Capricorn and Bunker Group - Geomorphology and Landscape

Geomorphology and Landscape

The Capricorn and Bunker Cays form part of a distinct geomorphic province at the southern end of the Great Barrier Reef. The cays and their reefs lie on the western marginal shelf, and are separated from the mainland by the Curtis Channel. The cays are not generally visible from the mainland, although Masthead Island may be viewed from Mount Larcom on a clear day.

Geologically the cays are young, having developed during the Holocene period, they are mostly around 5000 years old. The sea level was much lower during the last Ice Age (at the end of the Pleistocene period) and the coastal plane on which today’s reefs and cays developed was completely exposed. Early in the Holocene (around 10,000 years ago) the sea level began to rise, until it stabilised at its present level around 6000 years ago. Once the sea level stabilised, it was possible for reef flats to expand and provide potential sites for the formation of cays.

Four distinct reef types occur within the Capricorn and Bunker Groups

• 1. Platform Reefs:- Tyron, Wreck, North, Wilson, and Erskine Reefs.
• 2. Lagoonal Platform Reefs:- Heron, and One Tree Reefs.
• 3. Elongate Platform Reefs:- North West, and Masthead Reefs.
• 4. Closed Ring Reefs:- Lady Musgrave, Fairfax, and Hoskyn Reefs.

The cays occur on planar reefs of various sizes and with various levels of exposure to the prevailing winds. These factors have largely determined cay size and composition, which consists either of shingle, sand, or a mixture of both. Cay stability is profoundly affected by cyclonic disturbance.

The coral cays belong to two distinct types:

• 1. Vegetated sand cays:- North West, Heron, Masthead, Erskine, Wilson, Wreck, Tryon, North, Fairfax (West), and Hoskyn (West).
• 2. Shingle cays:- One Tree, Lady Musgrave, Fairfax (East), and Hoskyn (East).

The sand cays are all located to the lee of their reef flat, whereas the shingle cays, with the exception of Lady Musgrave, are located on the windward side. They vary in size, covering from one to 12 percent of the reef top surface area and there does not appear to be any correlation between island size, reef size, or stage of reef development.