The northern Great Barrier shelf can be divided into 3 major bathymetric zones: (1) the nearshore zone, extending to depths of 5 fm; (2) the inner shelf, extending to depths of 20 fm; and (3) the marginal shelf extending to the shelf edge at 40-50 fm. In the area studied, reef development is concentrated on the inner two thirds of the marginal shelf.

Sedimentary facies in this area reflect three major influences: (1) source (terrigenous and carbonate); (2) hydrography; and (3) relic facies. On the inner shelf, nearshore terrigenous sand is succeeded seaward by dominantly terrigenous mud which transgresses two pre-Recent facies: (1) terrigenous sand in the north and (2) shell gravel in the south. Mixing of pre-Recent sediments, terrigenous mud, and modern carbonates (molluscan and foraminiferal) complicates the basic pattern. Both pre-Recent facies extend to within 1 mi of the reef with no significant dilution by modern reef-derived debris.

Major interreef channels are floored by fine-grained, mixed terrigenous-carbonate sediment. The terrigenous material (30-50 percent) is dominantly silt and clay size. The identifiable, carbonate fraction is composed mainly of planktonic and benthonic Foraminifera, juvenile and larval mollusks, and skeletal debris. Most of this channel-floor sediment apparently is derived either from the inner shelf or from the open ocean; little appears indigenous.

The main reef complex is built on a 16-fm platform, and encloses a central area of about 64 sq mi. Much of the interior platform is covered by low coral growth or by dead coral with a thin cover of worn, broken, and stained skeletal grains, mainly large Foraminifera, Halimeda, and coralline algae. Major sediment accumulation occurs only in the lee of the main reef where quiet-water conditions allow the deposition of carbonate mud with up to 18 percent fine terrigenous detritus. The sparse coarse fraction is molluscan, Foraminifera increasing in abundance near the reef. Reef-derived sediment is abundant only on and directly adjacent to the reefs.

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