Episodic coral growth events during the building of Reunion and Mauritius shield volcanoes (Western Indian Ocean)

Field observations and hydrogeological drilling investigations carried out in Reunion and Mauritius Islands revealed marine coral-rich carbonate deposits interbedded with or trapped into lavas flows and (or) volcanic detritus in coastal sites. The times of deposition of these carbonates are inferred mainly from K/Ar dating and relative age estimates of associated volcanic rocks. In Reunion, three coral deposits were found beneath the western slopes of the older Piton des Neiges shield volcano. These were dated at 0.28-0.27 million years (Ma), 0.25 Ma, and about 37,000 years respectively, indicating that marine deposition occurred during Marine Isotope Stage (MIS) 8 low stand, MIS 7.5 high stand, and MIS 3.2 low stand. On Mauritius, seven marine carbonate units were identified, located in western and northern areas. The oldest are believed to be late Miocene (older than 5 Ma) to early Pliocene (4-4.5 Ma) in age. The younger were assigned to MIS 11 (0.44-0.35 Ma), MIS 9-8 (older than 0.26 Ma), and MISs 7.5 and 5.5 respectively. While the Piton des Neiges shield would have remained relatively stable since the earlier eruptive phases, Mauritius is shown to have experienced low rates of subsidence (0.015-0.033 mm/year). Identification and point-counting of the main skeletal components-including foraminiferal assemblages-from each carbonate unit, combined with statistical treatment (principal component analysis) of the relevant dataset, allowed different biological associations to be delineated, typified respectively by alcyonarians, ectoprocts, corals, coralline algae and foraminifera. By analogy with the compositional patterns of modern reefal sediments, these associations are shown to be referable to the outer-reef zones (i.e. lower fore-reef and upper fore-reef) and reef-crest to reef-flat zones. By contrast, in Reunion, coral gravels and boulders deposited during MIS 7.5 are interpreted as coastal, storm-generated sediments. The lack of recognized backreef deposits in drillholes in either Reunion or Mauritius suggest that the reef tracts represented may not have possessed well-differentiated backreef environments, but developed in the form of coralgal carpets or narrow fringing reefs such as those present along the west side of Reunion and western and northern Mauritius.