Absolute Dating of Strath Terraces along the Western High Plains Reveals Complicated History of Occupation and Incision

Friday, 18 December 2015
Poster Hall (Moscone South)
Melissa A Foster, University of Colorado at Boulder, INSTAAR and Geological Sciences, Boulder, CO, United States, Robert S Anderson, University of Colorado at Boulder, INSTAAR and Department of Geological Sciences, Boulder, CO, United States, Harrison J Gray, University of Colorado at Boulder, Earth Sciences, Boulder, CO, United States and Shannon Mahan, United States Geological Survey, Denver, CO, United States
Gravel-capped strath terraces are preserved along the western High Plains, adjacent to the Colorado Front Range. Terrace elevations appear to connect across the landscape, representing previous levels of the Denver Basin during fluvial exhumation. In this tectonically quiescent region, climatically-driven fluctuations in sediment supply are often interpreted to drive terrace occupation and basin exhumation.

We dated a series of strath terraces near Boulder, CO using 10Be/26Al cosmogenic radionuclides (CRNs) and optically stimulated luminescence. Results reveal: (1) a long history of fluvial occupation (0.5-1.0 Myr duration), with recent (<120 ka) fluvial abandonment of the highest surface; (2) a brief occupation of a narrow spillway, abandoned at 97.0 ± 4.2 ka, preserved because the stream was subsequently pirated; and (3) a long period of fluvial occupation of a lower terrace, recorded by the deposition of distinct gravel packages at 50-75 ka and 40.5 ± 5.0 ka.

In conjunction with absolute dating on nearby terraces, our study indicates the system is more complex than originally thought. Ages are not consistent among terraces of similar elevations deposited by different stream systems. The link between terrace age and climate is not straightforward, as terraces can be occupied through several glacial-interglacial cycles. When a surface is abandoned, incision to the next level appears to occur quite rapidly through the soft Cretaceous shales. Long-term incision rates may be derived from elevations and ages of high preserved surfaces, but such incision rates reflect long periods of no vertical erosion punctuated with short periods of rapid incision. Such interpretations would not be possible without paired CRNs or combination with other geochronologic techniques. This study therefore represents a cautionary tale for the interpretation of tectonic or climatic processes and process rates from single-method dating of fluvial terraces.