ONSET OF LATE CENOZOIC GLACIATION IN THE GULF OF ALASKA

The Yakataga Formation is a thick 5000+ m package of marine sediments that records changes in environmental conditions, as well as tectonic collision and mountain building in the Gulf of Alaska coast mountains over the last 6 million years. Detailed sedimentological and micropaleontological analyses to date have shown that the Yakataga Formation contains a record of fluctuating sea level, reworking of large amounts of glacigenic sediments, deposition of ice-rafted debris, and scouring by ice. Ongoing studies aim to refine the relative influence of climatic and tectonic controls on this succession as well as the overall tectonic evolution of the adjacent St. Elias/ Chugach mountains. The base of the Yakataga Formation is thought to record the onset of glacial conditions during the Late Cenozoic, though there is controversy over the age of this important boundary. Some suggest an early to mid Miocene age based on molluscan fauna, while others advocate a late Miocene (5.0 to 6.7 Ma) age based on foraminifera and magnetostratigraphy. Chronological constraints on the onset of glacial conditions in the region are key for these studies and for other paleoclimatic research in the NW Pacific.

The outcrops at Cape Yakataga comprise over 600 m of almost continuous exposure through the Yakataga Formation and the underlying Poul Creek Formation. Eighty five samples were collected for paleomagnetic analysis through both formations over a stratigraphic thickness of 380 m. The samples were analyzed for natural remanent magnetization and magnetic susceptibility. Principal component analysis was used to establish magnetic remanence directions, which were then plotted as normal or reversed directions against the stratigraphic log measured in the field. The resulting magnetostratigraphy overlaps part of the previously published magnetostratigraphy, but extends an additional 230 m into the underlying Poul Creek Formation. Where they overlap, the data are consistent. The polarity curve was used to test the two different age models and compared with other geochronological data from the region. The resulting magnetostratigraphy suggests that the onset of glaciation at this site was likely Early Pliocene (~ 4.997- 4.896 Ma) and therefore younger than either of the two scenarios proposed to date.