Holocene evolutionary history and accumulation rates for the granite based MacGillivray Reef System, Northern Great Barrier Reef

MacGillivray Reef (formerly known as Shadwell Reef) is located in the Northern Great Barrier Reef province, approximately 270km north of Cairns. Reef drilling was undertaken to investigate the Holocene evolution of the reef using radiocarbon dating of the cored coral material.

Two windward and two leeward cores were recovered from MacGillivray Reef. Two of the cores were initiated through thrown up coral blocks on the leeward margin which allowed a longer drilling window during the tidal range. Cores Mac 3 and 5 were abandoned due to the rising tide and equipment failure respectively; Mac 4 was abandoned after encountering a thick sand layer. The maximum depth reached was 11 m and 15 m an the windward and leeward margins respectively.

The cores were logged and photographed, and thin sections were made and petrographically examined. Corals that were determined to be in situ (by the orientation of their corallites) and free from internal cement and detritus by microscope examination were selected for radiocarbon dating.

Radiocarbon dating suggests that MacGillivray platform reef developed on a topographic high, which is assumed to be granite, at a depth of 15 m below the reef flat approximately 8.2 cal kyr BP. The reef only approached present day sea-level within the last 500 years (260 - 80 cal yr BP). Rates of vertical accretion ranged from 1.4 to 5.8 m/kyr.