The nature of the pre-Holocene surface, John Brewer Reef, with implications for the interpretation of Holocene reef development

Abstract

Seismic reflection profiles trace out a steep-sided antecedent platform, assumed to be the Holocene/Pleistocene interface, beneath the post-glacial (Holocene) coralline and sediment veneer of John Brewer Reef, a small mid-shelf reef within the central Great Barrier Reef Province. This platform, defined by a semi-continuous seismic reflector, is characterised by three essentially planar surfaces, at −20m, −32 m and −40 m depth with respect to present low water datum. The −20 m surface, which exhibits a maximum relief of ±2 m, is extensive and is postulated to represent either a senile reef which grew to maturity most probably during the 50 kyr interstadial, or the erosion and concurrent infilling, during this phase, of a near — flat reefal structure which dates from an earlier −20 m interstadial. The deeper surfaces are less extensive. That at −32 m is considered to represent a fringing reef which became established on the flanks of the antecedent platform during a brief late-Pleistocene sea-level stillstand. The −40 m surface appears to be predominantly erosional in origin and also of late-Pleistocene age. Whilst these surfaces are sites of major coral growth, Holocene reef development has occurred largely independent of antecedent platform morphology. Present reef morphology reflects the interaction between the rate of post-glacial sea-level rise, prevailing hydrodynamic conditions and biological activity, and bears no resemblance to that of the underlying antecedent surface. The present morphology of John Brewer Reef is consistent with a continuous post-glacial sea-level rise.