Depositional analysis of an overlooked permian unit in the Alexander Terrane in southeast Alaska.

摘要

In the Alexander terrane in the Prince of Wales region of Alaska, an overlooked, unnamed Permian carbonate unit crops out on the eastern shore of Suemez Island. The first mention of the unit appears in Buddington and Chapin's United States Geological Survey Bulletin 800 from 1929, but the unit was only mentioned --- not measured, described, or mapped. Fieldwork conducted in the summer of 2015 involved measuring and collecting fossil/lithological samples of the Permian section. The section has been measured where exposed, described, and the taxa are being identified and cataloged. The Suemez Island Permian unit is a slightly-folded, blue to gray limestone with cool or cold water origins that has undergone pervasive silicification. Macro-fossils are sparse but include bryozoans, brachiopods, echinoderms, and rugose corals. The dark- to light-gray alternating color of the beds in the unit may indicate seasonal storm events where macro-fossils are concentrated in beds with a chaotic orientation and a possible mixing zone of both siliceous and carbonate materials from the shelf slope. The overlooked unit is classified here as a Bryonoderm-extended carbonate which is specific to cool-water origins. The unit also shows a shallowing upwards trend, possibly due to changes in sea-level or a tectonically active region.;Utilizing the fossils and sedimentary structures, correlation of the overlooked unit to other Permian units in the Alexander terrane is possible. Karl et al. (2010) assigned the Suemez Island Permian unit to the Pybus Formation due to the similarities in the fauna. The Pybus Formation is the only other Permian age carbonate unit in the Alexander terrane, occurring 200 kilometers north of Suemez Island. However, this study provides new lithologic evidence indicating the Suemez Island Permian unit and the Pybus Formation have dissimilar facies though they likely are coeval.