dendrochronology. The tamarisk terrace (Ta) is still active and frequently flooded during large storm events.

Comparison of the Comb Wash chronology with southwestern fluvial chronologies indicates that most depositional/erosional cycles were driven by regional climatic events with human activity and internal basin morphology secondary. The timing of BCT deposition is the exception and may represent dominance by internal basin control over response to climate change.