THE ARROYO GRANDE SITE OF WESTERN GRAND CANYON—ARCHAIC TO PROTOHISTORIC CULTURAL FEATURES IN DETAILED SEDIMENTARY-CHRONOSTRATIGRAPHIC CONTEXT

Grand Canyon National Park has a varied archaeological record wherein geomorphic context plays a prominent role. The record at Arroyo Grande is amongst the most compelling. As part of BoR treatment work driven by erosion and dam-operation issues, we have investigated this important site, confirming that it has multiple components, including the oldest known cultural features along the river corridor in Grand Canyon.

The Holocene stratigraphic record at Arroyo Grande reflects changes in the balance between mainstem backwater-flood deposition with some eolian reworking, and a series of gully drainages off the local canyon slopes. Detailed stratigraphic and sedimentary description was undertaken along the eponymous gully walls, as well as at multiple archaeological study units. This is constrained with 14 new OSL ages (by both single-aliquot regenerative and single-grain methods) and 6 new AMS radiocarbon ages.

Sequence A is dominated by sandy flood deposits dating from B.C. ~4800-3100. A marker-living surface is preserved in the middle of this package, with a series of small basin-shaped thermal features as well as associated lithic artifacts. This Middle Archaic cultural occupation is dated to B.C. 3970-3650, and occupants utilized Colorado River sand bars during a time when flood activity was more dominant in the immediate area. The inset alluvium of sequence B, along with the partially disconformable and youngest sequence C, are notably different, with thinner beds of sand and silty mud drapes that interfinger laterally with local-slope gravels. These upper sediments generally record episodic erosion and infilling of a gully drainage similar in scale to the modern one. Sequence B dates to B.C. ~1000-200, and sequence A hosts sparse Puebloan artifacts and is dated to A.D. ~300-700. Prominent Protohistoric roasting features are dug into the terrace top.

This cultural chronostratigraphy correlates to some degree with that we have developed in eastern Grand Canyon. This suggests that millennial cycles of aggradation and incision have occurred along the length of the river. These are complexly constructed, and in some places inset, by paleoflood sequences of a river with steadier short-term grade. This tentative model is important for paleoenvironmental reconstruction, but requires further testing.