At least three marine transgressions of Pliocene age are recorded by littoral to inner-shelf sediments of the Gubik Formation, which mantles the Arctic Coastal Plain of northern Alaska. The three recognized transgressions were eustatic high sea levels that, from oldest to youngest, are informally named the Colvillian, Bigbendian, and Fishcreekian transgressions. The geochronology is based upon amino acid geochemistry, paleomagnetic studies, vertebrate and invertebrate paleontology, and strontium isotope age estimates. Pollen, plant macrofossils, and marine vertebrate and invertebrate remains indicate that these trangressions occurred when the Arctic was at least intermittently much warmer than it is now. The Colvillian transgression took place at sometime between 2.48 and 2.7 Ma, when adjacent coastal areas supported an open boreal forest or spruce-birch woodland with scattered pine and rare fir and hemlock. The Bigbendian transgression occurred about 2.48 Ma. Climatic conditions were probably slightly cooler than during the Colvillian transgression, but probably too warm for permafrost and too warm for even seasonal sea ice in the region. Nearby vegetation was open spruce-birch woodland or parkland, possibly with rare scattered pine. The Fishcreekian transgression took place sometime between 2.14 and 2.48 Ma and was also characterized by warm marine conditions without sea ice. During the waning stages of this transgression, however, terrestrial conditions were relatively cool, and coastal vegetation was herbaceous tundra with scattered larch trees in the vicinity. Other marine units from this time period occur around the Arctic Basin. The three oldest trangressions recognized from the Seward Peninsula may be broadly correlated with the three Pliocene transgressions of the Arctic Coastal Plain. The Tusatuvayam beds in Kamchatka possibly correlate with one of the two younger transgressions of northern Alaska. The non-marine Worth Point FoFation of Banks Island may be younger than all three of the transgressions of the Arctic Coastal Plain, and marine sediment of the Beaufort Formation on Meighen Island is slightly older than the Colvillian transgression. None of the Fliocene marine units on Baffin Island can be confidently correlated with the high sea level events of northern Alaska. The upper Kap Kobenhavn Formation and the upper Loden E h Formation of Greenland most likely correlate with the Fishcreekian transgression.

Introduction 1 Acknowledgments 2 Mammals of the Fish Creek Fauna 3 Genus Phenacomys Merriam 4 Phenacomys gryci Repenning, n. sp. 6 Genus Cromeromys Zazhigin 7 Ostracodes of the Fish Creek section 11 Basal assemblage 11 Upper assemblage-the Fish Creek Fauna 14 Mollusks of the Fish Creek Fauna 14 Flora of the Fish Creek section 18 Correlation in the Arctic Ocean Borderland 20 Age of the Fish Creek Fauna 20 Age of the Ocean Point Fauna 24 Age of the Cape Deceit Fauna 24 Age of the Krestovka Section 25 Age of the Kap Kobenhavn Fauna 25 Environment of the Arctic Ocean Borderland between 3.0 and 1.9 Ma 26 References cited 27 Index 29 PLATE [Plate follows index] 1. Scanning electron micrographs of diagnostic ostracode species from Fish Creek section. Contents Ill IV Contents