TRIASSIC STRATIGRAPHY AND FACIES RELATIONSHIPS IN A VOLCANIC ISLAND ARC, ALEXANDER TERRANE, KEKU STRAIT, SOUTHEAST ALASKA

Upper Triassic strata in the Keku Strait of southeast Alaska (Alexander terrane) represent a proximal to distal facies progression in a volcanic island arc. The Keku Volcanics includes mostly volcanic flow rock with subordinate volcaniclastics and conglomerate. The mainly felsic volcanics dominate the volcaniclastics, and the conglomerate contains reworked Permian and Carboniferous clasts. Both of these clastic units often show cross bedding and scouring structures. Downslope, the Keku interfingers with sandy to conglomeratic limestone and massive limestone of the Cornwallis Limestone. Both limestones contain reworked Permian clasts, minor volcaniclastics, and ooids. The clastic limestone can be cross-bedded and may have abundant plant debris, displaying closer proximity to land than the massive limestone, which has abundant marine fossils including corals, sponges, gastropods, and Stromatomorpha.

These shallower slope deposits grade into the Hamilton Island Limestone down slope. The Hamilton Island is fine-grained limestone with Halobia and ammonites and contains sporadic meter-thick debris flow deposits composed of reworked Permian and Triassic clasts. Its fine-grained texture, debris flows, absence of shallow water faunas, and pyrite all support deep-water deposition. Interbedded debris flow deposits of shallow water fossils represent the transition from Hamilton Island to Cornwallis. The Burnt Island Conglomerate underlies the Hamilton Island. This massive, basal conglomerate is composed mainly of reworked Permian clasts and represents early infill of the basin before extensive marine deposition.

This succession is overlain throughout the area by the Hound Island Volcanics. This unit comprises basaltic pillow lava, basaltic and andesitic volcanic debris flow breccias, tuff, and thin-bedded limestone. The limestone is very similar in lithology and fossil content to that of the Hamilton Island.

Despite the importance of stratigraphy in recognizing terranes, comprehensive stratigraphic studies are uncommon. Detailed stratigraphic characterization and revision of these strata assist in delineating the terrane boundaries of Alexander and improve terrane comparisons.