Drowned dolines - the blue holes of the Pompey Reefs, Great Barrier Reef

Blue holes occur at Cockatoo and Molar Reefs, in the Pompey hard line reefs. These two holes are roughly circular in shape, between 240-295 m in diameter, and 30-40 m deep. They are completely (Cockatoo) or partially (Molar) rimmed by profuse living coral and surrounded by lagoonal depths of 5-10 m. The inner slopes of the Cockatoo blue hole are 60-70° down to a depth of 25 m, below which coalescing sediment fans markedly reduce this angle. At the Molar blue hole, slopes are mainly gentler (45°) and sediment fans and terraces occur below 16 m. Distinct biological/sedimentary associations occur in both holes. Seismic refraction studies across the rim of the blue holes show a shallow (8.5-11 m) pre-Holocene surface beneath the rims. The balance of evidence suggests that the blue holes represent collapsed dolines which may have taken more than one low sea-level period to form. The original surface structures have been modified by subaerial solution processes, and subsequent sediment infill and coral growth following the Holocene transgression.