Relation of “Bonito” Paleo-channels and Base-level Variations to Anasazi Occupation, Chaco Canyon, New Mexico [No. 194]

Abstract

Late Holocene deposits of Chaco Canyon, rather well known from studies beginning in the 1800s, include a filled prehistoric arroyo that we call the Bonito channel. Extensive dating via detrital ceramics, confirmed by in situ archaeological sites, shows the channel filled from about A.D. 1025 to 1090, earlier than some previous authors thought. Channel cutting may have begun as early as A.D. 900. The development of both the Bonito and modern arroyos is due to the anomalous position of the valley floor in Chaco Canyon, which is perched 4-5 m above and separated from the rest of its drainage network by an eolian dune. This dune apparently formed an effective dam at some times (when valley-floor units formed) but was breached at others (when channels formed). Thus base-level change drove stratigraphic evolution. The Bonito channel system is dendritic, cut in the older Chaco and Gallo units that define the valley floor surface, and is filled to the valley-floor level with little indurated sand and lesser gravel. A single soil-flood plain unit, not as strongly developed as the multiple soils on older units, is present on Bonito channel fill. The timing of base-level change, governed by eolian vs. fluvial energy, is uncertain but seems consistent with dendroclimatic, cultural, and stratigraphic chronologies of Chaco Canyon (new local dendroclimatic data are presented herein). Probably because of the unusual, rather mechanical nature of controls there, the alluvial chronology of Chaco Canyon does not correlate well with others of the region. Anasazi activity seems to have been tuned to changes in the Bonito channel with regard to construction of pueblos, roads, and water control features. Relations between fluvial and cultural features were especially intricate during channel filling, about A.D. 1025-1090, a period of great Chacoan influence and complexity. The extraordinary Chacoan water-control features may have been initiated in response to the Bonito channel system, and at least three Chacoan great houses were built entirely or in part on filled Bonito channels.