Geologic setting and stratigraphy of the Ziegler Reservoir fossil site, Snowmass Village, Colorado

The geologic setting of the Ziegler Reservoir fossil site is somewhat unusual—the sediments containing the Pleistocene fossils were deposited in a lake on top of a ridge. The lake basin was formed near Snowmass Village, Colorado (USA) when a glacier flowing down Snowmass Creek Valley became thick enough to overtop a low point in the eastern valley wall and entered the head of Brush Creek Valley. When the glacier retreated at about 155–130 ka, near the end of Marine Oxygen Isotope Stage 6, the Brush Creek Valley lobe left behind a moraine that impounded a small alpine lake. The lake was initially ~10m deep and appears to have been highly productive during most of its existence, based on the abundant and exquisitely preserved organic material present in the sediments. Over time, the basin slowly filled with (mostly) eolian sediment such that by ~87 ka it contained a marsh or wetland rather than a true lake. Open-water conditions returned briefly between ~77 and 55 ka before the impoundment was finally breached to the east, establishing ties with the Brush Creek drainage system and creating an alpine meadow that persisted until historic times

Geologic setting and stratigraphy of the Ziegler Reservoir fossil site, Snowmass Village, Colorado

The geologic setting of the Ziegler Reservoir fossil site is somewhat unusual—the sediments containing the Pleistocene fossils were deposited in a lake on top of a ridge. The lake basin was formed near Snowmass Village, Colorado (USA) when a glacier flowing down Snowmass Creek Valley became thick enough to overtop a low point in the eastern valley wall and entered the head of Brush Creek Valley. When the glacier retreated at about 155–130 ka, near the end of Marine Oxygen Isotope Stage 6, the Brush Creek Valley lobe left behind a moraine that impounded a small alpine lake. The lake was initially ~10m deep and appears to have been highly productive during most of its existence, based on the abundant and exquisitely preserved organic material present in the sediments. Over time, the basin slowly filled with (mostly) eolian sediment such that by ~87 ka it contained a marsh or wetland rather than a true lake. Open-water conditions returned briefly between ~77 and 55 ka before the impoundment was finally breached to the east, establishing ties with the Brush Creek drainage system and creating an alpine meadow that persisted until historic times

Geochronology of Merrell Locality Strata and Regional Paleoenvironmental Contexts

An assessment of the age of the deposits and the fossils incorporated within them at the Merrell Locality is based on radiocarbon measurements from bone and tusk collagen and organic sediments (Table 3 and Figures 75-76), and luminescence measurements on sediments (Feathers, this report). Nine radiocarbon dates are available from the site; seven are finite and two are infinite. The finite dates range from ca. 49,000 to 19,000 14C yr. B. P. (Figure 75)

Geomorphic Features and History of the Lower Part of Logan Canyon, Utah

Logan Canyon is located east of Logan, Utah, in the Bear River Range. The lower part of Logan Canyon is considered that section of the canyon from its mouth upstream to Tony Grove Canyon, a distance of 22 miles, Figure 1 Some tributary canyons of the lower part of Logan Canyon have been included in this investigation because of their relationship to Logan Canyon. Grassy Flat Canyon, a south tributary of Logan Canyon 4.4 miles from Logan, exhibits several geomorphic features related to the geology of Logan Canyon. Because of its extensive use and close association with Logan Canyon, Tony Grove Canyon is also included. Tony Grove Canyon extends from Logan Canyon northwestward to the crest of the Bear River Range, a distance of about six miles. More than 20,000 feet of Paleozoic rocks ranging in age from Cambrian to Pennsylvanian are exposed in Logan Canyon . Cenozoic deposits are widespread in and near the canyon. The crest of the Bear River Range near Naomi Peak and Tony Grove Canyon was the center of glacial activity during the Pleistocene. During the glaciations of Tony Grove Canyon, Lake Bonneville extended into Logan Canyon and influenced the geomorphic development near the mouth of the canyon. Logan Canyon is vital to the economy of Cache Valley. The canyon is a large part of the Logan River watershed. Logan River passes through three hydroelectric plants in Logan Canyon and supplies culinary and irrigation water for the valley below. Animals and plants of a wide variety are abundant, providing fishing , hunting, and a harvest of forest products. U.S. highway 89 traverses the canyon and carries a large volume of traffic to points within the canyon, as well as to other areas. Logan Canyon is entirely within the Cache National Forest. Improved campsites and recreational facilities, which were used by almost 1.5 million visitors during 1963; according to the U. S. Forest Service, are located throughout the Canyon. Increased recreational and travel use of the canyon has resulted in a demand for more geologic work in this area

Late Pleistocene Glaciation in the Mosquito Range, Colorado, U.S.A.: Chronology and Climate

New cosmogenic 10Be surface exposure ages from seventeen moraine boulders in the Mosquito Range suggest that glaciers were at their late Pleistocene (Pinedale) maximum extent at ~21–20 ka, and that ice recession commenced prior to ~17 ka. These age limits suggest that the Pinedale Glaciation was synchronous within the Colorado Rocky Mountain region. Locally, the previous (Bull Lake) glaciation appears to have occurred no later than 117 ka, possibly ~130 ka allowing for reasonable rock weathering rates. Temperature-index modeling is used to determine the magnitude of temperature depression required to maintain steady-state mass balances of seven reconstructed glaciers at their maximum extent. Assuming no significant differences in precipitation compared to modern values, mean annual temperatures were ~8.1 and 7.5 °C cooler, respectively, on the eastern and western slopes of the range with quantifiable uncertainties of +0.8/–0.9 °C. If an average temperature depression of 7.8 °C is assumed for the entire range, precipitation differences - that today are 15-30% greater on the eastern slope due to the influence of winter/early spring snowfall - might have been enhanced. The temperature depressions inferred here are consistent with similarly derived values elsewhere in the Colorado Rockies and those inferred from regional-scale climate modeling