The Petrology of Oolitic Phosphorites from Esprit (Aldabra), Western Indian Ocean

• Published in: Philosophical transactions of the Royal Society of London. Series B, Biological sciences Biological sciences, 1980-05, Vol.288 (1032), p.511-540
• Main Author: Braithwaite, C. J. R.
• Format: Article
• Language: English
• Subjects: Apatites; Birefringence; Carbonates; Caves; Cements; Crystals; Fibrous minerals; Limestones; Phosphates; Sediments
• ISSN: 0962-8436; 0080-4622; 1471-2970; 2054-0280
• DOI: 10.1098/rstb.1980.0025

The island of Esprit, at the western end of Aldabra lagoon, is capped by two groups of phosphorites. Near the summit, bedded deposits rest on and in cavities within the subaerially eroded surface of the limestones forming the island. The limestones themselves have not been phosphatized. On the lower slopes, and derived from the phosphorites above, are small outcrops of coarse, phosphate-cemented, bioclastic sediments and large, irregular fans of phosphoritic conglomerates. The phosphorites can be divided into five petrographic groups. Oolitic phosphorites are the most common and are apparently primary. Associated with them are lithoclast-bearing rocks, fine-grained phosphorites, bioclastic deposits and internal sediments, all of which are also wholly phosphatic. There have been numerous reworking episodes in the history of these rocks, such that large volumes now consist only of phosphatic cement sequences (the linings of former cavities) and internal sediments. Up to fourteen changes in the depositional milieu can be seen. The distribution of both cements and internal sediments is restricted, recording the paths of particular transport streams in groundwaters. The minerals identified are carbonatian hydroxyfluorapatites. Most cements are multilayer colloform crusts with a radially fibrous structure, but, in addition, crystals show a range of morphologies, including hexagonal, monoclinic and cubic forms. Some cements are carbonate, but other minerals that may have been present have been pseudomorphed by phosphates. The primary source of the phosphate in these rocks is thought to have been avian guano, deposited on a limestone surface at a time when sealevel was 7-8 m above its present position. Phosphate-rich derivatives from this were carried downwards by surface-wash processes and precipitated in a series of caves in the limestones excavated at the water table and drained as sealevel fell. Solution pipes were formed when sealevel was at least 1-2 m below its present position, but marine coquinal sediments deposited within these suggest that it was subsequently higher. Continued erosion of the host limestones, destroying the caves, released both phosphorite detritus, redeposited as the low-level sediments and transported into beach calcarenites, and phosphatic solutions for precipitation as cements. The phosphorites are tentatively dated as deposited 170-230 ka B.P .