Paleosols in the Nanushuk Formation, Brooks Range Foothills, Alaska: Preliminary Observations and Sequence Stratigraphic Implications

By

P.J. McCarthy (University of Alaska, Fairbanks) and D.L. LePain (Alaska Division of Geological and Geophysical Surveys)

Paleosols are increasingly viewed as an important, component in nonmarine sequence stratigraphic analyses, where they mark interfluve sequence boundaries and provide at least qualitative information on relative rates of accommodation change within intervening alluvial strata. However, the application of detailed paleopedological analyses to nonmarine sequence stratigraphic studies is still relatively rare. This study focuses on alluvial and coastal plain deposits of the Cretaceous Nanushuk Formation exposed along the Kanayut River and at Arc Mountain, south of the National Petroleum Reserve, Alaska (NPRA). During the 2001 field season, these sites were logged and sampled for subsequent micromorphological, geochemical and mineralogical analyses. Preliminary observations indicate that pedogenic modification of alluvial deposits is common, however, rapid deposition of sediment and generally wet conditions resulted in a preponderance of weakly-developed and poorly-drained paleosols similar to modern entisols, inceptisols, gleysols and histosols. Sandstone and mudstone paleosols may contain a few large root traces, sometimes expressed as vertical to sub-horizontal coalified structures or as surface cradle knolls, but they maintain evidence of primary stratification and/or cumulic development. Coalified and carbonaceous root traces, organic fragments, blocky structures and abundant siderite nodules provide further evidence of waterlogged conditions. By combining field observations, paleosol micromorphology, geochemistry and mineralogy with regional stratigraphic information it is possible to pin-point the exact position of sequence boundaries in nonmarine rocks. Further work is planned in an attempt to understand the regional distribution of paleosols in the Nanushuk Formation in order to better characterize significant nonmarine depositional hiatuses (sequence boundaries) and Cretaceous high-latitude paleoenvironmental conditions.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.