New interpretations of alluvial and paleo-vegetation records from Chaco Canyon, New Mexico

Abstract

A BSTRACT — The alluvial stratigraphy, vegetation history, and paleoecology of Chaco Canyon are re-evaluated. The alluvial units are Fajada (middle-late Pleistocene), Pre-Gallo (undated), Gallo (6.7 to 2.8 ka), Chaco (2.1 to 1.0 ka), and Bonito (0.8 to 0.1 ka). A new alluvial unit, Pre-Gallo, is a small erosional remnant that may be late Pleistocene or early Holocene in age. The hot-dry climate of the mid-Holocene and accompanying low stream flow resulted in the accumulation of local sand in the canyon, forming the Gallo unit. A shift to less arid climate by 2.5 ka and increased stream flow flushed out much of the Gallo sand. Greater stream flow throughout the drainage basin resulted in clay overbank deposition across the canyon floor, forming the Chaco unit. The canyon floor provided a habitat for a molluscan fauna consisting of two freshwater snails and six terrestrial snails. The end of the accumulation of the Chaco unit occurred ca. 1.0 ka with a shift in climate to dry conditions and the downcutting of the Bonito channel. The Bonito channel was filling by ca. 0.8 ka, based on buried potsherds. The vegetation of the Chaco area has been a desert-shrub grassland since the end of the Pleistocene. Within that framework, a pinyon-juniper woodland thrived on sandstone substrates during the late Pleistocene-early Holocene. However, mid-Holocene aridity led to the demise of the woodlands, leaving behind a few individuals of pinyon pine and juniper on sandstone escarpments. Reduced rainfall, high evaporation, and increased soil alkalinity promoted the expansion of chenopod shrubs during the mid-Holocene. Moister conditions by 2.5 ka resulted in a reduction in chenopod shrubs and expansion of pinyon pine and juniper in isolated stands on local escarpments and in higher areas of sandy soils within the desert shrub grassland, similar to present-day vegetation patterns. Slight shifts in regional climate after ca. 1.0 ka to dry then wet conditions, while significant to fluvial geomorphology, does not show up in the local vegetation record.