Glaciers originating in the central Brooks Range extended south into the lower Alatna Valley during three major episodes of Illinoian and Wisconsin glaciation. During the oldest glaciation, coalescing glaciers formed a piedmont ice sheet that extended 60 miles south of the range and covered most of the Koyukuk Lowlands. The later Kobuk Glaciation, subdivided into two stades, was marked by a smaller lobe that terminated in the Koyukuk Lowlands, receded, then readvanced to a frontal position within the Alatna Valley 25 miles closer to the Brooks Range. Drift of the Itkillik Glaciation, which marks the final period of ice advance into the lower Alatna, is subdivided into four moraine belts. Siruk Creek moraines, the oldest and most extensive Itkillik deposits, form an arcuate belt that extends 30 miles south of the range front. The later Chebanika moraine marks a brief readvance or stillstand of glacier ice farther upvalley. Ice then stagnated in the Helpmejack Lakes area, and massive bodies of ice-contact stratified drift were deposited in a broad belt 8 to 15 miles south of the Brooks Range. Moraines at the range front represent the last major event of the Itkillik Glaciation. Subdivisions and correlations of the glacial sequence are based on extent and position of the drift sheets, inferred glacier regimens, postglacial modification of glacial deposits, radiometric dates, and ice limits within the upper Alatna Valley. The four episodes of Itkillik glaciation within the Alatna drainage system appear to be equivalent to four Itkillik stades in the Anaktuvuk Valley, and radiocarbon dates from both areas indicate a late Wisconsin age for Itkillik deposits. The preceding ice advance, correlated with the Kobuk Glaciation farther west, may represent the early Wisconsin. The oldest glaciation is correlated with an ice advance of Illinoian age that filled the Kobuk Valley and extended west into Kotzebue Sound.

Abstract The South Chukchi Basin separates the late Mesozoic Chukotka Fold Belt from the Wrangel Arch and represents the northwestern continuation of the Hope Basin of the United States sector of the Chukchi Sea, which is filled with middle Eocene–Quaternary nonmarine, marine, and lacustrine rocks. The main stages of South Chukchi Basin development in the Cenozoic are comparable to those of the Hope Basin, although the analysis of onshore data from Chukotka and Wrangel Island points to the beginning of sedimentation during the Aptian–Albian–Late Cretaceous. In the South Chukchi Basin, the sediment thickness seldom exceeds 3 to 4 km (1.9–2.5 mi)but can locally reach 5to 6 km(3.1–3.7 mi).The geometry of the faults indicates an extensional and/or transtensional setting for the South Chukchi Basin, although folds, reverse and thrust faults, pop-up and positive flower structures also occur, pointing to the local development of compressional and transpressional stress. Low-angle thrust faults predating the Aptian(?)–Paleogene extension (most likely of Late Jurassic–Neocomian age) are recognized at the base of the South Chukchi Basin. This could support the idea that the extension in the basin was driven by gravitational collapse of the Wrangel-Herald-Lisburne fold and thrust belt in the post-Neocomian. Based on the interpretation of new seismic data and analysis of published material, we believe that the hydrocarbon potential of the South Chukchi Basin may be significantly higher than what has been previously suggested.