Optical dating of arroyo-system deposits: Insights into cut-fill cycles in the southern Colorado piedmont

Optical ages are presented from a range of small-scale arroyo systems found across Fort Carson and Pinon Canyon Maneuver Site in southern Colorado. Recent developments in both single-aliquot and single-grain techniques combined with more rigorous statistical methods for analysing paleodose estimates now mean that optical dating is able to provide much more accurate and precise constraints on the timing of major depositional changes in these fluvial systems. The arroyos of the region display distinct and multiple paleochannels along their side walls. Optical dating of infill deposits from within these paleochannels and from intervening alluvial fill units provides the first detailed and accurate chronology of past and present arroyo epicycle histories in the region. Resulting ages suggest that cut-fill behaviour has been active to varying degrees over much of the Holocene, particularly from the mid-Holocene onwards. Evidence also points towards a strongly episodic pattern of arroyo incision, separated by more protracted periods of stability and/or aggradation. Correlation of arroyo incision and subsequent infilling around the mid-Holocene Altithermal event in several paleochannel sequences from one arroyo system (the "Little Grand Canyon", Fort Carson) provides evidence for external climatic control on major Holocene cut-fill cycles in the region. Dating sequences from different locations within the same systems, and from different systems within the area, enables an intercomparison of arroyo epicycles at a multitude of spatial scales. The complex spatio-temporal dynamics emerging suggest that time-transgressive responses may be common both within and between the arroyos of this region. These findings imply either that basin response times to external forcing vary significantly with basin size, or that local-scale forcing mechanisms have been dominant controls on past arroyo dynamics in this region.