Late Pleistocene Glacial Geology of the Okpilak-Kongakut Rivers Region, Northeastern Brooks Range, Alaska

Glacial mapping combined with cosmogenic exposure dating provides the first detailed assessment of the late Pleistocene glacial history in the northeastern Brooks Range, Alaska. Former ice limits were identified in the Okpilak, Jago, Aichillik, Egaksrak, and Kongakut River valleys. Relative-weathering data support our correlations with the wellstudied glacial sequence of the central Brooks Range. Of the 16 boulders from four moraines analyzed for Be, the exposure ages on two moraines are tightly clustered. The results indicate that the glacier in the Jago River valley reached its maximum extent during the Itkillik II glaciation by about 23.2 6 2.8 ka. The ice retreated rapidly, and a prominent, but local, readvance took place by 19.1 6 2.8 ka within a tributary valley. The relative size of glaciers and the difference in ice extent between the Itkillik I and Itkillik II glaciations varied among the river valleys, with the Aichillik River valley showing the greatest difference. A first-order trend surface fit to Itkillik II equilibrium lines rises toward the south, from 1400 to 1800 m. This is a reversal of the overall trend across the rest of the Brooks Range, and suggests that glaciers of the last glacial maximum in the northeastern sector, similar to today, benefited from the cold and possibly moister conditions associated with their proximity to the Beaufort Sea.