Offshore from Townsville in the central GBR (about 18.2-19.2°S), where most previous studies have been conducted, the continental shelf is about 120 km wide. On the outer 40-50 km of the shelf, an open matrix of coral reefs rises from 50-70 m depth. Between the coast and the reef matrix, a 75 km wide stretch of water known as the GBR Lagoon is almost devoid of reefs possibly due to the action of frequent cyclones.
North of 18°S, the shelf narrows (to about 50 km wide) and becomes shallower (typically 30 m, rarely >50 m). The outer shelf becomes delimited by a nearly continuous barrier of ribbon-like reefs; with steep descents to the abyss beyond. Platform reefs are common, often quite densely packed across the mid- and outer-shelf, and the GBR Lagoon becomes narrower (30 km to 20 km).
South of 19°S, to 22°S, the shelf broadens to as much as 250 km and generally becomes much deeper with substantial areas >70 m. Two lines of hard-packed reefs, the Pompeys, with narrow channels between, emerge from a shallow (30-50 m) relic limestone platform that dominates along much of the outer shelf. The Lagoon also broadens to >100 km and deepens (50-70 m) to the southeast, becoming the Capricorn Channel with depths in excess of 100 m at its mouth.
At the very southern end of the GBR (23.0-24.5°S), the Capricorn-Bunker Reefs rise from the outer areas of an 80 km wide sandy shelf of 30-40 m depth.
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