Study of Late Triassic biofacies and associated paleoecology reveals new silicified shallow-water corals and other fossils from new and previously known localities within the Alexander terrane (Keku Strait and Gravina Island, southeast Alaska) and Wrangellia (Wrangell Mountains, Alaska, and Vancouver Island, British Columbia). Twenty-five species of coral are identified from eight localities within the Alexander terrane and 34 species are identified from four localities in Wrangellia. Distributions of silicified shallow-water marine fossils contribute to Late Triassic (Norian–Rhaetian) paleoecology, biotic diversity, and terrane paleogeography. Depositional environments establish the conditions in which these organisms lived as well as provide evidence for lithological correlation between tectonically separate fragments. This study also confirms the presence of biostrome reef buildups in the southern Alexander terrane (Gravina Island), indicating warm, clear, and nutrient-free water with lots of sunlight; this differs from the central Alexander terrane (Keku Strait) and northern Wrangellia (Wrangell Mountains), where corals grow as individual colonies, not in a structured, reef-like buildup, and are accompanied by filter- and detritus-feeding organisms indicating warm, cloudy and nutrient-rich water in a back-reef environment. Paleobiogeographic results from silicified Upper Triassic corals show faunal similarity between Gravina Island and Keku Strait (Alexander terrane) and no similarity between northern and southern Wrangellia. Likewise, no similarity was found between the Alexander terrane and either northern or southern Wrangellia.

Upper Triassic rocks in the Keku Strait area of southeast Alaska record a variety of facies in an intra-arc setting. The Hyd Group consists of the Burnt Island Conglomerate, Keku sedimentary strata, Cornwallis Limestone, Hamilton Island Limestone, and the Hound Island Volcanics. The Burnt Island Conglomerate represents initial infill of the basin and underlies the Hamilton Island Limestone, which is coeval with the Cornwallis Limestone and Keku sedimentary strata. Volcanic and sedimentary rocks of the Hound Island Volcanics overlie the entire area. An improved biostratigraphic framework indicates deposition from early Carnian through late Norian time. Conodonts originating in the late Carnian include Metapolygnathus polygnathiformis , Metapolygnathus carpathicus , Metapolygnathus nodosus , Metapolygnathus sp. cf. M. reversus , Metapolygnathus sp. aff. M. zoae , Metapolygnathus sp. aff. M. nodosus , and Metapolygnathus primitius . Early Norian conodonts include Epigondolella quadrata , Epigondolella triangularis , Epigondolella sp. aff. E. triangularis , and the longer-ranging Neogondolella sp. and Misikella longidentata . Middle Norian conodonts include Epigondolella spiculata , Epigondolella transitia , Epigondolella matthewi , Epigondolella postera , and Neogondolella steinbergensis . Late Norian conodonts include Epigondolella bidentata , Epigondolella englandi , Epigondolella sp. aff. E. mosheri , and Epigondolella tozeri . This study resulted in three major accomplishments. Reworked Paleozoic conodonts in Upper Triassic rocks, combined with geologic evidence, suggest major preLate Triassic uplift due to compressional tectonics. Late Carnian and early Norian ages support the correlation between the Keku sedimentary strata, shallow-marine limestone of the Cornwallis Limestone, and deeper-water limestone of the Hamilton Island Limestone. Precise conodont biostratigraphy establishes the base of the Hound Island Volcanics as late early Norian, within the Epigondolella triangularis Zone.