ALLUVIAL TERRACE DEPOSITION AND SOIL FORMATION IN KEET SEEL CANYON, NAVAJO NATIONAL MONUMENT

Keet Seel Canyon, part of the Tsegi Canyon system in northeastern Arizona, contains several alluvial terraces deposited in repeated cycles of erosion and deposition in the Mid- to Late Holocene. Previous studies have correlated timing of deposition of alluvial terraces and subsequent arroyo formation from the Tsegi Canyon system to that of washes on nearby Black Mesa, demonstrating widespread, climatically-driven arroyo formation. The Tsegi Canyon system is the site of a number of large prehistoric pueblos dating to the thirteenth century, A.D., among which the best preserved is Keet Seel Pueblo, located in Navajo National Monument in Keet Seel Canyon. Previous work has found that depopulation of Keet Seel and other pueblos in the canyons occurred at c. 1300 A.D., a time that corresponds to alluvial downcutting, and was likely influenced by the loss of arable land in the canyon.

This study uses radiocarbon dating of a sequence of buried soils on the Tsegi-age terrace in Keet Seel Canyon, directly in front of Keet Seel Pueblo. Soil geomorphic data demonstrate that the terrace formed by rapid deposition of alluvial sediment followed by brief periods of stability and weak soil formation. Dates from bulk sediment and macrobotanicals demonstrate periods of stability at c. 2800±40, 2340±20, 1630±60, 1290±20, and 610±50 cal yr BP. Data presented here agrees with earlier work that dates the deposition of the Tsegi-age terrace in the Tsegi Canyon system to >5000-600 cal yr BP (3000 B.C.-A.D. 1350) followed by widespread downcutting between 1300-1340 A.D. This study demonstrates that the timing of downcutting in Keet Seel Canyon was consistent with that of the greater Tsegi Canyon system, and that the landscape in Keet Seel Canyon was a stable floodplain in front of the Keet Seel Pueblo until that time. The sequence of buried soils is very similar to a sequence of buried soils that have been documented on similar-age alluvial terraces on Black Mesa, further supporting models of a regional, climatic control on terrace deposition.

In addition, a radiocarbon date from a macrobotanical maize sample of 1460±70 presents additional evidence for very early use of the canyon by Basketmaker populations (1000 B.C.- A.D. 750). Sites of Basketmaker age are known for the region, but have only been dated in few locations in Keet Seel Canyon.