August GEOLOGY and GSA TODAY Media Highlights

Much controversy surrounds the mechanism(s) responsible for meter-scale relative sea-level oscillations in the Cretaceous, which have a frequency of much less than one million years. Various lines of evidence are converging to suggest that a small ice cap existed on the Antarctic craton during the Cretaceous 'greenhouse' period. Plint and Kreitner show that meter-scale deepening and shallowing successions can be traced for hundreds of kilometers across the Western Canada foreland basin, and include discrete high- and low-stand deposits. The extent, frequency, and geometry of these sequences strongly suggest an eustatic control, supporting the Antarctic ice cap hypothesis.

Pacing the post–Last Glacial Maximum demise of the Animas Valley glacier and the San Juan Mountain ice cap, Colorado
Zackry S. Guido et al., University of Colorado, Geology, INSTAAR, 1560 30th Street, 450 UCB, Boulder, CO 80309, USA. Pages 739-742.

The lengths of alpine glaciers quickly adjust to changes in temperature and precipitation, making them sensitive indicators of climate. Recording changes in length of current glaciers is a matter of visual observation. However, for glaciers that existed in the past, documenting the changes in length often requires rare geological and glaciological circumstances. The Animas River drainage of Colorado’s San Juan Mountains allows documentation of the demise of a large alpine valley glacier from its Last Glacial Maximum extent. Approximately 20,000 years before present (B.P.), the Animas Valley glacier lobe of the San Juan Ice cap extended 91 kilometers from the ice divide, and pervasively polished crystalline bedrock. To document the retreat history of this glacier, Guido et al. measured the concentration of cosmogenically produced 10Be in polished bedrock to deduce the duration of exposure to cosmic rays since the glacier receded past each of eight locations. This yielded a record of retreat that began at approximately 19,500 years B.P and ended at roughly 12,500 years B.P., when the San Juan Mountains became largely devoid of ice. This history implies that the demise of the Animas Valley glacier was protracted, coincided with a gradual rise in solar radiation, and was perhaps fastest during a time of regional drying recorded in shoreline elevations of lakes in western North America.

Tectonic uplift, threshold hillslopes, and denudation rates in a developing mountain range
Steven A. Binnie et al., University of Edinburgh, Institute of Geography, Drummond Street, Edinburgh, Midlothian EH8 9XP, UK. Pages 743-746.