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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 48 (1978)No. 3. (September), Pages 743-749

A Lagoonal Crustose Coralline Algal Micro-Ridge: Bahiret El Bibane, Tunisia

Scott E. Thornton (2), Orrin H. Pilkey, George W. Lynts


Colonies of Neogoniolithon notarisii (Dufour) Setchell and Mason found within a subtropical, hypersaline lagoon form a wave-resistant ridge 31 km long. The formation of algal ridges in a relatively low wave energy lagoonal setting under hypersaline conditions is highly unusual. Individual heads of mushroom-shaped colonies coalesce to differing degrees, probably in response to local variations in wave energies. Individual heads are circular to oval in plan that average 0.9 m in diameter, but may be as large as 2.0 m in diameter. Three growth modes appear to represent members of a continuum: 1) discontinuous ridges constructed of individual heads that commonly coalesce to form bands of two or three heads; 2) broader ridges of algal heads consisting of 7-10 coalescing heads; and 3 continuous ridges with lobate lagoonward margins. The composite ridge has formed on a wave-cut platform on the lagoonal side of an elongate lithified barrier. Initial growth of small cauliflower-shaped heads is directed upwards. Once the water surface is reached, lateral growth produces overhangs that average 25 cm, but may be as large as 60 cm. Lagoonward-facing rims are best developed, standing 4-8 cm in relief above the center of the colonies. These elevated lips are usually awash, but the remainder of the ridge is exposed only at lowest low water. A consistent overall growth height of 33 cm, measured on the lagoonward-facing rim, probably results from tidal control. Further dependence on wave activity for healthy growth is demonstrated by the disappearance of the ridges in the easte n part of the lagoon as fetch of the dominant summer winds is decreased. The ridge is strikingly sterile, with infrequent associations of Halimeda, serpulid worms, vermetid gastropods and boring clams. The existence of this coralline framework exemplifies a different type of environmental control than algal ridges elsewhere. Salinity and temperature limit potential grazers and other space competitors, so that Neogoniolithon notarisii totally dominates this ecological niche.

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