RODRIGUES ISLAND: CARBONATE DEPOSITION AND KARST PROCESSES AS INDICATORS OF PLATFORM STABILITY

Rodrigues Island is located in the western Indian Ocean approximately 600 km east of Mauritius, at 19°42' S and 63°24' E. It is a volcanic island with a small overlay of Quaternary eolian limestones, 108 km² of area, elongated ENE to WEW, 18 km long and 6.5 km wide, with a maximum elevation of 396 m. The last volcanic activity was 1.3 ma, and only one very small lava tube is known. The eolian calcarenites are restricted to a narrow segmented band on the east coast, and two larger patches, Plaine Caverne and Plaine Corail, on the south and southwest coast, respectively. No subtidal carbonate facies are known, and no flank margin caves are known, indicating that platform subsidence has been in excess of 5 m/100 ka to bring such features from the MIS 5e +6m sea-level highstand below modern sea level. The east coast eolianite outcrops contain rectangular incisions similar to the bokas found in the Netherland Antilles, consistent with a large watershed on volcanic rocks generating streams that traverse a thin coastal limestone outcrop. Plaine Caverne and Plaine Corail each contain large epigene cave passages developed by surface catchment from volcanic rocks being transported under high (up to 62 m) eolianites to the sea. The Plaine Corail caves are abandoned by streams today, are much segmented and modified by collapse such that dissolutional surfaces are absent, and contain massive calcite speleothems, all evidence of an age for the eolianite of perhaps 1 ma or more. Plaine Caverne contains two long, linear stream caves, mostly intact, with a 1 km long dry and abandoned cave having lost its recharge to a lower but parallel active stream cave. A pinnacled surface on these eolianites, up to 2 m high, indicates extensive denudation and therefore age for the eolianites; long-duration volcanic denudation is supported by a lack of lava tubes on Rodrigues. Mauritius has flank margin caves, subtidal facies above modern sea level, and abundant lava tubes (volcanic activity as recent as 26 ka), consistent with thermal inflation; that inflation is now long ended on subsiding Rodrigues.