Evolution of a Holocene, mixed-process, forced regressive shoreline: The Mitchell River delta, Queensland, Australia

Abstract

Subtle changes in local accommodation, sediment flux, and wave, tide and fluvial processes can result in significant changes in the depositional style and architecture of coastal depositional systems. The detailed study of modern mixed-process depositional systems provides an opportunity to examine how such forcing factors affect shoreline evolution. The Mitchell River delta is a morphologically complex system that has prograded during a Holocene forced regression in a low accommodation epicontinental seaway. Relatively minor anthropogenic modification of the delta has occurred during the last 200 yr such that the considerable temporal and spatial variability of a mixed-influence coastal system can be observed in a de facto natural state. Of special interest is the link between preserved delta morphology and process change. A recent coastal process classification system was applied to the delta using desktop mapping and field ground-truthing and sedimentological analyses. The distribution and extent of 3400 wave-, tide- and fluvial-derived depositional elements were mapped across over 500 km2. These elements were grouped into 7 distinct progradational Element Complex Sets (ECS), defined by major reorganisations of the shoreline. The process classification of the overall delta system is tide dominated, fluvial influenced, wave affected (Tfw); however, the delta has evolved through three geometrically discrete pulses of delta progradation. The delta has evolved from (i) a symmetrical, wave-dominated, fluvial-influenced, tide-affected (Wft) system (early Holocene), to (ii) a rapidly prograding asymmetrical, tide-dominated, fluvial-influenced, wave-affected (Tfw) system (mid-Holocene), to (iii) an asymmetrical, tide-dominated, wave-influenced, fluvial-affected (Twf) system (at the modern shoreline). When the delta commenced progradation (6 ka BP), high accommodation-to-sediment-supply ratio (A/S) resulted in the deposition of wave formed depositional elements. As sea level fell and effective precipitation (EP) increased, reduced A/S resulted in more rapid progradation of tidal deposits until approximately 2 ka BP. The subsequent decrease in EP (and increased A/S) resulted in reduced rates of delta progradation and prompted multiple channel avulsions. Limited evidence suggests increased progradation rates at the modern channel mouth since 0.2 ka BP, which may be linked to increased sediment supply from anthropogenic catchment disturbance.