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 Palynological and Core Data Suggest Middle-Late Turonian Age for the Avak Impact, Barrow, Alaska*


Arthur C. Banet1,  James P. Fenton2, Thomas P. Walsh3, Thomas C. Morahan3 and

Peter Stokes3


Search and Discovery Article #30099 (2009)

Posted August 26, 2009


*Adapted from oral presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009


1Self, Anchorage, AK   (mailto:[email protected])

2Fugro Robertson Limited, Llandudno, United Kingdom

3Petrotechnical Resources Associates, Anchorage, AK




The Avak impact feature in northernmost Alaska is an integral part of the trapping mechanism for the South, East and Sikulik pools of the Barrow Gas field. The pools are at the periphery of the impact rim. The gas production comes from the Lower Jurassic Barrow sands at the base of the Kingak Formation. Logs and cores show that the upper and lower Barrow sands are coarsening upwards shoreface parasequences and that the sand-rich facies are restricted to the Barrow Arch area.


Barrow Field peculiarities include shallow burial depths (<2,250 ft.), approximately 55% water saturation, and reservoir temperatures between about 55 and 65 degrees F. Gas geochemistry (Pixler Plots) show that the gas at Barrow is comparatively dry; drier than other North Slope gas pools. Reservoir studies show the East Barrow Field is within the methane hydrate stability field and that hydrates contribute to the gas charge. The East Barrow Field also initially had anomalously high Helium concentrations which may be related to the impact origin from deep seated faulting.


Core data from the Avak well, which is at the center of the Avak impact and between the East and South pools, shows uplifted, out of sequence and disrupted stratigraphy, shatter cones and shocked quartz which is typical of impacts. Basement lithologies, Barrow sand and Pebble Shale clasts have been found and described from several of the Simpson Core test wells which are 50 to 70 km southeast of the Barrow Gas Field. These macroscopic exotic clasts are found in the Seabee mudstones. These clasts appear to be ejecta from the Avak impact. Palynological analyses of the core matrix and clasts suggest the Avak impact occurred in the middle to late Turonian.


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Figure 1. Location of the Barrow Structure and distribution of oil types across the North Slope of Alaska.


Figure 2. Location of Late Cretaceous impact sites in North America.


Figure 3. Structure map of Avak Crater area.


Figure 4. Seismic section across Avak Crater.


Figure 5. Model of Avak impact event and subsequent infill deposition.


Figure 6. Model of Avak impact event and subsequent infill deposition.




Collins, F.R. and R.M. Robinson, 1967, Subsurface stratigraphic, structural, and economic geology, northern Alaska: U. S. Geological Survey Open-file Report, 252 p.


Collins, F.R., 1961, Core tests and test wells, Barrow area, Alaska: U S Geological Survey Professional Paper: Report # P, 0305-K, p.569-644.


Kirschner, C.E., A.Grantz, and M.W. Mullen, 1992, Impact origin of the Avak Structure and genesis of the Barrow Gas Fields: AAPG Bulletin v. 75, p. 651-679.


Leeman, B., Extraterrestrial impactors, ESCI 108 Lecture #17, Rice University: Web accessed 29 July 2008.


Osinski, G.R., 2008, Meteorite impact structures: the good and the bad: Geology Today, v. 24/1, p. 13-19.


Robinson, F.M., 1964, Core tests, Simpson area, Alaska: U S Geological Survey Professional Paper: USGS PP Report # P, 0305-L, p. 645-730.


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