The origin of beach sediments on the North Queensland coast

Abstract

Petrographic and granulometric analyses of North Queensland beach sediments indicate their affinity with sediments delivered to the coast by rivers, and it is shown that the beaches are largely derived from fluvial sediment reworked, sorted and distributed by the dominant south-easterly waves in coastal waters. Beach sediments are generally quartzose, with subordinate felspars and admixtures of coralline sediment near fringing reefs and lithic material near river mouths and rocky shore sectors. The prevailing northerly drift of shore sediment is reduced, and locally reversed, on sectors sheltered from the dominant south-easterly waves by headlands, reefs and islands. Variations in beach sediment are related to wave conditions, distance from river-mouth sources, and patterns of drift. Four Mile Beach, near Port Douglas, is identified as anomalous in its morphological and sedimentological characteristics. It has been cut off from former sources of sediment, both fluvial and longshore, as a result of reef extension around the mouth of Mowbray River, and is now essentially a relict beach system attaining sedimentological maturity.