The evolution of an atoll: the depositional and erosional history of Aldabra

Abstract

The earliest sediments identified of late Pleistocene age are quiet-water calcarenites, containing abundant molluscs but few corals. As waters shallowed these were replaced by emergent intertidal sediments, probably colonized by mangroves. Further emersion into subaerial conditions resulted in cementation and solution of the limestones to form a dissected karst topography. Perhaps concurrent with this, phosphates, probably originally derived from sea birds, were deposited in open cavities. Further marine withdrawal caused the erosion and re-deposition of these phosphatic sediments at a lower level. A succeeding event was the formation of a sand cay at the western end of the atoll. Its emergent surface was colonized by tortoises, birds, crocodiles and snails, and the youngest deposits indicate the presence of at least seasonal bodies of standing fresh water. This terrestrial interlude was followed by inundation by a sea, perhaps 2-3 m higher than the present level, in which was deposited a fairly uniform limestone over the whole area of proto-Aldabra. The limestone consists largely of calcareous algae set in a calcilutite-calcarenite matrix, in which both corals and molluscs are low in numbers and diversity. The emergence that followed caused widespread erosion and the island formed, surrounded by low cliffs, was subject to solution fretting and a limited deposition of soils. Renewed submergence carved proto-Aldabra into a broad shallow bank, with an almost flat ring-like rim around the periphery, while re-emergence allowed deposition on this bank of a series of sandbanks, beaches and terrestrial soils. These deposits were truncated by a marine erosion surface which marked the beginning of a sea-level rise that continued to a level at least 8 m above present datum, proto-Aldabra (125000 years b.p.) then consisting of a shallow bank covered by coral-rich calcarenite. Coral growth was more prolific around the rim, but coral knolls were also common within the rim to the northwest, where both coral and mollusc diversities are highest. The dominant sediment in the southeast is a Halimeda -rich sand with relatively little coral growth, but the presence of abundant Acropora around the southeast rim suggests that the dominant wind was from this quarter. Early in the deposition of this rock unit a sea-level oscillation resulted in temporary emergence, causing a change in facies or a break in deposition. A subsequent drop in sea level resulted in the cutting of a terrace at about 8 m and a more conspicuous terrace at around 4 m. Aldabra then stood as a ring of narrow, low, rocky islets surrounding a broad shallow lagoon. Deposits with a land-fauna including crocodiles, tortoises and lizards possibly date from this period. Continued depression of the sea level as a result of the Wurm/Wisconsin glaciation left Aldabra as a steep-sided rocky island, perhaps up to 100 m high. This was well vegetated, and a high rainfall caused much solutional erosion. Sea level, in response to post-Glacial climatic warming, has risen to its present position, a principal consequence being the breaching of the land rim and the flooding of the lagoon to reduce the land area by almost 60 %. Small cavity-fill deposits, caves, and stromatolitic coatings have been formed since the last interglacial, but it is difficult to place them in sequence. A restless history, of changes in the size, shape and character of Aldabra had important effects upon both the marine and terrestrial biotas. The giant tortoise, Testudo gigantea , was eliminated and has recolonized the area on at least two occasions; and the faunas and floras characterizing the sediments of each marine event are very different in composition. The evidence presented does not accord with the popular view of reef growth as steady accretion. The limestones were formed as discrete increments, often with dramatic facies changes between one unit and the next.