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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 54 (1970)

Issue: 5. (May)

First Page: 836

Last Page: 836

Title: Geomorphology of Reef Complexes: ABSTRACT

Author(s): Arthur L. Bloom

Article Type: Meeting abstract


Quaternary reef complexes are primarily Darwinian oceanic or shelf reefs on slowly subsiding foundations. "Glacial control" of reef morphology has been important, but not in the sense that Daly visualized. Corals dated by the uranium-series method prove that many, perhaps most, reefs were not truncated at the low sea levels of the last glacial age. Dates from such scattered and classic reef localities as Western Australia, the Tuamotu Archipelago, Eniwetok Atoll, the Florida Keys, the Bahamas, and Barbados demonstrate that reef limestones and eolianites can be emerged for 100,000 years or more, several hundred feet above the glacially lowered sea, without being dissolved away. The permeability of reef frameworks is the major reason for their durability under subaerial tro ical weathering. Although Pleistocene and older reefs were not eroded to sea level during glacial ages, the extensive drowned karst topography on them, as identified from Florida, Bermuda, the Bahamas, and the Marshall Islands, probably has been a primary control in the post-glacial evolution of modern reef complexes.

Reef growth probably was not suppressed, in the tropical Pacific at least, during glacial low sea levels. Recent calculations of the change in O18 isotopic composition of seawater by continental ice sheets minimize the effect attributable to cooling of the sea surface. The central Pacific was neither too cold nor too turbid for coral-reef development even during a full glacial age. Pelagic carbonate productivity in the equatorial Pacific actually increased during glacial ages, and it is likely that reefs flourished on the exposed flanks of emerged atolls. The steep flanks of reef-rimmed Pacific islands and atolls may provide evidence by which we can measure the true Pleistocene fluctuations of sea level, and thereby infer the amount of Quaternary warping of continental shel es.

The concept that postglacial sea level higher than the present is necessary to cause modern reef morphology needs to be rigorously reexamined. Rather than being relict from a "10-ft stand of the sea" a few thousand years ago, modern reefs show every indication of being in dynamic equilibrium with the forces that now act on them. Modern reef-building organisms have reoccupied the weathered surfaces of reef limestones that have been exposed for nearly 100,000 years and have veneered the tops of ancient atolls or eolianite dune fields. As the postglacial rise of sea level has approached its present level, upward growth has slowed and reefs have expanded laterally to resharpen the precipitous upper slopes of eroded ancient platforms. Primary productivity by reef builders is generally adeq ate to keep pace with rising sea level but, at several localities, atolls were "drowned" and have not reached present sea level and may be in the process of being preserved.

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