Holocene Depositional History and Anasazi Occupation in McElmo Canyon, Southwestern Colorado [No. 188]

Abstract

McElmo Canyon in southwestern Colorado, which drains the Montezuma basin into the San Juan River, contains excellent exposures of Holocene sequences that underlie a broad valley-bottom terrace system. These exposures are the vehicle for this study of the stratigraphy and geometry of fluvial deposits and their contained archaeological remains. Anasazi sites in alluvium range from Basketmaker III to Pueblo III in age, thus providing age guides for the period AD. 500-1300. Fluvial deposits include channel, floodplain, and tributary alluvial fan facies. During times when (and at locales where) the system aggraded, these facies are interbedded and gradational in a way that suggests a braided channel, in contrast to degrading episodes that suggest a meandering channel. Local deposition rate was as great as about three meters in 100 years where distal fan deposits on the northern side of the valley are interbedded with main-channel floodplain deposits. Two main depositional packages are present, separated by an unconformity that mostly formed during the Pueblo I period. The age of this high relief unconformity is apparently diachronous, and the overlying package is certainly diachronous, both suggesting upstream migration of about five kilometers in 200 years. Our stratigraphic record of migrating loci of entrenchment and aggradation corresponds to studies of modern drainages, in which such changes are internal drainage adjustments. However, the broader time intervals of dominant erosion versus deposition are similar to alluvial chronologies elsewhere in the region and are thought to be controlled by climate change. An intricate feedback system apparently operated between sedimentary and geomorphic events on one hand, and Anasazi agriculture and habitation on the other. Agricultural water-control features show the importance of actively aggrading toes of northside fans in Anasazi agriculture. Habitation, situated on adjacent quasi-stable landforms, closely tracked loci of aggradation as these loci migrated. No habitation adjacent to valley segments suffering coeval entrenchment was found. The relation of migrating entrenchment loci and observed Anasazi habitation patterns suggest that the deleterious effects of entrenchment on Anasazi floodland agriculture probably resulted only in migration to nearby loci of deposition. The floodland component of Anasazi agriculture in this region may explain some Anasazi migration patterns that are otherwise anomalous. Adjacent floodlands and uplands, both in zones favorable for agriculture, may be required for successful habitation at certain times. The locations of the zones favorable for each agricultural strategy may vary through time somewhat independently of one another.