ABSTRACT: Holocene Platform Carbonates of the Northern Great Barrier Reef, Australia

Richard Orme, Lars Skjold

Emerging from analyses of high-resolution seismic reflection profiles and shallow vibrocores of the northern Great Barrier Reef between lat. 12°S and 15°S is a stratigraphic/sedimentary model of Holocene platform carbonates. The high carbonate production zone occupies most of the outer reef-rimmed shelf. Holocene carbonates overlie a prominent subbottom reflector representing a pre-Holocene erosion surface, regarded as the Holocene/Pleistocene unconformity. Differences in seismic response distinguish reef, back reef (coralgal), carbonate (Halimeda) bank and interreef facies. A study of overall carbonate grain size distribution shows that only a limited portion of the sediments has been transported any great distance. This contrasts with the small proportion of t rrigenous siliciclastics transported in suspension.

The most spectacular carbonate buildup is the carbonate bank facies that occupies more than one-quarter of the total shelf width, and reaches a maximum thickness of 33 m in the northern part of the region. Bank sediments are typically wackestone or packstone; the gravel and sand fractions are composed of 96% Halimeda with minor contributions from bivalves, gastropods and foraminifers. General temporal and spatial trends in texture and composition probably relate to marked cross-shelf changes apparent in seismic profiles. The bedded nature of the banks is evident in most profiles, the internal bedding usually being concordant with bank morphology, suggesting progressive upward growth. Time stratigraphic units in bank deposits probably relate to variations in texture, carbonate producti n rates, and sea level changes. The distribution and thickness of the banks are likely to be controlled by upwelling nutrients from the Coral Sea, tidal and wind-driven currents, and mud supply--the latter contributing to the banks' vertical accretion. Holocene outer-shelf carbonates began to accumulate approximately 9000 yr ago, and represent a replication of Pleistocene outer shelf conditions.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990