ABSTRACT: Impact Origin of the Avak Structure, Arctic Alaska, and Genesis of the Barrow Gas Fields

Charles E. Kirschner, Arthur Grantz

Geophysical and subsurface geologic data confirm that the Avak structure, which underlies the coastal plain 12 km southeast of Barrow, is an impact crater. The structure is a roughly circular area of chaotically deformed Upper Triassic to Lower Cretaceous sedimentary rocks 8 km in diameter bounded by a ring of anastomozing, inwardly dipping, listric normal faults. Beyond the ring, these rocks are almost flat. Basement is strongly deformed Ordovician and Silurian argillite. Strong density and seismic velocity contrasts between the argillite and the overlying strata produce gravity and seismic reflection signatures that define ring anticlines around the disturbed zone and a structural high at its center.

The Mesozoic strata are about 760 m thick in the adjacent Barrow gas fields, where the Neocomian pebble shale unit and the gas-producing Lower Jurassic Barrow sandstone lie at average subsea depths of 488 m and 670 m, respectively. In the Avak well, drilled on the central high, the pebble shale unit and Barrow sandstone lie near the surface, documenting more than 500 m of uplift at the high. The cores in this well also show steep dips (30-90°), abundant tectonic breccia with argillite clasts 90 m above basement, fractured quartz grains, and shatter cones resembling those found in well-documented meteorite craters.

Stratigraphic data suggest that the Avak meteorite struck a late Early Cretaceous marine shelf, produced peripheral highs that trapped gas in the Barrow fields, and triggered massive landslides on the adjacent outer shelf. The age of the landslides dates the impact at about 105 Ma.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990