Sedimentation trends in the lee of outer (ribbon) reefs, Northern Region of the Great Barrier Reef Province

Abstract

A programme of surface sediment sampling, continuous high resolution seismic reflexion profiling, and side-scan sonar surveys, carried out in the Northern Region of the Great Barrier Reef Province between latitudes 14° 31' S and 14° 45' S, extending between the continental shelf edge and a line of mid-shelf continental islands, has demonstrated the diversity of sedimentation factors controlling sediment distribution patterns, and revealed complex petrological variations and unsuspected stratigraphic relations. The granulometric characteristics of many inter-reef sediments are imparted chiefly by mixing, whereas sorting under high energy conditions, and the effectiveness of the * Sorby Principle ’ are primarily responsible for the granulometric characteristics of the reef-top sediments. Terrigenous end-members of mainland and/or continental island provenance are im portant on the southwestern margin of the area, but Halimeda debris dominates sediments coinciding with luxuriant Halimeda growth on back-reef ‘banks’ and ridges. A major disconformity indicating marine regression and shelf emergence is the most prominent seismic reflector, which outcrops near Cook’s Passage to coincide with the rocky, current swept, seabed at — 45 m, and which is incised near the shelf edge to — 69 m. The succeeding transgression resulted in the filling of the incised channels with sediment, followed by rapid sedimentation in the back-reef area and the establishment of platform reefs on this surface. Periods of still-stand or minor regressions are indicated by marine terraces at —22.5 m and —30 m, and by erosion surfaces within back-reef sedimentary deposits and reef masses. The seismic profiles suggest that the Holocene sedimentation pattern is a repetition of an ancient trend.