UPPER TRIASSIC PALEONTOLOGY AND STRATIGRAPHY IN THE ALEXANDER TERRANE, KEKU STRAIT, SOUTHEAST ALASKA

Upper Triassic sedimentary and volcanic rock from Keku Strait between Kuiu and Kupreanof islands, southeast Alaska, belong to the Alexander terrane. Rock types in the Keku Volcanics and Hyd Group include limestone, calcareous mudstone, conglomerate, pillow basalt, volcanic breccia, and marine tuff. These Carnian and Norian rocks represent a wide range of depositional environments from an island arc complex in a volcanically and tectonically active sedimentary basin. Facies relationships within and between most formations display an eastward shallow to deep water trend in depositional environment.

In addition to deeper water ammonoids and the bivalve Halobia, the sedimentary rocks contain diverse, often silicified, assemblages of shallow-water invertebrate fossils. These include sponges, corals, spongiomorphs, articulate brachiopods, gastropods, bivalves, ammonoids, and echinoids. Rocks previously dated as Carnian and Norian by ammonites and Halobia can be further constrained using conodonts retrieved from calcareous units. Conodonts help unravel the complex age and facies relationships from an island arc setting and aid identification of contemporaneous faunas. Taxonomic study of the fossil groups, particularly the corals, sponges, hydrozoans, and gastropods, reveals complex paleobiogeographic affinities, which are expected in oceanic islands.

This study permits comparison to other Cordilleran terranes, including Wrangellia, Wallowa, Quesnel, Eastern Klamath, Farewell, and Chulitna. Findings confirm similarity to the Alexander terrane faunas of Gravina Island. Detailed study of the stratigraphy, facies relationships, taxonomy, and paleoecology, helps resolve many stratigraphic problems in these Triassic units. Furthermore, comparison of fossil groups and the stratigraphic succession from Keku Strait with other terranes, especially Wrangellia, allows reassessment of tectonic relationships, such as the accretion and amalgamation of Alaskan terranes.