Abstract

A comparison was made between geologic, radiometric, and paleomagnetic methods of dating and stratigraphic correlation on the Pribilof Islands of Alaska. Age relationships based on geomorphology and paleontology agree with those determined using potassium-argon dating and paleomagnetic reversals, permitting the following synthesis of the geologic and geomagnetic history of the Pribilofs: The two islands formed during two pulses of volcanism separated by about 1 m.y. St. George Island was formed during the earlier pulse, which lasted from 2.2 to at least 1.6 m.y. ago. During this short interval of 0.6 m.y. the geomagnetic field changed polarity from an initial reversed direction to a normal direction and then back again to a reversed direction. The normal flows, which are both underlain and overlain by reversed flows, have ages between 2.0 and 1.8 m.y. They were extruded during the Olduvai normal event, a relatively brief interval of worldwide normal geomagnetic polarity within the otherwise reversed Matuyama polarity epoch, which lasted from 2.5 to 0.7 m.y. ago. A second pulse of volcanism, which began about 1 m.y. after the first, produced St. Paul Island. This pulse has continued for the past one third m.y., an interval of normal polarity.

Fossiliferous marine sedimentary rocks deposited during the Beringian marine transgression are covered on St. George Island by a reversely magnetized pillow lava 2.14 ± 0.08 m.y. old. Fossiliferous marine sedimentary rocks deposited during a transgression younger than the Anvilian and older than the Kotzebuan transgressions are bracketed at the Einahnuhto Bluffs, St. Paul, between normally magnetized lava flows and are less than 0.4 and more than 0.1 m.y. old.

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