Gladstone Harbour forms the central portion of the Port Curtis Estuary, a drowned river valley on the east coast of Queensland. Physiographically, the harbour consists of an axial region of deep tidal channels and linear sandbanks, and this is flanked on both sides by zones of tidal flats and mangrove swamp.
Throughout the area, textural variation of the bottom sediments is controlled primarily by areal differences in the hydrography. Accordingly, coarse lag sediments' characterize the deep channels of the axial zone, where swift tidal currents occur during flood and ebb tides; finer sediment, winnowed from these sites by the currents, is deposited upon the axial and lateral shoa1s. Moreover, sediment differentiation is enhanced by tidal concentration of muddy sediment towards the peripheral shoals; muds are deposited in these areas in sheltered parts which are not subjected to strong currents and waves. Locally, certain areas of the axial zone are characterized by texturally immature sediments, and these are probably related to zones of intense bottom scour which occur mainly at ebb tide. Secondary currents-sets of alternating right and left helical vortices which have horizontal axes parallel to the channels-also occur at this time and are thought to have an important influence upon the redistribution of suspended sediment and upon the growth and maintenance of linear sandbanks.
Textural study of the bottom sediments has delineated four broad sediment facies. They comprise a sandy facies, a muddy facies, a mixed or intermediate facies, and· a carbonate facies associated with a small area of fringing coral-algal reefs. In the deeper channel areas, modem marine sands are underlain by fluvial sediments which were probably deposited during the waning stages of Pleistocene glacial periods as gradual drowning of the region took place.