# Aspects of the Geological History and Structure of the Northern Great Barrier Reef

G. R. Orme , J. P. Webb , N. C. Kelland , G. E. G. Sargent

## Abstract

Continuous high resolution seismic profiling of the continental shelf near Cairns between latitudes 16 degrees 15$^{\prime}$ S and 16 degrees 55$^{\prime}$ S to depths of 70-120 m revealed a sequence of varied lithologies with major and minor disconformities in both reef and non-reef accumulations. The most conspicuous sub-bottom reflector (I) is a complex surface formed through marine regression, shelf emergence, and subaerial erosion, which has been dissected by an ancient drainage system represented by sediment filled channels. It occurs at approximately -67 m near Spur and Onyx reefs, has been deeply channelled below Trinity Opening, and generally rises towards the mainland. This surface marks a major disconformity representing a considerable hiatus in the development of the reef province, and has strongly influenced the distribution and thickness of both reefs and sedimentary accumulations formed during the succeeding marine transgression. While the age of reflector I is conjectural, the higher parts of this surface may have been emergent and non-depositional for longer periods than the lower parts, so that overlying sedimentary accumulations and reefs in contact with it may not be exactly contemporaneous. A continental slope terrace at approximately -116 m may reflect a late Wurm low stand of sea level, and periods of still stand during the Holocene transgression, or minor regressions may be indicated by minor disconformities, and by the marine terraces and changes in slope which occur at several levels, the most conspicuous occurring at -30 m on reefs of the marginal shelf. The seismic profiles illustrate the form and internal structure of some sedimentary units, and indicate that the present viable outer reefs are merely remnants of more extensive precursors.