Seismic refraction studies have been conducted in close proximity to shallow boreholes previously drilled at Bewick (Northern Great Barrier Reef), Hayman (Central Great Barrier Reef) and Heron Islands (Southern Great Barrier Reef). Results show a seismic discontinuity at depths similar to the unconformity separating Holocene and pre-Holocene carbonates in the boreholes. Extensive seismic refraction studies have been conducted in the Capricorn/Bunker Reefs to assess the effects of substrate on modern reef growth, and the classification of modern reefs in the area. One hundred and twenty-four profiles were completed on six reefs. A marked seismic discontinuity, detected at depths ranging from 8-23 m, is equated with the Holocene/ pre-Holocene unconformity identified in the boreholes. Results show that the pre-Holocene surface forms a central depression with a raised rim around the perimeter beneath those reefs which have lagoons. This also appears to be the case beneath those reefs which presently have small or no lagoons, the inference being that Holocene sedimentation and growth have infilled the original depressions. The geophysical evidence does not support the claim that reefs of the southern Great Barrier Reef follow a latitudinal genetic sequence, but rather that the varied growth forms of the Holocene reefs have been determined to a considerable extent by the shape of the pre-Holocene surface.
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