EVIDENCE FROM THE FRONT RANGE, COLORADO, INDICATES THAT PINEDALE GLACIATION BEGAN BEFORE 31,000 YR AGO

Cosmogenic ages of boulders on terminal moraines indicate that the last glacial maximum (LGM) occurred at about the same time (≈ 20 ± 1.5 ka) in several ranges of the Southern Rocky Mountains, including the Front Range, Colorado. However, three AMS ages of microscopic plant and animal material in sediment of the last phase of glacial Lake Devlin indicate that by 31 ka Pinedale valley glaciers on the east slope of the Front Range were nearly as large as they were during the LGM. Hence, we conclude that the beginning of the Pinedale glaciation was much earlier than previously supposed.

Lake Devlin was at least 1.5 km long and initially as much as 60 m deep. It formed when the North Boulder Creek glacier blocked drainage from a tributary at a point 2.2 km up valley from the Pinedale glacial limit. The upper reaches of the tributary contained two small valley glaciers. Bedload in outwash from these glaciers was trapped in small basins up valley from Lake Devlin, but the suspended load (almost entirely silt and clay) accumulated in Lake Devlin, attaining thicknesses >36 m. Only the upper 19 m of lacustrine sediment was cored. Attempts to core through a 6 m-thick gravel bed that separates sediment of an earlier phase of Lake Devlin from that of the late phase were unsuccessful.

Chironomids indicate that at its inception, the last phase of Lake Devlin was oligotrophic and very cold. Material from the basal 1 m of the same core, sampled at three levels (6–9 cm, 14–16 cm, and 59-61 cm) above the underlying gravel, provided ages of 25,125 ± 110 (WW-6078), 23,830 ± 80 (Aeon-246), and 22,460 ± 90 (Aeon-247) ¹⁴C yr BP, respectively. The 2σ cal-yr ranges of these ages (using CALIB 6.01) are 30,288–29,568; 29,081–28,211; and 27,734–26,682. The last phase of Lake Devlin is estimated to have begun between 31,570 and 30,850 cal yr BP. This estimate is arrived at by extrapolation using the sedimentation rate determined for the interval between the lower two AMS ages (7.5 cm in 1280 cal yr). The interval between the lowest AMS age and the gravel is also about 7.5 cm. Thus, the base of the lacustrine sediment is estimated to be about 1280 yr older than the oldest AMS age.