ABSTRACT

Recent surface sediments from the Capricorn Reef Complex at the southern end of the Great Barrier Reef have been described and analyzed for their texture, constituent particle, mineralogic, and minor element composition. These results have been interpreted with respect to the characteristics of the environment, the faunal distribution, and the dependent relationships between the variables.

Sand sized detritus is the dominant component in all but a few of the samples collected. Mud seldom exceeds 2 percent of any of the sediments but gravel does constitute a significant proportion of some. Gravel is common in sediments near the reefs where it is largely a lag-coral material and also on the deep outer shelf where it is believed to be a Pleistocene or older remnant.

Because of the heterogeneous nature of the source material most of the sediments have widely dispersed grain-size distributions. The most dispersed sediments are those closest to the reefs or reefal shoals and those on the outer shelf. Nearly all of the sediments are coarse skewed and those closest to the reefs or reefal shoals are the most coarsely skewed. Extreme values of kurtosis have been found to be characteristic of mixed sediments, particularly those near reef-edge breaks where reef top lagoonal sediments have been mixed with off-reef sediments.

Skeletal detritus from five groups of organisms--corals, coralline algae, Halimeda, foraminifers, and molluscs--constitute the bulk of the constituent particles. Their distribution is the result of two factors--(a) distribution of living organisms (the source) and (b) hydraulic agents (waves, currents, and quiet water). The various grain types react in different ways to hydraulic forces and thus are sorted and deposited under different conditions. Detritus from coral and coralline algae is most abundant in the near-reef sediments, whereas foraminifers and molluscs are more plentiful in those more distant from the reefs. Halimeda detritus is variable.

The distribution of the three carbonate minerals, aragonite, high-magnesium calcite, and low-magnesium calcite, and the two minor elements, strontium and magnesium, is controlled by the skeletal materials in the sediments. The near-reef sediments tend to be enriched in aragonite and strontium whereas the two calcites and magnesium are more plentiful in those sediments further from the reefs.