STRATIGRAPHY AND DEPOSITIONAL SETTING OF SUBSURFACE NONMARINE TERTIARY (MIOCENE) SEDIMENTARY ROCKS, FORT YUKON, ALASKA

Tertiary nonmarine sedimentary rocks beneath Fort Yukon, Alaska were drilled and cored in 2004 as part of a joint effort between the Alaska Division of Geological & Geophysical Surveys, U.S. Geological Survey, U.S. Bureau of Land Management-Alaska, University of Alaska Fairbanks, and U.S. Department of Energy-Arctic Energy Office to evaluate the shallow gas potential of lignite seams beneath Fort Yukon. This effort reentered a 1994 U.S. Geological Survey drill hole that was originally drilled and cored to 1,281 feet to study Tertiary paleoclimates. The 2004 drilling operations drilled to a total depth of 2,287 feet and recovered about 650 feet of core from core drilling, and cuttings were collected from about 330 feet of rotary drilling. We studied the 2004 lower Fort Yukon core in order to describe its stratigraphy and interpret its depositional settings.

The 1994 and 2004 drilling combined encountered two major coal zones, an upper coal zone about 58 feet thick (from 1,257 to 1,315 feet) and a lower 45-ft-thick coal zone (from 1,875 to 1,920 feet). The upper coal zone is middle Miocene in age (16-18 Ma) and the age of the lower coal zone is likely Miocene in age as well (T. Ager, USGS, personal communication). Lithologies within the Fort Yukon core are organized into eight general lithofacies that are: coal (lignite), carbonaceous shale, claystone, silty claystone, siltstone, silty sandstone, sandstone, and pebble sand. Grain types within the silty claystone to sandstone include polycrystalline and monocrystalline quartz, chert, white mica, lithic clasts, and traces of plagioclase. Beginning at ~2,170 feet are pebbly sand interbeds of unknown thickness that coincide with a prominent reflector recognized in a 2001 shallow seismic study of Fort Yukon.

Generally, the lower Fort Yukon core consists of stacked sequences of sandstone, siltstone and claystone, ±carbonaceous shale and/or coal. We interpret these sediments to represent a meandering river to lacustrine and sometimes poorly drained swamp system. The fluvial settings fine upwards from sand- to silt-dominated facies into eventual lake and sometimes a poorly-drained swamp environment above, represented by carbonaceous shale or lignite. Parallel laminations in some claystone horizons suggest varved lake deposits representing seasonal variations in sedimentation.