Dynamics of sedimentation and geomorphic history of Chaco Canyon National Monument, New Mexico
— David W. Love

Chaco Canyon, located in the middle of the San Juan Basin in northwestern New Mexico, is famous for spectacular multistoried stone pueblos built about 850 to 1000 years ago. The reasons for the flourishing and subsequent disappearance of these prehistoric people remain obscure. In order to provide answers about the way the people lived in what is presently a semiarid, uncompromising environment, the National Park Service Chaco Center of the University of New Mexico initiated a multidisciplinary investigation of the archaeology, geology and ecology of Chaco Canyon National Monument. As a result, several new reports discuss the geology of the area. Siemers and King (1974) described the bedrock geology and paleontology of the canyon. DeAngelis (1972) summarized the physical geography of the Chaco drainage. Hall (1975) investigated the palynology of the alluvial fill. Nichols (1975) tested the possibility of magnetically dating the sediments of the canyon floor and Ross (in progress) is examining the depth of the canyon fill and the detailed sedimentology of well cores of the alluvium. Investigations by the U.S. Geological Survey concern geomorphic processes and water quality in the canyon (Malde, personal commun.; Lyford, personal commun.).

 

The purpose of this paper is to summarize the results of my investigation of the dynamics of Quaternary sedimentary deposits in Chaco Canyon. The study included an investigation of the relationships between climate and geomorphic and sedimentologic conditions in the upper Chaco River drainage basin. Consequently, the conditions involved in filling former arroyos are interpreted from arroyo fills exposed in the walls of the present arroyo. The possible conditions in the canyon during the period of extensive occupation by prehistoric peoples, until about 850 years ago, are interpreted from sedimentologic constraints and from tree rings. Besides arroyo cuts-and-fills which predate the pueblos, there is evidence of other geomorphic regimes in the canyon which extends the geomorphic history of Chaco Canyon.

 

The most famous report on the geology of Chaco Canyon is by Bryan (1954). Besides summarizing the bedrock geology of Chaco Canyon, Bryan interpreted the history of the formation of the canyon and its subsequent alluvial fill. He explored the relationships between climate, vegetative' cover and cycles of arroyo cut-and-fill. Bryan interpreted a buried arroyo containing pottery in the base of the channel as a major factor in the abandonment of the pueblos. His interpretations of the history of Chaco Canyon have been used by archaeologists and geologists for decades, but the results of the present investigation do not substantiate Bryan's conclusions about the relation-ships between stream behavior and climate, about the initiation of the modern arroyo and about the conditions in the canyon during the occupation of the pueblos.