HR: 1340h
AN: PP33B-1272    [Abstracts]
TI: Moraine pebbles and boulders yield indistinguishable 10Be ages: A case study from Colorado, USA
AU: * Briner, J
EM: jbriner@buffalo.edu
AF: University at Buffalo, Department of Geology 876 NSC, Buffalo, NY 14221, United States
AB: Cosmogenic exposure dating of moraines over the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1-2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of collections of pebbles as a sample type for exposure dating. Twenty-one ¹⁰Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Eleven ¹⁰Be ages from a single-crested Pinedale lateral moraine have an unexpected bimodal age distribution; one mode is 22.0±1.4 ka (3 boulders, 2 pebble collections), the other is 15.0±0.9 ka (1 boulder, 5 pebble collections). Ten ¹⁰Be ages from a single-crested Bull Lake lateral moraine are much more scattered, ranging between 3 and 72 ka, with no statistical difference between pebble (n=5) and boulder (n=5) ages. The preliminary interpretation of the two age modes from the Pinedale moraine is that two glacier maxima of similar extent were attained during the late Pleistocene. Unlike the Pinedale moraine, there is no cluster of ¹⁰Be ages from the Bull Lake moraine, suggesting that moraine degradation has led to anomalously young exposure ages. Regardless of interpretations of moraine age, that ¹⁰Be ages from pebble collections and boulder tops are indistinguishable on moraines of two different ages, and in two different modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.
DE: 0720 Glaciers
DE: 1105 Quaternary geochronology
DE: 4918 Cosmogenic isotopes (1150)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting