The Santa Cruz River terraces near Tubac, Santa Cruz County, Arizona

Abstract

Nine Quaternary surfaces along the Santa Cruz River reflect degradation subsequent to basin filling. Five alluvial-geomorphic surfaces represent stages of aggradation in a net trend of channel downcutting. Upstream of the mountain-bounding fault, longitudinal surface profiles do not converge in a downstream direction and downstream of this zone there are no thicker sediment wedges. This indicates that the mountain front was tectonically inactive and climatic change initiated shifts from aggradation to degradation. Terrace soils indicate that four major climatic changes have occurred in the region. These changes resulted in downcutting events during the early Pleistocene, mid-Pleistocene, late Pleistocene and early Holocene. The timing of last movement on two piedmont faults indicates that they were active during the late Pleistocene. Oxalate to dithionite extractable iron ratios are a maximum for early Holocene soils, declining to minimum values for early Pleistocene soils.