Boundary Bay tidal flats on the inactive southern flank of the Fraser Delta have surface sediments consisting almost entirely of very fine to fine, well to very well sorted sands which show a gradual fining-shorewards trend. Five floral/sedimentological zones form distinct biofacies. These are, from the shoreline seaward, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. The lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from ∼ 12 to 40 days. Similarly, at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to ∼ 2 days, and at the upper limit of the eelgrass zone from ∼ 0·5 to ∼ 0·8 days. These correlations between exposure and zonation are suggested to be causal.

In the algal mat and eelgrass zones microtopography of biogenic origin, only a few centimetres high, creates lateral heterogeneity within the zonal biofacies. In the upper sand wave zone, very low amplitude (a∼ 0·1 m) symmetrical sand waves (λ∼ 30 m) of probable storm-wave origin have a similar effect. In the lower sand wave zone, sand waves (a∼ 0·5 m, λ∼ 60 m) are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones.

The densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects: Callianassa californiensis and Upogebia pugettensis, both thalassinidean shrimps; three polychaete worms, Abarenicola sp., Spio sp. and Clymenella sp.; the bivalve Mya arenaria and the gastropods Batillaria attramentaria and Nassarius mendicus. Callianassa excavate unlined temporary feeding burrows, whereas Upogebia build mud-lined permanent dwelling burrows. Upogebia are restricted to below mean sea-level where continuous exposure is < 0·5 days, whereas Callianassa extend up to a level, just below mean higher high water, where maximum continuous exposure rises abruptly from 4 to 9 days. This difference in range is probably due to the latter's greater anoxia tolerance—a necessary adaptation for life in an unlined feeding burrow.