Abstract This field-trip guide outlines the glacial history of the upper Arkansas River valley, Colorado, and builds on a previous GSA field trip to the area in 2010. The following will be presented: (1) new cosmogenic 10 Be exposure ages of moraine boulders from the Pinedale and Bull Lake glaciations (Marine Isotope Stages 2 and 6, respectively) located adjacent to the Twin Lakes Reservoir, (2) numerical modeling of glaciers during the Pinedale glaciation in major tributaries draining into the upper Arkansas River, (3) discharge estimates for glacial-lake outburst floods in the upper Arkansas River valley, and (4) 10 Be ages on flood boulders deposited downvalley from the moraine sequences. This research was stimulated by a new geologic map of the Granite 7.5′ quadrangle, in which the mapping of surficial deposits was revised based in part on the interpretation of newly acquired LiDAR data and field investigations. The new 10 Be ages of the Pinedale terminal moraine at Twin Lakes average 21.8 ± 0.7 ka ( n = 14), which adds to nearby Pinedale terminal moraine ages of 23.6 ± 1.4 ka ( n = 5), 20.5 ± 0.2 ka ( n = 3), and 16.6 ± 1.0 ka ( n = 7), and downvalley outburst flood terraces that date to 20.9 ± 0.9 ka ( n = 4) and 19.0 ± 0.6 ka ( n = 4). This growing chronology leads to improved understanding of the controls and timing of glaciation in the western United States, the modeling of glacial-lake outburst flooding, and the reconstruction of paleotemperature through glacier modeling.

Abstract This field guide focuses on the glacial geology and paleoecology beginning in the Paradise Valley and progressing southward into northern Yellowstone National Park. During the last (Pinedale) glaciation, the northern Yellowstone outlet glacier flowed out of Yellowstone Park and down the Yellowstone River Valley into the Paradise Valley. The field trip will traverse the following Pinedale glacial sequence: (1) deposition of the Eightmile terminal moraines and outwash 16.5 ± 1.4 10 Be ka in the Paradise Valley; (2) glacial recession of ~8 km and deposition of the Chico moraines and outwash 16.1 ± 1.7 10 Be ka; (3) glacial recession of 45 km to near the northern Yellowstone boundary and moraine deposition during the Deckard Flats readjustment 14.2 ± 1.2 10 Be ka; and (4) glacial recession of ~37 km and deposition of the Junction Butte moraines 15.2 ± 1.3 10 Be ka (this age is a little too old based on the stratigraphic sequence). Yellowstone's northern range of sagebrush-grasslands and bison, elk, wolf, and bear inhabitants is founded on glacial moraines, sub-glacial till, and outwash deposited during the last glaciation. Floods released from glacially dammed lakes and a landslide-dammed lake punctuate this record. The glacial geologic reconstruction was evaluated by calculation of basal shear stress, and yielded the following values for flow pattern in plan view: strongly converging—1.21 ± 0.12 bars ( n = 15); nearly uniform—1.04 ± 0.16 bars ( n = 11); and strongly diverging—0.84 ± 0.14 bars ( n = 16). Reconstructed mass balance yielded accumulation and ablation each of ~3 km 3 /yr, with glacial movement near the equilibrium line altitude dominated by basal sliding. Pollen and charcoal records from three lakes in northern Yellowstone provide information on the postglacial vegetation and fire history. Following glacial retreat, sparsely vegetated landscapes were colonized first by spruce parkland and then by closed subalpine forests. Regional fire activity increased significantly with the development of closed subalpine forests as a result of increased fuel biomass and warmer summers. Warm dry conditions prevailed at low elevations during the early Holocene, as indicated by the presence of steppe and open mixed conifer forest. At the same time, closed subalpine forests with low fire frequency were present at higher elevations, suggesting relatively wet summer conditions. Douglas fir populations expanded throughout northern Yellowstone in the middle Holocene as a result of effectively drier conditions than before, and a decline of mesophytic plant taxa during the late Holocene imply continued drying, even though fire frequency decreased in recent millennia.