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Geological setting and surficial sediments of Fatty Basin, a shallow inlet on the west coast of Vancouver Island, British Columbia. Wiese, Wolfgang


Fatty Basin and Useless Inlet result from modification by water and ice erosion of depressions caused by Early Tertiary faulting. Uplift of the land due to post-glacial rebound is in excess of 6 m (20 ft) for the last 7000 years. Shallow entrance sills cause the inlets to act as traps for organic detritus brought in by tidal action. The rate of deposition of fine-grained suspended debris is in the range of 900 g/m² /year, with maximum deposition during late summer, when phytodetritus is most abundant. Five sedimentary environments exist in Fatty Basin, namely mud zones, rock slopes, beaches, deltas, and zones of strong currents. In addition to boulder accumulations and bedrock exposures, general categories of sediments are pebbles and gravels, terrestrial sands, shell debris, muds, and shell-gravel mixtures. Statistical analysis of the size distribution of 125 samples resulted in recognition of 8 groups of sediments, which were then subdivided into 13 types on the basis of composition and grain shape. Olive-green mud rich in organic matter covers almost three-quarters of the bottom surface in the Basin. Coarse terrestrial sands are derived mainly from bedrock exposures within about 300 ft of the shore, whereas most of the fine sands, silts, and clays originate from glacial sediments. The source area for glacial debris is in the Henderson Lake region, underlain dominantly by Karmutsen basalts. Shell debris, notably barnacle plates and calcareous worm tubes, is essentially confined to the rock-slope environment, where it accumulates in a narrow zone along the base of steep slopes. The rock-slope environment represents a preferred habitat for lobsters, because it offers better shelter and food supply than the other environments. In Fatty Basin, the total area most suitable to lobsters amounts to about 38,000 m² (=7% of the bottom surface), in Useless Inlet, this area covers 135,000 m² (= 5% of the bottom surface).

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