Abstract:

Stone capped calcareous pillars, rising 10-70 cm above the surrounding reef surface are to be found at Aqaba (Jordan) and on several other fringing reefs of the Northern Red Sea from intertidal down to 3 m depth. Terrigenous (non-calcareous) cobbles and boulders are fixed on top of coral limestone. At Aqaba it is assumed that the stones were once introduced by fishermen and came to rest in depressions of the reef. Afterwards the surrounding reef limestone was eroded so that only the substrate underlying the granite stones has been left as singular towers. 14C-dating of a column sample provided an age of 345-560 years. Several interpretations are possible: the respective reef part did not grow since then, or younger layers were removed by bioerosion. Some suggestions are provided based on erosion data of sea urchins (Diadema setosum) and fish, gathered at that site. Two more examples of partial reef decline are presented from a reefflat at the Sinai coast north of Dahab indicating uplift along the margin of the Gulf of Aqaba and from the bay Marsa Bareika at the southern tip of Sinai where big boulders are interpreted as ballast stones discharged in an antique harbour.