Radiocarbon evidence for the timing and rate of Island development, beach-rock formation and phosphatization at Lady Elliot Island, Queensland, Australia
1986; Elsevier BV; Volume: 69; Issue: 3-4 Linguagem: Inglês
10.1016/0025-3227(86)90043-5
ISSN1872-6151
AutoresAllan R. Chivas, John Chappell, Henry Polach, Brad Pillans, Peter Flood,
Tópico(s)Geological formations and processes
ResumoLady Elliot Island (Great Barrier Reef) is a coral cay constructed from near-concentric shingle ridges composed of coral, Tridacna shells and bioclastic sand bound by guano-derived phosphatic cement. Radiocarbon dating of individual Tridacna samples is used to investigate the evolution of the island and place constraints on the variations in storminess of the last 3200 years. A shallow platform reef existed at Lady Elliot since at least 6500 yrs B.P. Shortly before 3200 yrs B.P. island growth by shingle progradation commenced and proceeded to the present at a rather uniform lateral rate (60 m ka−1 to leeward and 90 m ka−1 windward). Phosphatization may have occurred continuously or episodically at any time throughout the last 3200 years. The youngest cemented ridge top is less than 770 years, whereas at the base of uncemented ridges, phosphatization is a modern (∼1950 A.D.) process. Similarly, low-level beach rock has continued to form within the last 25 years. A significant period of instability occurred less than 500 years ago during which marine erosion truncated the southwestern extremity of Lady Elliot Island, formed an eastern "spit" and stranded high-level beach rock. However, the radiocarbon ages of Tridacna samples from shingle ridges from Lady Elliot and Curacoa Islands are uniformly distributed (Kolmogorov-Smirnov test) throughout the last 4000 years. We interpret this to indicate no significant variation in ridge-forming processes or fluctuation in average storminess during this interval.
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