Carbonate Concepts from the Maldives, Indian Ocean
The remoteness of the Maldives seems generally to have deterred visits by scientists concerned with coral reef phenomena. The little geology that is known about the Maldives has been provided chiefly by the early work of Gardiner (1903-1906) and Sewell (1936), and the more recent studies of Stod- dart (1966) and Davies et al. (1971). The early work is characterized by an emphasis on zoological studies; the later, by emphasis of marine geomorphology and reef zonation. Considerations of the origin of Maldives atolls have been made by visitors and non-visitors alike, but these have been little more than guesses because of the absence of pre-Holocene outcrops and subsurface information. All these considerations had as their starting point the plan-view reef configurations and bathymetric soundings provided by British Admiralty Charts 66a, b and c. These have been updated a number of times since they were first published in 1839 and constitute the descriptive base for the hypotheses of Darwin (1842), Gardiner (1903), Daly (1915), and Davis (1928), among others, to explain the unusual attributes of Maldives atolls within the framework of a general theory of atoll evolution. The purpose of the present volume is to present the subsurface information provided by a single exploration well and seismic reflection profiles, respectively.
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A combination of ground truth, provided by a single exploration well, and geometries apparent on seismic lines, provides new insight on the geologic development of the Maldives. The single exploration well documents the occurrence of more than 2000 m of Tertiary carbonate rock overlying volcanic basement. Seismic lines demonstrate that the shallow water Paleogene part of the drilled section is generally horizontally bedded, whereas the overlying upper Oligocene-Miocene carbonates have a distinctly prograding pattern. Significantly, the prograding pattern is directed inward from the edges of the present platform rather than outward into the thousands of meters of Indian Ocean water depths. The punctuated lateral expansion of the carbonate platforms is therefore bilaterally opposing, i.e., away from either of the Indian Ocean edges of the platform and toward each other. The same opposing directions of expansion are apparent in Saya de Malha bank, where, unlike the Maldives, coalescence has been completed in the form of a large carbonate bank. In both cases, the opposing directions of progradation appear to be the result of a structural change in depositional architecture of the kind usually associated with the thermal contraction development of sag basins.
Within this overall structural and stratigraphic framework, there is evidence that seismic stratigraphic relationships can vary considerably from atoll to atoll, indicating important local variations of the regional stratigraphy.
A second significant depositional change resulted from the onset of Pliocene-Pleistocene glacial fluctuations in sea level.