TEMPORAL AND SPATIAL PATTERNS OF DEGLACIATION AND ALLUVIAL FAN DEPOSITION IN THE SOUTHERN ROCKY MOUNTAINS, COLORADO, USA

Alpine glaciers are commonly viewed as sensitive indicators of past climatic variability. We use moraine boulder surface exposure ages from two valleys in the southern Rocky Mountains, Colorado, and detailed (24k-scale) surficial geologic mapping, to constrain the timing of deglaciation and compare moraine chronologies to numeric geochronology of alluvial fan surfaces below a glaciated catchment.

We collected 17 boulder samples from 5 moraines for ¹⁰Be exposure ages; a down-valley lateral moraine (n=6) and high-elevation till (n=2) in Little Cochetopa (LC) valley, southern Sawatch Range, and an up-valley lateral moraine (n=3) and pair of inset down-valley lateral moraines (n=3, n=3) at Zapata Creek (ZC), Sangre de Cristo Range.

At LC, boulders from the lateral moraine have an error weighted mean age and standard deviation of 21.8 ± 2.1 (48.2 ± 3.3, outlier age in parentheses) ka. Two boulders on the till show evidence of substantial surface erosion and have minimum ages of 127.6 ± 8.1 and 93.1 ± 5.9 ka. Mapping at ZC demonstrates clear stratigraphic relations between moraine and alluvial fan units. Previous work used U-Th series of pedogenic carbonate to determine that intermediate and old alluvial fan surfaces on the southern side of the fan are older than 26.2 ± 1.2 ka and 115.1 ± 1.2 ka, respectively. In ZC valley, which debouches on the north side of the fan, our exposure ages show down-valley moraines are 17.5 ± 0.1 (29.5 ± 1.3) ka on the S side of the valley and 20.6 ± 2.1 (39.2 ± 2.5) ka on the N side of the valley. On the higher, outer moraine, 3 boulders have ages of 0.8 ± 0.1, 0.7 ± 0.1, and 55.4 ± 3.5 ka, which we consider unrepresentative of the moraine age. We use the surface stratigraphy and ages of the lower moraines in a Bayesian model to refine our interpretations of depositional ages at ZC.

Our geochronology and mapping demonstrate that 1) LGM deglaciation at LC and at ZC ~115 km to the SE was synchronous and is consistent other LGM exposure age chronologies in the S and N Rocky Mountains, and 2) intermediate and old alluvial fan surfaces on the southern part of the ZC alluvial fan were active during glacial conditions and abandoned prior to deglaciation at ~20-17 ka. This abandonment may have been a result of deposition of a thick end moraine complex that may have blocked the valley and pushed Zapata Creek to the north side of the alluvial fan.