An Invertebrate Macrofauna from an Early Holocene Fjord-Delta System, North Norway

During early Holocene, the Breivikeidet Valley of Troms County was part of a fjord system. Prodelta and delta slope deposits were formed. overlain by fluvial deposits, and are presently exposed in river sections. The depositional setting was influenced by sediment suspension and traction processes. Our geochemical analyses, together with implications from macrofauna and sedimentology, indicate an overall change from occasionally oxygen deficiency in marine waters to an overall fall in salinity, these changes finally leading to brackish and freshwater conditions. Radiocarbon dates confirm an early Holocene age (Preboreal, Boreal and Atlantic) of the deposits. Shells reworked from older deposits indicate Allerod and Younger Dryas ages. Bivalves. gastropods and barnacles are common and well preserved in the muddy prodelta deposits, whereas they are uncommon and poorly preserved in the delta slope sands. Faunal composition of the delta slope sands is numerically characterised by suspension-feeding endobenthos that have moderate to fast burrowing ability. The fossil macrofauna of the muddy prodelta deposits, on the other hand, can be much more diverse, including mobile to stationary endobenthos and epibenthos. The prodelta muds were close to normal marine in their salinity. The delta slope sands were probably somewhat brackish, while the topsets were formed in fluvial freshwater. Faunal composition indicates water depth of the ancient habitat having been equivalent to nearshore shelf to fjord-deltaic setting. A fjord-deltaic setting is evident by the sedimentology and adjacent steep topography. We conclude that the fossil macrofauna is in composition comparable with modern marine communities of the high-boreal province.