ABSTRACT: Seismic Stratigraphy of the Great Barrier Reef Shelf Edge/Upper Slope--Initiation of Reef Growth Adjacent to ODP Sites 819 to 821

FEARY, DAVID A., PHILIP A. SYMONDS, PETER J. DAVIES, and CHRISTOPHER J. PIGRAM, Bureau of Mineral Resources, Canberra, Australia

Sequence stratigraphic analysis of the high resolution seismic grid on the Great Barrier Reef shelf edge and upper slope at ODP Sites 819-821 shows that sedimentary geometries are characterized by shifting loci of sedimentation and erosion, controlled by relative sea level fluctuations and by the initiation of reef growth. Progradational sequences at the base of this Pleistocene succession are overlain by aggradational sediment packages. At different stages during deposition of this sequence, sedimentation was concentrated on the inner shelf edge; on the outer shelf edge; on the upper slope; and in broad submarine valleys.

The progradational sequences consist of broadly coarsening-upward cycles, which include substantial smaller scale grain-size variations within each cycle. The geometries of these sediment packages show that the sediment progressively infilled broad submarine valleys, with the locus of sedimentation for each package progressively

prograding further offshore. At this stage, the nature of any shore-parallel barrier system that may have acted to funnel sediment into the broad submarine valleys is unknown.

The basal sequence of the entirely aggradational portion of the succession consists of lobate sediment packages that extend seaward from the reef/barrier edge. These lobes are onlapped by relatively flat-lying sediment forming the outer shelf edge, but with complex channeling toward the outermost shelf edge. Aggradation of the uppermost sequences on the shelf edge reflects both original depositional controls and erosional processes.

The major break in the sequence corresponds to the transition from progradational to aggradational sediment geometry, and from coarsening-upward cycles to thick sequences of sandy calcareous mud interbedded with discrete muddy sand units. This is likely to reflect initiation of reef growth for the central Great Barrier Reef at about 1 m.y.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)