Dating of Late Pleistocene and Holocene Fill Terraces: a Record of Fluvial Deposition and Incision, Colorado Front Range

Cosmogenic 26Al, 10Be, and 14C dating of fluvial fill terraces in steep canyons of the Colorado Front Range provides a temporal framework for analysing episodic aggradation and incision. Results from Boulder Canyon show that terrace heights above the modern channel (grade) can be divided into: (1) Bull Lake ( 100 ka; 20–15 m above grade); (2) Pinedale (32–10 ka; 15–4 m above grade); and (3) Holocene age (<4 m above grade). No pre-Bull Lake deposits are preserved along Boulder Canyon, and only three small remnants >15 m above grade record Bull Lake deposition. Wellpreserved terraces of Pinedale age suggest that the range of terrace height above grade reflects short-term fluctuations in the river profile during periods of rapidly changing stream load and power. Net river incision apparently occurred during transitions to interglacial periods. Soil development and stratigraphic position, along with limited cosmogenic and 14C dating, suggest that 3⁄4130 ka terraces in Boulder Canyon correlate with the Louviers Alluvium, and that 32 to 10 ka fills in the canyon correlate with the Broadway Alluvium on the adjacent High Plains. Late Pleistocene incision rates (3⁄40Ð15 m ka 1) along Boulder Canyon exceed pre-late Pleistocene incision rates, and are higher than middle to late Pleistocene incision rates (3⁄40Ð04 m ka 1) on the High Plains. This study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments. Copyright  2002 John Wiley & Sons, Ltd.