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Kaufman, D.S., 1991

Pliocene-Pleistocene chronostratigraphy, Nome, Alaska

Bibliographic Reference

Kaufman, D.S., 1991, Pliocene-Pleistocene chronostratigraphy, Nome, Alaska: University of Colorado, Boulder, Ph.D. dissertation, 297 p., illust., maps.

Abstract

Surficial deposits blanketing the coastal plain at Nome, Alaska, and adjacent continental shelf record multiple sea-level and glacier-ice fluctuations during the Pliocene and Pleistocene. Amino acid epimerization ratios (aIle/Ile) measured in 295 fossil molluscan shells of three genera, including 27 fragmented shells classified based on their amino acid compositions, were used to evaluate the number and timing of high-sea-level events. Clusters in the aIle/Ile data, each presumably representing an interval of marine inundation, were identified using frequency-distribution analysis, and a new approach based on ranked-data distribution. At least four and as many as six events were recognized, including: (1) a late Miocene/early Pliocene event known only from the base of boreholes drilled in the Nome nearshore area; (2) the middle to late Pliocene Beringian transgression, which was a complex and protracted event that may have included three distinct high-sea-level phases, and was followed by a lengthy period of non-marine deposition; (3) the middle Pleistocene Anvilian transgression, which is considerably younger than previously thought, perhaps as young as marine oxygen-isotope stage 11 (410 ka); and (4) the last interglacial Pelukian transgression. The amino acid data together with an empirical parabolic model of epimerization kinetics, and provisional correlations with marine units on the Arctic Coastal Plain, indicate that the amount of effective diagenetic temperature (EDT) lowering between middle Pliocene and late Pleistocene time was about 4 to 5 degrees C at both sites, and that the temperature difference separating the two sites existed prior to the middle Pliocene. The strontium isotopic composition (87Sr/86Sr) of 12 molluscan shells from Nome was measured to assess the absolute ages of the shells. 87Sr/86Sr ratios were considerably higher than expected, demonstrating that such data cannot be used for chronostratigraphic purposes until criteria for screening altered shells are devised. Glaciers expanded onto the coastal plain at Nome at least twice; the age of the youngest extensive ice advance (Nome River glaciation) is constrained by laser-fusion 40Ar/39Ar dates that average 470 +/- 190 ka on lava that overlies Nome River drift. Snowline depressions during subsequent intramontane advances were considerably smaller.

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