NEW CHRONOSTRATIGRAPHIC WORK ON HOLOCENE COLORADO RIVER ALLUVIUM HOSTING CULTURAL SITES IN GRAND CANYON NATIONAL PARK—A RECORD OF CUT-AND-FILL CYCLES OR OF INCREMENTAL FLOODING?

Luminescence and radiocarbon geochronology of Colorado River deposits in Grand Canyon National Park, Arizona, provides new insight into the style and timing of alluviation at three important cultural sites. Preliminary OSL (optically stimulated luminescence) ages indicate these preserved terrace deposits are roughly coeval between distinct cultural areas of eastern and western Grand Canyon. Results also indicate the record is longer than previously thought, with deposition beginning in the early Holocene before Archaic peoples' arrival in the Canyon, and continuing intermittently until sometime after Late Formative (Pueblo II) cultures occupied terraces along the river. These PII sites were buried by one or a few major overbank flood deposits before terrace deposition ceased and the river apparently entered its historic erosional regime.

Previous researchers described the stratigraphy as a series of inset cut-and-fills with overall lowering of river grade beginning before PII time. However, inset buttress unconformities have not been conclusively observed. Alternatively, other previous researchers have interpreted the deposits as a more continuous stack, recording deposition of increasingly large floods throughout the Holocene. The refined luminescence and radiocarbon geochronology will provide a test of these two hypotheses relating to the fundamental river behavior and processes that formed the context for cultural activity along the river corridor.