Coral variations in two deep drill cores from the Northern Great Barrier Reef: significance for the Pleistocene development of the Great Barrier Reef

Variations in lithology and coral assemblages in drill cores from outer- and inner-shelf reefs are used to characterize the Pleistocene development of the Great Barrier Reef. Based on petrographic, isotopic and seismic characteristics, the outer-shelf core from Ribbon Reef 5 is divided into three sections: (1) a main reef section from 0 to 96 m is composed of six reef units, (2) a rhodolith section from 96 to 158 m is interbedded with two thin reef units and (3) a basal section from 158 to 210 m is composed of non-reefal skeletal grainstones and packstones. Two distinct coral assemblages identified in this core represent a shallow, high-energy community and lower-energy community. These two assemblages are repeated throughout the main reef section, with some units recording transitions between assemblages, and others composed of only a single assemblage. These coral assemblage data also correlate with transitions recorded by coralline algae. Using similar criteria, the inner-shelf core from Boulder Reef is divided into two sections: (1) an upper carbonate-dominated section from 0 to 34 m is comprised of four reef units and (2) an underlying mud section from 34 to 86 m is composed of siliciclastics and two thin, coral-bearing units. The four reef units in the upper section are dominated by a single coral assemblage representing a community typical of low energy, turbid environments. Taken together, these data indicate that: (1) reef growth on the inner shelf initiated later than on the outer shelf, (2) true reef ‘turn-on’ in outer shelf areas, as represented by the main reef section in Ribbon Reef 5, was preceded by a transitional period of intermittent reef development and (3) the repeated occurrence of similar coral assemblages in both drill cores indicates that the Great Barrier Reef has been able to re-establish itself, repeatedly producing reefs of similar composition over the last 500 ky, despite major environmental fluctuations in sea level and perhaps temperature. D 2003 Elsevier Science B.V. All rights reserved