HOLOCENE ALLUVIAL HISTORY AND GEOARCHAEOLOGY OF CARRIZO WASH, NEW MEXICO

The Holocene alluvial record of Carrizo Wash, a tributary of the Little Colorado River, has a complex history of deposition and erosion that appears to be driven primarily by climate change. Twelve allostratigraphic units have been identified, although only four to seven are typically present in any given sub-basin. Late Holocene deposits dominate the record; earlier deposits are only locally preserved. Alluvial cycles represented by units predating 2500 cal yr B.P. generally span 1000 to 2000 years, whereas latest Holocene aggradation cycles are of ~500 years duration. Most of the allostratigraphic units consist of couplets of sand capped by weakly soil-altered mud. Infilled paleochannels represent past periods of piedmont and valley stream entrenchment 4800, 1100, and 600 cal yr B.P. Less conclusive evidence also exists for arroyo formation at 2500 and 1500 cal yr B.P. These events are roughly synchronous with arroyo cutting episodes documented over much of the American Southwest, suggesting the arroyo cycles are related to climate change. Very localized organic-rich cienega deposits imply especially wet conditions from 4000–3000 cal yr B.P. and 2000–1600 cal yr B.P. When the timing of Carrizo Wash deposition is compared to past findings of regional paleoenvironmental studies, a tentative pattern emerges. Periods dominated by sand deposition generally coincide with peaks in flood frequency and/or enhanced El Niño or monsoon conditions, whereas mud deposition and soil formation loosely correlate with relatively drier, less stormy periods.

Numerous deeply buried archaeological sites have been documented in arroyo exposures, suggesting that surface site distributions provide an incomplete picture of prehistoric land-use patterns. Except for one late Paleoindian feature, all of the buried archaeological sites date to the late Holocene. Many of the dated features at these sites contain maize, and almost half occur in thick deposits of sandy alluvium that rapidly accumulated 4000–3000 years ago in piedmont contexts during the early Neoglacial, a period of increased winter precipitation. Several other features date 2200–2000 cal yr B.P., coincident with latest Holocene intensification of El Niño conditions.