Geometry of Pleistocene facies on the Great Barrier Reef outer shelf and upper slope - seismic stratigraphy of sites 819, 820, and 821

Seismic stratigraphic analysis of the sedimentary succession intersected in Ocean Drilling Program (ODP) Sites 819, 820, and 821, on the outer shelf and upper slope seaward of the Great Barrier Reef, provides a clear indication of the importance of sediment supply and depositional base-level as two of the fundamental parameters that control the geometry of seismic sequences. The nine predominantly unconformity-bounded seismic sequences display two different geometric styles: obliquely progradational sequences at the base of the succession are succeeded by purely aggradational sequences. The distribution of the aggradational sequences shows that deposition was concentrated on the outer shelf, with the locus of sedimentation varying from the shelf edge to immediately adjacent to the Great Barrier Reef. The locus of deposition of the underlying obliquely progradational sequences was predominantly within broad submarine valleys immediately below the paleoshelf edge. Depositional base-level fluctuations controlled the textural and compositional characteristics of sediment that was deposited on the outer shelf and upper slope, and formed sequence and subsequence boundaries characterized by offlap and/or onlap. However, it was the initiation of reef growth to form an outer reef barrier, at approximately 0.75 to 1 Ma, that restricted sediment supply to the outer shelf and resulted in the two fundamentally different types of seismic sequence geometry.