CARBONATES IN ALASKA? MODERN SEDIMENTS IN COOL-TEMPERATE KACHEMAK BAY

We discovered modern calcareous sediments in the intertidal zones of small islands in Kachemak Bay, Alaska. These cool-temperate deposits are some of the first documented examples of recently formed carbonates in the state. Kachemak Bay is a subarctic fjord in southcentral Alaska with seawater temperatures ~2-11℃ and tidal ranges of up to 8.5 m. The bay straddles the boundary between the Cenozoic-modern Cook Inlet forearc basin to the northwest and the rugged, recently deglaciated Mesozoic accretionary wedge (McHugh Complex) to the southeast. Isostatic rebound and the nearby Aleutian subduction zone drive uplift in this region. These sediments are not found in a classic carbonate setting; they occur in cold waters of a tectonically active area. The southeastern shore of Kachemak Bay is composed of graywacke, chert, and basaltic cliffs, boulders, and beaches with small accumulations of calcareous sediments.

Heterozoan gravels and sands of these shorelines are mainly formed from the remains of barnacles, epilithic and infaunal bivalves, chitons, gastropods, echinoids, and siliceous sponges. These invertebrates are found living in abundance in adjacent rocky intertidal habitats and kelp forests. The kelp and rocky intertidal carbonate factories are widespread in the shallows surrounding these islands, but the calcareous sediments are found concentrated in small patches (<200 m²) on beaches and along the tops of isthmuses that are only exposed during low tides. These patchy distributions may be due to longshore tidal currents and storms. Skeletal carbonates have higher intraskeletal porosities and lower densities than the volcaniclastic sediments with which they are intermixed. These lighter grains may be more easily swept towards these beaches and isthmuses than the denser grains, concentrating them in the shallow intertidal zones. Carbonates can build up on the shores of these islands, despite cold waters and tectonic activity, due to limited local terrigenous sediment sources on islands without rivers and streams, and high carbonate production by organisms in adjacent carbonate factories.