Great Barrier Reef: Regional Variation in a Terrigenous-Carbonate Province

Abstract

The Great Barrier Reef Province is distinguished from other carbonate areas by (1) very extensive development of shelf reefs, (2) widespread terrigenous facies, and (3) a relatively deep shelf with free water circulation. The Province occupies 100,000 sq mi of the continental shelf and varies in width from 180 mi in the south to 10 mi at latitude 14° S. Regional deepening from north to south, together with regional variation in reef morphology, permits division of the Province into Northern, Central, and Southern Regions, with boundaries at 16° S., and 20° to 21° S. Geologically, the shelf is an integral part of the Tasman Geosynclinal belt, consisting of submeridional basins and highs. Main reef development has occurred along the tectonic highs. Prolific reef growth in the Northern and Southern Regions is enhanced by oceanic currents impinging directly on the shelf, whereas in the Central Region, sparse growth results partly from less effective oceanic circulation and partly from weaker tidal activity. Local hydrological conditions influence reef morphology, since reefs expand in directions where water (and substrate) conditions are most favorable. Reefs vary in form from concentric platform reefs with shallow lagoons to linear reefs which tend to recurve at the ends, ultimately forming closed ring reefs, similar to platform reefs but characterized by deep lagoons. “Resorbed” reefs are found in areas of unfavorable water conditions. The Northern Region is dominated by linear shelf-edge reefs, the Central Region by platform and resorbed reefs and the Southern Region by a massive development of lagoonal and elongate platform reefs and ring reefs. The regional facies pattern reflects a western terrigenous source, an eastern carbonate source, and a central shelf zone which shows minor influence from either source. Local divergence from this is caused by variation in (1) shelf width and depth, (2) rate of supply and type of material delivered to the shelf, and (3) age and history of the local area. Thus, in the Southern Region where the wide shelf has a deep axial trough, a poor source of terrigenous sediments, and a massive source of carbonate, the facies pattern is characterized by a band of nearshore coarse, relict, terrigenous sands, an axial zone of modern terrigenous muds and a Marginal Shelf with high carbonate (80 percent) sediments. In the Northern Region, the shelf is narrow and has a relatively strong terrigenous source; coarse, coastal terrigenous sands are followed seaward by a wedge of terrigenous muds prograding over coarse relict quartz sands which occur within one mile of individual reefs; inter-reef sediments contain both fine terrigenous and carbonate components. In the Great Barrier Reef Province, contemporary dispersal processes appear to be ineffective, and there is a general lack of accumulation of modern sediment over most of the shelf. Substantial modern deposition is represented only by terrigenous muds adjacent to the coast in the north and in the axial shelf area in the south, and by carbonate sediments on actively forming reefs, in localized areas associated with specific reefs, and in major inter-reef passages. Elsewhere, deposition is limited. This pattern probably will remain stable until advent of a regressive sea-level phase. Marked physical and chemical differences exist between the Great Barrier Reef Province and the Florida-Bahamas Province. In the Bahamas, diverse grain types reflect local environmental variation; in the Great Barrier Reef Province, carbonate grains are almost entirely of skeletal origin and environmental differences are reflected in relative abundance of different skeletal elements. On the other hand, sediment texture and composition as well as reef distribution in both areas reflect primary hydrologic control.