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Bioclastic Carbonate Sedimentation on a High-latitude, Tide-dominated Shelf: Northeast Orkney Islands, Scotland (1)
George E. Farrow (2), N. Henry Allen (2), Etie Ben Akpan (3)
Shell-sands and gravels cover much of the shallow Orkneys shelf at 59°N, accumulating locally into 30-m-high banks at rates up to 540 g/m2/yr (67 cm/l,000 yr). Overall, the Orkney shelf sedimentation rate is approximately 10 cm/l,000 yr, compared with 3 cm/1,000 yr for the entire Scottish continental shelf. Major sandbanks are located off headlands that produce circulation loops in the tidal flow. Regional sandwave orientations reveal a clockwise transport of sediment around the islands, probably resulting from storm-wave reinforcement of the tidal asymmetry combined with the net inflow of Atlantic water into the North Sea.
Carbonate production is high from the Modiolus epifauna. Modiolus shell gravels, commonly containing Glycymeris, pass laterally into comminuted shell-sands. Within the euphotic zone (down to 40 m) dead shells are weakened by echinoid biting, algal boring, and limpet grazing. Boring by fungi and clionid sponges and grazing by chitons are common but not depth-restricted.
Sediments contain 89-95% carbonate on the level bottom offshore, but 94-99% in sandwaves. Mean values for the main skeletal components are--bivalves 46%, barnacles 18%, bryozoans 11%, and serpulids 7%. Calcareous algal gravels occur in sheltered areas less than 20 m deep. Bryozoa typify lower-energy offshore environments, while more durable barnacle and serpulid debris is concentrated in sandwave fields. The sediments are dominantly calcitic and have a high preservation potential.
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