GEOPHYSICAL EXPRESSION OF THE YUKON FLATS BASIN, EAST-CENTRAL ALASKA

Yukon Flats is a large area of lowland and rolling hills along the Yukon River in east-central Alaska. Because the region is mostly covered by Quaternary alluvium and there are no deep wells to constrain physical properties of the rocks, geophysical studies can do much for interpreting the subsurface geology. The area is geophysically characterized by an isostatic gravity low that is about 250 km east-west and 100 km north-south with an anomaly of about 25 mGal. There are numerous smaller gravity lows of about 25 mGal within the large low that indicate the presence of lower density rocks such as silicic intrusives or basin-filling sedimentary rocks. Seismic interpretations have helped resolve the nature of a number of these lows, but some are still poorly understood. A seismic reflection survey, along with gravity measurements along the seismic lines, was used to evaluate one of these lows for petroleum potential. Rocks outcropping outside the area suggest that the Yukon Flats sedimentary rocks may be Tertiary fluvial and lacustrine deposits and basement rocks may be oceanic in origin and Devonian to Jurassic in age. A forward gravity model constructed from interpreted seismic reflection data indicates what could be a fault-bounded 7,600 m (25,000 f) deep basin filled with low to medium density, probably sedimentary rocks adjacent to a 1,200-2,700 m (4,000-9,000 f) thick section of fairly flat-lying low density sediments. It is probable that the rocks filling the deep basin are thicker accumulations of the same rocks in the surrounding flat-lying sediments. It is also presumed that oceanic rocks lay beneath these sedimentary sections. The boundaries of the model bodies were derived from interpreted seismic reflectors from a depth-migrated section. Converting depth-migrated velocities to densities yielded initial model densities for the bodies.