Terrigenous sediments as influences upon Holocene nearshore coral reefs, central Great Barrier Reef, Australia

Abstract

Sedimentary studies of inner shelf reefs on the Great Barrier Reef shelf increasingly reveal evidence for coral growth in turbid, shallow coastal environments, where coral survival is aided by prevailing hydrodynamic and sedimentological conditions. New models of coral reef growth on the Great Barrier Reef are needed which explicitly include the role of terrigenous sediments in influencing sites of potential reef initiation and conditions of growth. We outline potential interrelationships of sediment dynamics, terrigenous sedimentation and coral reef growth on the Great Barrier Reef over the last 9000 years, by integrating measurements of hydrodynamic and sedimentary processes at inner shelf coral reefs near Townsville, with an assembled dataset of 324 radiocarbon‐dated corals. In Halifax Bay, ‘coastal turbid‐zone reefs’ occur in water less than ∼4 m deep, where coastal turbidity is high, sometimes over 100 NTU (nephelometric turbidity units) as a result of wave‐induced resuspension, and wind‐driven longshore currents prevent accumulation of fine‐grained sediment. Net accumulation of sediments at these sites over the last 5500 years has been less than 1 m. A model of mid‐ and inner shelf evolution is presented, which describes a suite of potential conditions and sites of reef initiation on the central Great Barrier Reef, related to coastal geomorphology and the resulting sedimentary regime under a rising sea‐level. We distinguish progradational, erosional and indented island coast‐lines, which, by virtue of their likely sedimentary character, may have distinct patterns of coral reef growth in time and space. At some stages during the transgression, the presence of inner shelf sediment wedges may have delayed initiation of nearshore reefs for a period of a few centuries.