PALEONTOLOGIC AND STRATIGRAPHIC FIELD STUDY OF THE TRIASSIC KEKU VOLCANICS AND HYD GROUP IN THE KEKU STRAIT AREA, SOUTHEAST ALASKA (ALEXANDER TERRANE)

The Keku Strait area of southeast Alaska contains an extensive marine sedimentary succession of Triassic age, making it valuable for regional terrane comparison and correlation. The Keku Volcanics and the Hyd Group are relatively unmetamorphosed. The Keku Volcanics contain felsic and mafic volcanic rock intercalated with volcaniclastic and some carbonate rock. The Triassic Hyd Group comprises the Burnt Island Conglomerate, Hamilton Island Limestone, Cornwallis Limestone, and the Hound Island Volcanics. Abrupt facies changes, variable rock types, and varying thicknesses in the Triassic support the idea of an island arc terrane.

Biostratigraphy reveals differences in the Carnian and Norian ages than previously reported. Rather than shallow water or reef facies, some of the reefal faunas and limestone of the Cornwallis represent reworked storm beds, often among deeper water facies, and small in situ biostromes of corals and hydrozoans. These facts, coupled with fossils and rock types in the Keku Volcanics that are comparable to those of the Cornwallis Limestone, and the overlapping facies relationships, suggest that some rocks previously considered integral units need reclassification.

Key localities at Keku Strait with silicified fossils offer great potential for taxonomic and paleogeographic study. Besides conodonts, fossils include corals, spongiomorphs, gastropods, bivalves, brachiopods, ammonites, and giant, calcified, biostromal sponges. Diagnostic Norian fossils identified include the Primitius and Bidentata conodont zones, the Kerri ammonite zone, the gastropods Chulitnacula alaskana and Spinidelphinulopsis whaleni, the corals Chondrocoenia schafhaeutli and Crassistella cf. C. juvavica, and the hydrozoan Heterastridium. The coral and spongiomorph composition indicates more similarity to other Alexander terrane faunas, and faunas from the Wallowa, Eastern Klamath, and Quesnellia terranes, than faunas from Alaskan Wrangellia. Gastropods in this area are quite distinct from coeval faunas in Alaskan Wrangellia and the Wallowa terrane, but are similar to other Alexander terrane faunas and those of the Farewell and Chulitna terranes of western Alaska.