ABSTRACT

Surface sediment from the inner continental shelf and island beaches in the Sitka Sound area of the Alexander Archipelago contains between 94 and 37 percent by weight carbonate, the remainder being silicate rock fragments and minerals derived from Jurassic-Cretaceous marine clastics. Sand-size pumice is a minor constituent. These sediments are bi- and polymodal coarse skewed gravelly sands. Mud is present in trace amounts and is mostly carbonate. The carbonate fraction is biogenic; sand-size grains are composed of fragmented skeletons of barnacles, echinoids, bryozoans and mollusks. Stony corals, crustose coralline algae and articulate brachiopods are present, but are not major sediment producers. Sand-size carbonate minerals are mostly low Mg-calcite with high Mg-calcite and aragonit as minor constituents. These carbonates are believed to be modern because skeletons found in the sand fraction come from communities of carbonate-secreting organisms now living in the same environment. Carbonate sediments are known from the continental shelf (264 feet) west of Chichagof Island and in Symonds Bay (72 feet) at Biorka Island and beaches on the lee side of islands at Pirates Cove, Three Entrance Bay and Taigud. Aerial reconnaissance in the Sitka Sound area suggests that these carbonates are more widespread than formerly supposed.