Marine stratigraphy and amino-acid geochronology of the Gubik Formation, western Arctic Coastal Plain, Alaska

The Pliocene/Pleistocene Gubik Formation that mantles the Arctic Coastal Plain includes wedges of marine sediment that represent one of the most complete records of high sea level stands in Arctic North America. These marine sequences have been subdivided into six members, each of which corresponds to a specific aminozone representing a depositional time period when sea level was above present. The oldest marine units, the Nulavik and Killi Creek Members, are the most poorly preserved along Skull Cliff and represent high sea level stands that took place > 2.2 and between 1.7 and 2.2 m.y. ago, respectively. An early Pleistocene transgression represented by deposits up to 33-36 m a.s.l. of the Tuapaktushak Member occurred sometime between 0.7 and 1.7 m.y. B.P. Extralimital mollusk faunas in these latter deposits suggest that coastal conditions along northern Alaska were warmer than they are today and that the Arctic Ocean may have lacked perennial sea ice during a portion of the early Pleistocene. The Karmuk Member consists of marine sediments deposited when sea level reached about 23 m a.s.l. about 500 ka B.P. and is characterized by marine conditions similar to today. About 12b ka B.P. sea level reached 8 to 10 m a.s.l. and beach and lagoonal sediments of the Walakpa Member were deposited. Marine conditions in the Chukchi Sea at that time were similar to today: however, several mollusks that today reach their northern limits in the Chukchi Sea expanded their ranges into the Beaufort Sea at this time.