GEOARCHAEOLOGY ALONG THE COLORADO RIVER IN GRAND CANYON—CULTURALLY RICH ALLUVIAL STRATIGRAPHY AND THE ISSUE OF PALEOFLOODS VERSUS CHANGING GRADE IN A BEDROCK CANYON

Ongoing geoarchaeology research along the Colorado River in Grand Canyon involves the excavation of several cultural sites with an alluvial context. The Holocene river-landscape history is being refined through detailed chronostratigraphic and sedimentologic analysis of natural exposures and excavation trenches, with an eye towards how the changing river relates to cultural occupation. A primary research question, inherited from previous workers, is whether the stratigraphy in this alluvium-floored canyon represents cycles of changing river form and grade or instead paleoflood variations of a relatively static channel. This dichotomy is key to understanding settlement along the river, including early agriculture.

New work at five sites with stratigraphic relations constrained by OSL and radiocarbon ages reveals a record episodically spaning the entire Holocene, to at least 11 ka. Three or more older sedimentary packages are characterized by interbedded facies of local-slope and mainstem river deposits, along with rare evidence of insipient buried soils. These sequences core the primary terrace landform, are variously preserved at the same, overlapping range of heights above the river, and are separated by often subtle unconformities. We interpret these as representing at least local aggradation and degradation of the fluvial system at millennial timescales, based upon key examples of environmentally distinct facies found at stratigraphic positions that are very difficult to explain assuming a static channel grade. Furthermore, these packages may be correlatable across multiple reaches of the river, but study at more sites will be necessary to confirm this.

The primary late-Archaic through Pueblo II (A.D. 0-1250) cultural features occupy the upper few meters of the youngest of these millennial packages. Complex palimpsest relations of occupation horizons within the upper terrace stratigraphy suggest that this cultural transition occurred when landforms inhabited where relatively free of flood inundation. Then, stratigraphically above and inset into these older deposits are younger packages of thickly bedded, purely mainstem-flood deposits—in places burying the Pueblo II features. These represent recent large floods that may ultimately have low preservation potential.