TIMING OF LATE CENOZOIC TIDEWATER GLACIATION IN THE FAR NORTH PACIFIC

The onset of late Cenozoic glacial events in the far North Pacific Oce an is recorded by ice-rafted debris in the Yakataga Formation in the G ulf of Alaska. The dating of these events is controversial. Ages based on molluscs suggest that initial late Cenozoic tidewater glaciation o ccurred in the early middle Miocene (15-16 Ma). Previous work on plank tic formaminifera indicates that this event is no older than late Mioc ene, probably latest Miocene (5-6 Ma). Resolution of this problem is i mportant because the Yakataga Formation is the thickest, most complete , and best exposed repository of late Cenozoic glaciomarine rocks in t he northern hemisphere. Investigation of this record has broader impli cations for global temperature gradients, paleoceanographic developmen t of the far North Pacific, and northern hemisphere glacial history. N ew planktic foraminiferal data from onshore and offshore Yakataga Form ation sections in the Gulf of Alaska are compared to a regional deposi tional and chronostratigraphic framework in order to test whether the Yakataga record is anomalous or consistent with the paleoclimatic reco rd of the North Pacific. This comparison reconfirms that initial glaci omarine rocks of the Yakataga Formation are no older than late Miocene . New paleomagnetic data suggest that the base of the Yakataga Formati on is within the lower Gilbert polarity chron, consistent with the pla nktic foraminiferal dating and K/Ar dates on glauconite. Correlation o f the Yakataga record to key offshore sections in the far North Pacifi c (Deep Sea Drilling Project [DSDP] Sites 178, 183, 192) provides addi tional insights into the paleoclimatic significance of these rocks. Up lift of the Alaska coastal ranges, a necessary prelude to alpine glaci ation, occurred during late Miocene time. Diatom biofacies (DSDP Sites 183, 192) indicate warm middle Miocene conditions, climatic deteriora tion during the late middle and early late Miocene time, and initial t idewater glaciation between 5.0 and 6.7 Ma. A relatively warm mid-Plio cene interval succeeded this initial glaciation, which was, in turn, f ollowed by hemisphere-scale glaciation beginning at approximately 2.5 Ma. This second phase of Yakataga glaciation may have begun as early a s 3.0-3.5 Ma. Glaciomarine rocks in the basal Yakataga Formation mark an important climatic event in the North Pacific Ocean. They are lates t Miocene in age, consistent with regional climatic information, and n ot a middle Miocene climatic anomaly.